From ca884980ee8e6fe1269f5abdb803519d51aa09c0 Mon Sep 17 00:00:00 2001
From: Oswaldo Ferreira <oswaldo@gitlab.com>
Date: Sun, 7 Apr 2019 15:35:16 -0300
Subject: [CE] Support multiple assignees for merge requests

Backports https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10161
(code out of ee/ folder).
---
 .../javascripts/issuable_bulk_update_actions.js    |   3 -
 .../sidebar/components/assignees/assignees.vue     |  33 ++++-
 app/assets/stylesheets/pages/merge_requests.scss   |  10 ++
 app/assets/stylesheets/pages/projects.scss         |   2 +
 app/controllers/concerns/issuable_actions.rb       |   7 +-
 app/controllers/concerns/issuable_collections.rb   |  10 +-
 .../merge_requests/application_controller.rb       |   2 +-
 app/finders/issuable_finder.rb                     |  30 ++--
 app/finders/issues_finder.rb                       |  14 --
 app/helpers/boards_helper.rb                       |   2 +-
 app/helpers/form_helper.rb                         |  39 ++++-
 app/helpers/issuables_helper.rb                    |  11 +-
 app/mailers/emails/merge_requests.rb               |   6 +-
 app/mailers/notify.rb                              |   2 +
 app/models/concerns/deprecated_assignee.rb         |  86 +++++++++++
 app/models/concerns/issuable.rb                    |  41 ++++--
 app/models/issue.rb                                |  22 ---
 app/models/merge_request.rb                        |  51 +------
 app/models/project.rb                              |   4 +
 app/serializers/issuable_sidebar_extras_entity.rb  |   2 +
 app/serializers/issue_sidebar_extras_entity.rb     |   1 -
 app/serializers/merge_request_assignee_entity.rb   |   7 +
 app/serializers/merge_request_basic_entity.rb      |   3 +-
 app/serializers/merge_request_serializer.rb        |   4 +-
 .../merge_request_sidebar_basic_entity.rb          |  11 --
 .../merge_request_sidebar_extras_entity.rb         |   7 +
 app/services/issuable_base_service.rb              |  18 ++-
 app/services/issues/base_service.rb                |  22 +--
 app/services/issues/update_service.rb              |   2 +-
 app/services/merge_requests/base_service.rb        |   6 +-
 app/services/merge_requests/update_service.rb      |  18 +--
 app/services/notification_recipient_service.rb     |  24 +---
 app/services/notification_service.rb               |  20 +--
 app/services/system_note_service.rb                |  10 +-
 app/services/todo_service.rb                       |  16 +--
 .../notify/_reassigned_issuable_email.html.haml    |  10 ++
 .../notify/closed_merge_request_email.text.haml    |   2 +-
 app/views/notify/issue_due_email.html.haml         |   2 +-
 app/views/notify/issue_due_email.text.erb          |   2 +-
 .../notify/merge_request_status_email.text.haml    |   2 +-
 .../merge_request_unmergeable_email.text.haml      |   2 +-
 .../notify/merged_merge_request_email.text.haml    |   2 +-
 app/views/notify/new_issue_email.html.haml         |   2 +-
 app/views/notify/new_issue_email.text.erb          |   2 +-
 .../notify/new_mention_in_issue_email.text.erb     |   2 +-
 .../new_mention_in_merge_request_email.text.erb    |   2 +-
 app/views/notify/new_merge_request_email.html.haml |   4 +-
 app/views/notify/new_merge_request_email.text.erb  |   2 +-
 app/views/notify/reassigned_issue_email.html.haml  |  11 +-
 .../reassigned_merge_request_email.html.haml       |  11 +-
 .../notify/reassigned_merge_request_email.text.erb |   4 +-
 app/views/projects/issues/_issue.html.haml         |   2 +-
 .../merge_requests/_merge_request.html.haml        |   4 +-
 .../boards/components/sidebar/_assignee.html.haml  |   2 +-
 app/views/shared/issuable/_assignees.html.haml     |   6 +-
 .../shared/issuable/_bulk_update_sidebar.html.haml |   5 +-
 .../shared/issuable/_sidebar_assignees.html.haml   |  56 ++------
 .../form/_merge_request_assignee.html.haml         |  31 ----
 app/views/shared/issuable/form/_metadata.html.haml |   7 +-
 .../form/_metadata_issuable_assignee.html.haml     |  11 ++
 .../form/_metadata_issue_assignee.html.haml        |  11 --
 db/fixtures/development/10_merge_requests.rb       |   2 +-
 lib/api/entities.rb                                |   6 +-
 lib/api/merge_requests.rb                          |   4 +
 .../reference_parser/merge_request_parser.rb       |   2 +-
 lib/gitlab/hook_data/issuable_builder.rb           |   6 +-
 lib/gitlab/hook_data/merge_request_builder.rb      |   5 +-
 locale/gitlab.pot                                  |  18 ---
 qa/qa/page/merge_request/new.rb                    |   2 +-
 .../projects/merge_requests_controller_spec.rb     |   6 +-
 spec/features/dashboard/issuables_counter_spec.rb  |   4 +-
 spec/features/dashboard/merge_requests_spec.rb     |   4 +-
 spec/features/groups/merge_requests_spec.rb        |   2 +-
 spec/features/issues/form_spec.rb                  |   4 +-
 .../user_creates_merge_request_spec.rb             |   4 +-
 .../features/merge_request/user_creates_mr_spec.rb |  15 +-
 spec/features/merge_request/user_edits_mr_spec.rb  |  18 ++-
 .../user_filters_by_assignees_spec.rb              |   2 +-
 .../user_filters_by_multiple_criteria_spec.rb      |   2 +-
 .../user_lists_merge_requests_spec.rb              |   4 +-
 .../merge_requests/user_mass_updates_spec.rb       |   3 +-
 .../search/user_uses_header_search_field_spec.rb   |   4 +-
 spec/finders/issues_finder_spec.rb                 |  64 +++------
 spec/finders/merge_requests_finder_spec.rb         |  49 +++++--
 .../api/schemas/entities/merge_request_basic.json  |  12 +-
 .../api/schemas/public_api/v4/merge_request.json   |   5 +
 spec/javascripts/sidebar/assignees_spec.js         |  85 +++++++++++
 spec/lib/gitlab/hook_data/issuable_builder_spec.rb |   6 +-
 .../gitlab/hook_data/merge_request_builder_spec.rb |   1 +
 spec/lib/gitlab/import_export/all_models.yml       |   4 +
 .../gitlab/import_export/safe_model_attributes.yml |   4 +
 spec/lib/gitlab/issuable_metadata_spec.rb          |   4 +-
 spec/mailers/notify_spec.rb                        |  18 +--
 spec/models/ci/pipeline_spec.rb                    |   8 +-
 spec/models/concerns/deprecated_assignee_spec.rb   | 160 +++++++++++++++++++++
 spec/models/concerns/issuable_spec.rb              |   7 +-
 spec/models/event_spec.rb                          |   2 +-
 spec/models/merge_request_spec.rb                  |  60 ++------
 spec/models/user_spec.rb                           |   6 +-
 spec/requests/api/events_spec.rb                   |   4 +-
 spec/requests/api/merge_requests_spec.rb           | 124 +++++++++++++---
 spec/services/issuable/bulk_update_service_spec.rb |  16 +--
 spec/services/issuable/destroy_service_spec.rb     |   2 +-
 spec/services/members/destroy_service_spec.rb      |   4 +-
 spec/services/merge_requests/close_service_spec.rb |   2 +-
 .../create_from_issue_service_spec.rb              |   2 +-
 .../services/merge_requests/create_service_spec.rb |  30 ++--
 .../merge_requests/ff_merge_service_spec.rb        |   2 +-
 spec/services/merge_requests/merge_service_spec.rb |   4 +-
 .../merge_requests/merge_to_ref_service_spec.rb    |   2 +-
 .../merge_requests/post_merge_service_spec.rb      |   2 +-
 .../services/merge_requests/reopen_service_spec.rb |   2 +-
 .../services/merge_requests/update_service_spec.rb |  42 +++---
 spec/services/notification_service_spec.rb         |  55 +++----
 .../quick_actions/interpret_service_spec.rb        |   4 +-
 spec/services/system_note_service_spec.rb          |   6 +-
 spec/services/todo_service_spec.rb                 | 110 +++++++-------
 spec/services/users/destroy_service_spec.rb        |   4 +-
 .../merge_requests_finder_shared_contexts.rb       |   6 +-
 .../shared_contexts/merge_request_create.rb        |  26 ++++
 spec/support/shared_contexts/merge_request_edit.rb |  28 ++++
 .../creatable_merge_request_shared_examples.rb     |  31 +---
 .../editable_merge_request_shared_examples.rb      |  28 +---
 .../multiple_assignees_mr_shared_examples.rb       |  47 ++++++
 .../finders/assignees_filter_spec.rb               |  49 +++++++
 .../projects/merge_requests/edit.html.haml_spec.rb |   6 +-
 .../projects/merge_requests/show.html.haml_spec.rb |  13 --
 127 files changed, 1189 insertions(+), 817 deletions(-)
 create mode 100644 app/models/concerns/deprecated_assignee.rb
 create mode 100644 app/serializers/merge_request_assignee_entity.rb
 delete mode 100644 app/serializers/merge_request_sidebar_basic_entity.rb
 create mode 100644 app/serializers/merge_request_sidebar_extras_entity.rb
 create mode 100644 app/views/notify/_reassigned_issuable_email.html.haml
 delete mode 100644 app/views/shared/issuable/form/_merge_request_assignee.html.haml
 create mode 100644 app/views/shared/issuable/form/_metadata_issuable_assignee.html.haml
 delete mode 100644 app/views/shared/issuable/form/_metadata_issue_assignee.html.haml
 create mode 100644 spec/models/concerns/deprecated_assignee_spec.rb
 create mode 100644 spec/support/shared_contexts/merge_request_create.rb
 create mode 100644 spec/support/shared_contexts/merge_request_edit.rb
 create mode 100644 spec/support/shared_examples/features/multiple_assignees_mr_shared_examples.rb
 create mode 100644 spec/support/shared_examples/finders/assignees_filter_spec.rb

diff --git a/app/assets/javascripts/issuable_bulk_update_actions.js b/app/assets/javascripts/issuable_bulk_update_actions.js
index b844e4c5e5b..ccbe591a63e 100644
--- a/app/assets/javascripts/issuable_bulk_update_actions.js
+++ b/app/assets/javascripts/issuable_bulk_update_actions.js
@@ -81,9 +81,6 @@ export default {
     const formData = {
       update: {
         state_event: this.form.find('input[name="update[state_event]"]').val(),
-        // For Merge Requests
-        assignee_id: this.form.find('input[name="update[assignee_id]"]').val(),
-        // For Issues
         assignee_ids: [this.form.find('input[name="update[assignee_ids][]"]').val()],
         milestone_id: this.form.find('input[name="update[milestone_id]"]').val(),
         issuable_ids: this.form.find('input[name="update[issuable_ids]"]').val(),
diff --git a/app/assets/javascripts/sidebar/components/assignees/assignees.vue b/app/assets/javascripts/sidebar/components/assignees/assignees.vue
index d1a396182b3..ce378e24289 100644
--- a/app/assets/javascripts/sidebar/components/assignees/assignees.vue
+++ b/app/assets/javascripts/sidebar/components/assignees/assignees.vue
@@ -74,8 +74,7 @@ export default {
       }
 
       if (!this.users.length) {
-        const emptyTooltipLabel =
-          this.issuableType === 'issue' ? __('Assignee(s)') : __('Assignee');
+        const emptyTooltipLabel = __('Assignee(s)');
         names.push(emptyTooltipLabel);
       }
 
@@ -90,6 +89,27 @@ export default {
 
       return counter;
     },
+    mergeNotAllowedTooltipMessage() {
+      const assigneesCount = this.users.length;
+
+      if (this.issuableType !== 'merge_request' || assigneesCount === 0) {
+        return null;
+      }
+
+      const cannotMergeCount = this.users.filter(u => u.can_merge === false).length;
+      const canMergeCount = assigneesCount - cannotMergeCount;
+
+      if (canMergeCount === assigneesCount) {
+        // Everyone can merge
+        return null;
+      } else if (cannotMergeCount === assigneesCount && assigneesCount > 1) {
+        return 'No one can merge';
+      } else if (assigneesCount === 1) {
+        return 'Cannot merge';
+      }
+
+      return `${canMergeCount}/${assigneesCount} can merge`;
+    },
   },
   methods: {
     assignSelf() {
@@ -154,6 +174,15 @@ export default {
       </button>
     </div>
     <div class="value hide-collapsed">
+      <span
+        v-if="mergeNotAllowedTooltipMessage"
+        v-tooltip
+        :title="mergeNotAllowedTooltipMessage"
+        data-placement="left"
+        class="float-right cannot-be-merged"
+      >
+        <i aria-hidden="true" data-hidden="true" class="fa fa-exclamation-triangle"></i>
+      </span>
       <template v-if="hasNoUsers">
         <span class="assign-yourself no-value">
           No assignee
diff --git a/app/assets/stylesheets/pages/merge_requests.scss b/app/assets/stylesheets/pages/merge_requests.scss
index 86b58c1b1b2..709940ba6c8 100644
--- a/app/assets/stylesheets/pages/merge_requests.scss
+++ b/app/assets/stylesheets/pages/merge_requests.scss
@@ -498,6 +498,16 @@
       flex: 1;
     }
 
+    .issuable-meta {
+      .author-link {
+        display: inline-block;
+      }
+
+      .issuable-comments {
+        height: 18px;
+      }
+    }
+
     .merge-request-title {
       margin-bottom: 2px;
 
diff --git a/app/assets/stylesheets/pages/projects.scss b/app/assets/stylesheets/pages/projects.scss
index 792c618fd40..6594f20456d 100644
--- a/app/assets/stylesheets/pages/projects.scss
+++ b/app/assets/stylesheets/pages/projects.scss
@@ -1158,6 +1158,8 @@ pre.light-well {
 .cannot-be-merged:hover {
   color: $red-500;
   margin-top: 2px;
+  position: relative;
+  z-index: 2;
 }
 
 .private-forks-notice .private-fork-icon {
diff --git a/app/controllers/concerns/issuable_actions.rb b/app/controllers/concerns/issuable_actions.rb
index 85aeecbf90b..065d2d3a4ec 100644
--- a/app/controllers/concerns/issuable_actions.rb
+++ b/app/controllers/concerns/issuable_actions.rb
@@ -192,12 +192,7 @@ module IssuableActions
 
   def bulk_update_params
     permitted_keys_array = permitted_keys.dup
-
-    if resource_name == 'issue'
-      permitted_keys_array << { assignee_ids: [] }
-    else
-      permitted_keys_array.unshift(:assignee_id)
-    end
+    permitted_keys_array << { assignee_ids: [] }
 
     params.require(:update).permit(permitted_keys_array)
   end
diff --git a/app/controllers/concerns/issuable_collections.rb b/app/controllers/concerns/issuable_collections.rb
index 6d6e0cc6c7f..91e875dca54 100644
--- a/app/controllers/concerns/issuable_collections.rb
+++ b/app/controllers/concerns/issuable_collections.rb
@@ -190,15 +190,15 @@ module IssuableCollections
                          end
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def preload_for_collection
+    common_attributes = [:author, :assignees, :labels, :milestone]
     @preload_for_collection ||= case collection_type
                                 when 'Issue'
-                                  [:project, :author, :assignees, :labels, :milestone, project: :namespace]
+                                  common_attributes + [:project, project: :namespace]
                                 when 'MergeRequest'
-                                  [
-                                    :target_project, :author, :assignee, :labels, :milestone,
-                                    source_project: :route, head_pipeline: :project, target_project: :namespace, latest_merge_request_diff: :merge_request_diff_commits
-                                  ]
+                                  common_attributes + [:target_project, source_project: :route, head_pipeline: :project, target_project: :namespace, latest_merge_request_diff: :merge_request_diff_commits]
                                 end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 end
diff --git a/app/controllers/projects/merge_requests/application_controller.rb b/app/controllers/projects/merge_requests/application_controller.rb
index 6045ee4e171..eb469d2d714 100644
--- a/app/controllers/projects/merge_requests/application_controller.rb
+++ b/app/controllers/projects/merge_requests/application_controller.rb
@@ -20,7 +20,6 @@ class Projects::MergeRequests::ApplicationController < Projects::ApplicationCont
   def merge_request_params_attributes
     [
       :allow_collaboration,
-      :assignee_id,
       :description,
       :force_remove_source_branch,
       :lock_version,
@@ -35,6 +34,7 @@ class Projects::MergeRequests::ApplicationController < Projects::ApplicationCont
       :title,
       :discussion_locked,
       label_ids: [],
+      assignee_ids: [],
       update_task: [:index, :checked, :line_number, :line_source]
     ]
   end
diff --git a/app/finders/issuable_finder.rb b/app/finders/issuable_finder.rb
index 64c88505a16..88ec77426d5 100644
--- a/app/finders/issuable_finder.rb
+++ b/app/finders/issuable_finder.rb
@@ -439,22 +439,6 @@ class IssuableFinder
   end
   # rubocop: enable CodeReuse/ActiveRecord
 
-  # rubocop: disable CodeReuse/ActiveRecord
-  def by_assignee(items)
-    if filter_by_no_assignee?
-      items.where(assignee_id: nil)
-    elsif filter_by_any_assignee?
-      items.where('assignee_id IS NOT NULL')
-    elsif assignee
-      items.where(assignee_id: assignee.id)
-    elsif assignee_id? || assignee_username? # assignee not found
-      items.none
-    else
-      items
-    end
-  end
-  # rubocop: enable CodeReuse/ActiveRecord
-
   def filter_by_no_assignee?
     # Assignee_id takes precedence over assignee_username
     [NONE, FILTER_NONE].include?(params[:assignee_id].to_s.downcase) || params[:assignee_username].to_s == NONE
@@ -478,6 +462,20 @@ class IssuableFinder
   end
   # rubocop: enable CodeReuse/ActiveRecord
 
+  def by_assignee(items)
+    if filter_by_no_assignee?
+      items.unassigned
+    elsif filter_by_any_assignee?
+      items.assigned
+    elsif assignee
+      items.assigned_to(assignee)
+    elsif assignee_id? || assignee_username? # assignee not found
+      items.none
+    else
+      items
+    end
+  end
+
   # rubocop: disable CodeReuse/ActiveRecord
   def by_milestone(items)
     if milestones?
diff --git a/app/finders/issues_finder.rb b/app/finders/issues_finder.rb
index cb44575d6f1..e6a82f55856 100644
--- a/app/finders/issues_finder.rb
+++ b/app/finders/issues_finder.rb
@@ -144,18 +144,4 @@ class IssuesFinder < IssuableFinder
 
     current_user.blank?
   end
-
-  def by_assignee(items)
-    if filter_by_no_assignee?
-      items.unassigned
-    elsif filter_by_any_assignee?
-      items.assigned
-    elsif assignee
-      items.assigned_to(assignee)
-    elsif assignee_id? || assignee_username? # assignee not found
-      items.none
-    else
-      items
-    end
-  end
 end
diff --git a/app/helpers/boards_helper.rb b/app/helpers/boards_helper.rb
index be1e7016a1e..1640f4fc93f 100644
--- a/app/helpers/boards_helper.rb
+++ b/app/helpers/boards_helper.rb
@@ -69,7 +69,7 @@ module BoardsHelper
   end
 
   def board_sidebar_user_data
-    dropdown_options = issue_assignees_dropdown_options
+    dropdown_options = assignees_dropdown_options('issue')
 
     {
       toggle: 'dropdown',
diff --git a/app/helpers/form_helper.rb b/app/helpers/form_helper.rb
index 8b3d270e873..f7c7f37cc38 100644
--- a/app/helpers/form_helper.rb
+++ b/app/helpers/form_helper.rb
@@ -17,8 +17,8 @@ module FormHelper
     end
   end
 
-  def issue_assignees_dropdown_options
-    {
+  def assignees_dropdown_options(issuable_type)
+    dropdown_data = {
       toggle_class: 'js-user-search js-assignee-search js-multiselect js-save-user-data',
       title: 'Select assignee',
       filter: true,
@@ -28,8 +28,8 @@ module FormHelper
         first_user: current_user&.username,
         null_user: true,
         current_user: true,
-        project_id: @project&.id,
-        field_name: 'issue[assignee_ids][]',
+        project_id: (@target_project || @project)&.id,
+        field_name: "#{issuable_type}[assignee_ids][]",
         default_label: 'Unassigned',
         'max-select': 1,
         'dropdown-header': 'Assignee',
@@ -39,5 +39,36 @@ module FormHelper
         current_user_info: UserSerializer.new.represent(current_user)
       }
     }
+
+    type = issuable_type.to_s
+
+    if type == 'issue' && issue_supports_multiple_assignees? ||
+        type == 'merge_request' && merge_request_supports_multiple_assignees?
+      dropdown_data = multiple_assignees_dropdown_options(dropdown_data)
+    end
+
+    dropdown_data
+  end
+
+  # Overwritten
+  def issue_supports_multiple_assignees?
+    false
+  end
+
+  # Overwritten
+  def merge_request_supports_multiple_assignees?
+    false
+  end
+
+  private
+
+  def multiple_assignees_dropdown_options(options)
+    new_options = options.dup
+
+    new_options[:title] = 'Select assignee(s)'
+    new_options[:data][:'dropdown-header'] = 'Assignee(s)'
+    new_options[:data].delete(:'max-select')
+
+    new_options
   end
 end
diff --git a/app/helpers/issuables_helper.rb b/app/helpers/issuables_helper.rb
index 52c49498e9b..9a12db258d5 100644
--- a/app/helpers/issuables_helper.rb
+++ b/app/helpers/issuables_helper.rb
@@ -15,11 +15,14 @@ module IssuablesHelper
     sidebar_gutter_collapsed? ? _('Expand sidebar') : _('Collapse sidebar')
   end
 
-  def sidebar_assignee_tooltip_label(issuable)
-    if issuable.assignee
-      issuable.assignee.name
+  def assignees_label(issuable, include_value: true)
+    label = 'Assignee'.pluralize(issuable.assignees.count)
+
+    if include_value
+      sanitized_list = sanitize_name(issuable.assignee_list)
+      "#{label}: #{sanitized_list}"
     else
-      issuable.allows_multiple_assignees? ? _('Assignee(s)') : _('Assignee')
+      label
     end
   end
 
diff --git a/app/mailers/emails/merge_requests.rb b/app/mailers/emails/merge_requests.rb
index 9ba8f92fcbf..63148831a24 100644
--- a/app/mailers/emails/merge_requests.rb
+++ b/app/mailers/emails/merge_requests.rb
@@ -24,10 +24,12 @@ module Emails
     end
 
     # rubocop: disable CodeReuse/ActiveRecord
-    def reassigned_merge_request_email(recipient_id, merge_request_id, previous_assignee_id, updated_by_user_id, reason = nil)
+    def reassigned_merge_request_email(recipient_id, merge_request_id, previous_assignee_ids, updated_by_user_id, reason = nil)
       setup_merge_request_mail(merge_request_id, recipient_id)
 
-      @previous_assignee = User.find_by(id: previous_assignee_id) if previous_assignee_id
+      @previous_assignees = []
+      @previous_assignees = User.where(id: previous_assignee_ids) if previous_assignee_ids.any?
+
       mail_answer_thread(@merge_request, merge_request_thread_options(updated_by_user_id, recipient_id, reason))
     end
     # rubocop: enable CodeReuse/ActiveRecord
diff --git a/app/mailers/notify.rb b/app/mailers/notify.rb
index efa1233b434..0b740809f30 100644
--- a/app/mailers/notify.rb
+++ b/app/mailers/notify.rb
@@ -4,6 +4,7 @@ class Notify < BaseMailer
   include ActionDispatch::Routing::PolymorphicRoutes
   include GitlabRoutingHelper
   include EmailsHelper
+  include IssuablesHelper
 
   include Emails::Issues
   include Emails::MergeRequests
@@ -24,6 +25,7 @@ class Notify < BaseMailer
   helper MembersHelper
   helper AvatarsHelper
   helper GitlabRoutingHelper
+  helper IssuablesHelper
 
   def test_email(recipient_email, subject, body)
     mail(to: recipient_email,
diff --git a/app/models/concerns/deprecated_assignee.rb b/app/models/concerns/deprecated_assignee.rb
new file mode 100644
index 00000000000..7f12ce39c96
--- /dev/null
+++ b/app/models/concerns/deprecated_assignee.rb
@@ -0,0 +1,86 @@
+# frozen_string_literal: true
+
+# This module handles backward compatibility for import/export of Merge Requests after
+# multiple assignees feature was introduced. Also, it handles the scenarios where
+# the #26496 background migration hasn't finished yet.
+# Ideally, most of this code should be removed at #59457.
+module DeprecatedAssignee
+  extend ActiveSupport::Concern
+
+  def assignee_ids=(ids)
+    nullify_deprecated_assignee
+    super
+  end
+
+  def assignees=(users)
+    nullify_deprecated_assignee
+    super
+  end
+
+  def assignee_id=(id)
+    self.assignee_ids = Array(id)
+  end
+
+  def assignee=(user)
+    self.assignees = Array(user)
+  end
+
+  def assignee
+    assignees.first
+  end
+
+  def assignee_id
+    assignee_ids.first
+  end
+
+  def assignee_ids
+    if Gitlab::Database.read_only? && pending_assignees_population?
+      return Array(deprecated_assignee_id)
+    end
+
+    update_assignees_relation
+    super
+  end
+
+  def assignees
+    if Gitlab::Database.read_only? && pending_assignees_population?
+      return User.where(id: deprecated_assignee_id)
+    end
+
+    update_assignees_relation
+    super
+  end
+
+  private
+
+  # This will make the background migration process quicker (#26496) as it'll have less
+  # assignee_id rows to look through.
+  def nullify_deprecated_assignee
+    return unless persisted? && Gitlab::Database.read_only?
+
+    update_column(:assignee_id, nil)
+  end
+
+  # This code should be removed in the clean-up phase of the
+  # background migration (#59457).
+  def pending_assignees_population?
+    persisted? && deprecated_assignee_id && merge_request_assignees.empty?
+  end
+
+  # If there's an assignee_id and no relation, it means the background
+  # migration at #26496 didn't reach this merge request yet.
+  # This code should be removed in the clean-up phase of the
+  # background migration (#59457).
+  def update_assignees_relation
+    if pending_assignees_population?
+      transaction do
+        merge_request_assignees.create!(user_id: deprecated_assignee_id, merge_request_id: id)
+        update_column(:assignee_id, nil)
+      end
+    end
+  end
+
+  def deprecated_assignee_id
+    read_attribute(:assignee_id)
+  end
+end
diff --git a/app/models/concerns/issuable.rb b/app/models/concerns/issuable.rb
index 17f94b4bd9b..3232c51bfbd 100644
--- a/app/models/concerns/issuable.rb
+++ b/app/models/concerns/issuable.rb
@@ -67,13 +67,6 @@ module Issuable
              allow_nil: true,
              prefix: true
 
-    delegate :name,
-             :email,
-             :public_email,
-             to: :assignee,
-             allow_nil: true,
-             prefix: true
-
     validates :author, presence: true
     validates :title, presence: true, length: { maximum: 255 }
     validate :milestone_is_valid
@@ -88,6 +81,19 @@ module Issuable
     scope :only_opened, -> { with_state(:opened) }
     scope :closed, -> { with_state(:closed) }
 
+    # rubocop:disable GitlabSecurity/SqlInjection
+    # The `to_ability_name` method is not an user input.
+    scope :assigned, -> do
+      where("EXISTS (SELECT TRUE FROM #{to_ability_name}_assignees WHERE #{to_ability_name}_id = #{to_ability_name}s.id)")
+    end
+    scope :unassigned, -> do
+      where("NOT EXISTS (SELECT TRUE FROM #{to_ability_name}_assignees WHERE #{to_ability_name}_id = #{to_ability_name}s.id)")
+    end
+    scope :assigned_to, ->(u) do
+      where("EXISTS (SELECT TRUE FROM #{to_ability_name}_assignees WHERE user_id = ? AND #{to_ability_name}_id = #{to_ability_name}s.id)", u.id)
+    end
+    # rubocop:enable GitlabSecurity/SqlInjection
+
     scope :left_joins_milestones,    -> { joins("LEFT OUTER JOIN milestones ON #{table_name}.milestone_id = milestones.id") }
     scope :order_milestone_due_desc, -> { left_joins_milestones.reorder('milestones.due_date IS NULL, milestones.id IS NULL, milestones.due_date DESC') }
     scope :order_milestone_due_asc,  -> { left_joins_milestones.reorder('milestones.due_date IS NULL, milestones.id IS NULL, milestones.due_date ASC') }
@@ -104,6 +110,7 @@ module Issuable
 
     participant :author
     participant :notes_with_associations
+    participant :assignees
 
     strip_attributes :title
 
@@ -270,6 +277,10 @@ module Issuable
     end
   end
 
+  def assignee_or_author?(user)
+    author_id == user.id || assignees.exists?(user.id)
+  end
+
   def today?
     Date.today == created_at.to_date
   end
@@ -314,11 +325,7 @@ module Issuable
       end
 
       if old_assignees != assignees
-        if self.is_a?(Issue)
-          changes[:assignees] = [old_assignees.map(&:hook_attrs), assignees.map(&:hook_attrs)]
-        else
-          changes[:assignee] = [old_assignees&.first&.hook_attrs, assignee&.hook_attrs]
-        end
+        changes[:assignees] = [old_assignees.map(&:hook_attrs), assignees.map(&:hook_attrs)]
       end
 
       if self.respond_to?(:total_time_spent)
@@ -355,10 +362,18 @@ module Issuable
   def card_attributes
     {
       'Author'   => author.try(:name),
-      'Assignee' => assignee.try(:name)
+      'Assignee' => assignee_list
     }
   end
 
+  def assignee_list
+    assignees.map(&:name).to_sentence
+  end
+
+  def assignee_username_list
+    assignees.map(&:username).to_sentence
+  end
+
   def notes_with_associations
     # If A has_many Bs, and B has_many Cs, and you do
     # `A.includes(b: :c).each { |a| a.b.includes(:c) }`, sadly ActiveRecord
diff --git a/app/models/issue.rb b/app/models/issue.rb
index 261935fd054..5ecd2019222 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -49,10 +49,6 @@ class Issue < ApplicationRecord
 
   scope :in_projects, ->(project_ids) { where(project_id: project_ids) }
 
-  scope :assigned, -> { where('EXISTS (SELECT TRUE FROM issue_assignees WHERE issue_id = issues.id)') }
-  scope :unassigned, -> { where('NOT EXISTS (SELECT TRUE FROM issue_assignees WHERE issue_id = issues.id)') }
-  scope :assigned_to, ->(u) { where('EXISTS (SELECT TRUE FROM issue_assignees WHERE user_id = ? AND issue_id = issues.id)', u.id)}
-
   scope :with_due_date, -> { where.not(due_date: nil) }
   scope :without_due_date, -> { where(due_date: nil) }
   scope :due_before, ->(date) { where('issues.due_date < ?', date) }
@@ -75,8 +71,6 @@ class Issue < ApplicationRecord
   attr_spammable :title, spam_title: true
   attr_spammable :description, spam_description: true
 
-  participant :assignees
-
   state_machine :state, initial: :opened do
     event :close do
       transition [:opened] => :closed
@@ -155,22 +149,6 @@ class Issue < ApplicationRecord
     Gitlab::HookData::IssueBuilder.new(self).build
   end
 
-  # Returns a Hash of attributes to be used for Twitter card metadata
-  def card_attributes
-    {
-      'Author'   => author.try(:name),
-      'Assignee' => assignee_list
-    }
-  end
-
-  def assignee_or_author?(user)
-    author_id == user.id || assignees.exists?(user.id)
-  end
-
-  def assignee_list
-    assignees.map(&:name).to_sentence
-  end
-
   # `from` argument can be a Namespace or Project.
   def to_reference(from = nil, full: false)
     reference = "#{self.class.reference_prefix}#{iid}"
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 458c57c1dc6..0a39a720766 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -16,6 +16,7 @@ class MergeRequest < ApplicationRecord
   include LabelEventable
   include ReactiveCaching
   include FromUnion
+  include DeprecatedAssignee
 
   self.reactive_cache_key = ->(model) { [model.project.id, model.iid] }
   self.reactive_cache_refresh_interval = 10.minutes
@@ -69,8 +70,7 @@ class MergeRequest < ApplicationRecord
   has_many :suggestions, through: :notes
 
   has_many :merge_request_assignees
-  # Will be deprecated at https://gitlab.com/gitlab-org/gitlab-ce/issues/59457
-  belongs_to :assignee, class_name: "User"
+  has_many :assignees, class_name: "User", through: :merge_request_assignees
 
   serialize :merge_params, Hash # rubocop:disable Cop/ActiveRecordSerialize
 
@@ -79,10 +79,6 @@ class MergeRequest < ApplicationRecord
   after_update :reload_diff_if_branch_changed
   after_save :ensure_metrics
 
-  # Required until the codebase starts using this relation for single or multiple assignees.
-  # TODO: Remove at gitlab-ee#2004 implementation.
-  after_save :refresh_merge_request_assignees, if: :assignee_id_changed?
-
   # When this attribute is true some MR validation is ignored
   # It allows us to close or modify broken merge requests
   attr_accessor :allow_broken
@@ -188,19 +184,14 @@ class MergeRequest < ApplicationRecord
   end
   scope :join_project, -> { joins(:target_project) }
   scope :references_project, -> { references(:target_project) }
-  scope :assigned, -> { where("assignee_id IS NOT NULL") }
-  scope :unassigned, -> { where("assignee_id IS NULL") }
-  scope :assigned_to, ->(u) { where(assignee_id: u.id)}
   scope :with_api_entity_associations, -> {
-    preload(:author, :assignee, :notes, :labels, :milestone, :timelogs,
+    preload(:assignees, :author, :notes, :labels, :milestone, :timelogs,
             latest_merge_request_diff: [:merge_request_diff_commits],
             metrics: [:latest_closed_by, :merged_by],
             target_project: [:route, { namespace: :route }],
             source_project: [:route, { namespace: :route }])
   }
 
-  participant :assignee
-
   after_save :keep_around_commit
 
   alias_attribute :project, :target_project
@@ -337,31 +328,6 @@ class MergeRequest < ApplicationRecord
     Gitlab::HookData::MergeRequestBuilder.new(self).build
   end
 
-  # Returns a Hash of attributes to be used for Twitter card metadata
-  def card_attributes
-    {
-      'Author'   => author.try(:name),
-      'Assignee' => assignee.try(:name)
-    }
-  end
-
-  # These method are needed for compatibility with issues to not mess view and other code
-  def assignees
-    Array(assignee)
-  end
-
-  def assignee_ids
-    Array(assignee_id)
-  end
-
-  def assignee_ids=(ids)
-    write_attribute(:assignee_id, ids.last)
-  end
-
-  def assignee_or_author?(user)
-    author_id == user.id || assignee_id == user.id
-  end
-
   # `from` argument can be a Namespace or Project.
   def to_reference(from = nil, full: false)
     reference = "#{self.class.reference_prefix}#{iid}"
@@ -682,15 +648,6 @@ class MergeRequest < ApplicationRecord
     merge_request_diff || create_merge_request_diff
   end
 
-  def refresh_merge_request_assignees
-    transaction do
-      # Using it instead relation.delete_all in order to avoid adding a
-      # dependent: :delete_all (we already have foreign key cascade deletion).
-      MergeRequestAssignee.where(merge_request_id: self).delete_all
-      merge_request_assignees.create(user_id: assignee_id) if assignee_id
-    end
-  end
-
   def create_merge_request_diff
     fetch_ref!
 
@@ -1208,7 +1165,7 @@ class MergeRequest < ApplicationRecord
       variables.append(key: 'CI_MERGE_REQUEST_PROJECT_URL', value: project.web_url)
       variables.append(key: 'CI_MERGE_REQUEST_TARGET_BRANCH_NAME', value: target_branch.to_s)
       variables.append(key: 'CI_MERGE_REQUEST_TITLE', value: title)
-      variables.append(key: 'CI_MERGE_REQUEST_ASSIGNEES', value: assignee.username) if assignee
+      variables.append(key: 'CI_MERGE_REQUEST_ASSIGNEES', value: assignee_username_list) if assignees.any?
       variables.append(key: 'CI_MERGE_REQUEST_MILESTONE', value: milestone.title) if milestone
       variables.append(key: 'CI_MERGE_REQUEST_LABELS', value: label_names.join(',')) if labels.present?
       variables.concat(source_project_variables)
diff --git a/app/models/project.rb b/app/models/project.rb
index e2869fc2ad5..1493dc95fa2 100644
--- a/app/models/project.rb
+++ b/app/models/project.rb
@@ -674,6 +674,10 @@ class Project < ApplicationRecord
     { scope: :project, status: auto_devops&.enabled || Feature.enabled?(:force_autodevops_on_by_default, self) }
   end
 
+  def multiple_mr_assignees_enabled?
+    Feature.enabled?(:multiple_merge_request_assignees, self)
+  end
+
   def daily_statistics_enabled?
     Feature.enabled?(:project_daily_statistics, self, default_enabled: true)
   end
diff --git a/app/serializers/issuable_sidebar_extras_entity.rb b/app/serializers/issuable_sidebar_extras_entity.rb
index d60253564e1..fb35b7522c5 100644
--- a/app/serializers/issuable_sidebar_extras_entity.rb
+++ b/app/serializers/issuable_sidebar_extras_entity.rb
@@ -11,4 +11,6 @@ class IssuableSidebarExtrasEntity < Grape::Entity
   expose :subscribed do |issuable|
     issuable.subscribed?(request.current_user, issuable.project)
   end
+
+  expose :assignees, using: API::Entities::UserBasic
 end
diff --git a/app/serializers/issue_sidebar_extras_entity.rb b/app/serializers/issue_sidebar_extras_entity.rb
index 7b6e860140b..dee891a50b7 100644
--- a/app/serializers/issue_sidebar_extras_entity.rb
+++ b/app/serializers/issue_sidebar_extras_entity.rb
@@ -1,5 +1,4 @@
 # frozen_string_literal: true
 
 class IssueSidebarExtrasEntity < IssuableSidebarExtrasEntity
-  expose :assignees, using: API::Entities::UserBasic
 end
diff --git a/app/serializers/merge_request_assignee_entity.rb b/app/serializers/merge_request_assignee_entity.rb
new file mode 100644
index 00000000000..6849c62e759
--- /dev/null
+++ b/app/serializers/merge_request_assignee_entity.rb
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class MergeRequestAssigneeEntity < ::API::Entities::UserBasic
+  expose :can_merge do |assignee, options|
+    options[:merge_request]&.can_be_merged_by?(assignee)
+  end
+end
diff --git a/app/serializers/merge_request_basic_entity.rb b/app/serializers/merge_request_basic_entity.rb
index 178e72f4f0a..973e971b4c0 100644
--- a/app/serializers/merge_request_basic_entity.rb
+++ b/app/serializers/merge_request_basic_entity.rb
@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 class MergeRequestBasicEntity < Grape::Entity
-  expose :assignee_id
   expose :merge_status
   expose :merge_error
   expose :state
@@ -9,7 +8,7 @@ class MergeRequestBasicEntity < Grape::Entity
   expose :rebase_in_progress?, as: :rebase_in_progress
   expose :milestone, using: API::Entities::Milestone
   expose :labels, using: LabelEntity
-  expose :assignee, using: API::Entities::UserBasic
+  expose :assignees, using: API::Entities::UserBasic
   expose :task_status, :task_status_short
   expose :lock_version, :lock_version
 end
diff --git a/app/serializers/merge_request_serializer.rb b/app/serializers/merge_request_serializer.rb
index 4cf84336aa4..6f589351670 100644
--- a/app/serializers/merge_request_serializer.rb
+++ b/app/serializers/merge_request_serializer.rb
@@ -8,9 +8,9 @@ class MergeRequestSerializer < BaseSerializer
     entity =
       case opts[:serializer]
       when 'sidebar'
-        MergeRequestSidebarBasicEntity
+        IssuableSidebarBasicEntity
       when 'sidebar_extras'
-        IssuableSidebarExtrasEntity
+        MergeRequestSidebarExtrasEntity
       when 'basic'
         MergeRequestBasicEntity
       else
diff --git a/app/serializers/merge_request_sidebar_basic_entity.rb b/app/serializers/merge_request_sidebar_basic_entity.rb
deleted file mode 100644
index 0ae7298a7c1..00000000000
--- a/app/serializers/merge_request_sidebar_basic_entity.rb
+++ /dev/null
@@ -1,11 +0,0 @@
-# frozen_string_literal: true
-
-class MergeRequestSidebarBasicEntity < IssuableSidebarBasicEntity
-  expose :assignee, if: lambda { |issuable| issuable.assignee } do
-    expose :assignee, merge: true, using: API::Entities::UserBasic
-
-    expose :can_merge do |issuable|
-      issuable.can_be_merged_by?(issuable.assignee)
-    end
-  end
-end
diff --git a/app/serializers/merge_request_sidebar_extras_entity.rb b/app/serializers/merge_request_sidebar_extras_entity.rb
new file mode 100644
index 00000000000..7276509c363
--- /dev/null
+++ b/app/serializers/merge_request_sidebar_extras_entity.rb
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class MergeRequestSidebarExtrasEntity < IssuableSidebarExtrasEntity
+  expose :assignees do |merge_request|
+    MergeRequestAssigneeEntity.represent(merge_request.assignees, merge_request: merge_request)
+  end
+end
diff --git a/app/services/issuable_base_service.rb b/app/services/issuable_base_service.rb
index 7a4ccf0d178..26132f1824a 100644
--- a/app/services/issuable_base_service.rb
+++ b/app/services/issuable_base_service.rb
@@ -34,14 +34,20 @@ class IssuableBaseService < BaseService
   end
 
   def filter_assignee(issuable)
-    return unless params[:assignee_id].present?
+    return if params[:assignee_ids].blank?
 
-    assignee_id = params[:assignee_id]
+    unless issuable.allows_multiple_assignees?
+      params[:assignee_ids] = params[:assignee_ids].first(1)
+    end
+
+    assignee_ids = params[:assignee_ids].select { |assignee_id| assignee_can_read?(issuable, assignee_id) }
 
-    if assignee_id.to_s == IssuableFinder::NONE
-      params[:assignee_id] = ""
+    if params[:assignee_ids].map(&:to_s) == [IssuableFinder::NONE]
+      params[:assignee_ids] = []
+    elsif assignee_ids.any?
+      params[:assignee_ids] = assignee_ids
     else
-      params.delete(:assignee_id) unless assignee_can_read?(issuable, assignee_id)
+      params.delete(:assignee_ids)
     end
   end
 
@@ -352,7 +358,7 @@ class IssuableBaseService < BaseService
   end
 
   def has_changes?(issuable, old_labels: [], old_assignees: [])
-    valid_attrs = [:title, :description, :assignee_id, :milestone_id, :target_branch]
+    valid_attrs = [:title, :description, :assignee_ids, :milestone_id, :target_branch]
 
     attrs_changed = valid_attrs.any? do |attr|
       issuable.previous_changes.include?(attr.to_s)
diff --git a/app/services/issues/base_service.rb b/app/services/issues/base_service.rb
index ef08adf4f92..48ed5afbc2a 100644
--- a/app/services/issues/base_service.rb
+++ b/app/services/issues/base_service.rb
@@ -20,7 +20,7 @@ module Issues
     private
 
     def create_assignee_note(issue, old_assignees)
-      SystemNoteService.change_issue_assignees(
+      SystemNoteService.change_issuable_assignees(
         issue, issue.project, current_user, old_assignees)
     end
 
@@ -31,26 +31,6 @@ module Issues
       issue.project.execute_services(issue_data, hooks_scope)
     end
 
-    # rubocop: disable CodeReuse/ActiveRecord
-    def filter_assignee(issuable)
-      return if params[:assignee_ids].blank?
-
-      unless issuable.allows_multiple_assignees?
-        params[:assignee_ids] = params[:assignee_ids].take(1)
-      end
-
-      assignee_ids = params[:assignee_ids].select { |assignee_id| assignee_can_read?(issuable, assignee_id) }
-
-      if params[:assignee_ids].map(&:to_s) == [IssuableFinder::NONE]
-        params[:assignee_ids] = []
-      elsif assignee_ids.any?
-        params[:assignee_ids] = assignee_ids
-      else
-        params.delete(:assignee_ids)
-      end
-    end
-    # rubocop: enable CodeReuse/ActiveRecord
-
     def update_project_counter_caches?(issue)
       super || issue.confidential_changed?
     end
diff --git a/app/services/issues/update_service.rb b/app/services/issues/update_service.rb
index cec5b5734c0..cb2337d29d4 100644
--- a/app/services/issues/update_service.rb
+++ b/app/services/issues/update_service.rb
@@ -39,7 +39,7 @@ module Issues
       if issue.assignees != old_assignees
         create_assignee_note(issue, old_assignees)
         notification_service.async.reassigned_issue(issue, current_user, old_assignees)
-        todo_service.reassigned_issue(issue, current_user, old_assignees)
+        todo_service.reassigned_issuable(issue, current_user, old_assignees)
       end
 
       if issue.previous_changes.include?('confidential')
diff --git a/app/services/merge_requests/base_service.rb b/app/services/merge_requests/base_service.rb
index 8a9e5ebb014..b8334a87f6d 100644
--- a/app/services/merge_requests/base_service.rb
+++ b/app/services/merge_requests/base_service.rb
@@ -49,9 +49,9 @@ module MergeRequests
       MergeRequestMetricsService.new(merge_request.metrics)
     end
 
-    def create_assignee_note(merge_request)
-      SystemNoteService.change_assignee(
-        merge_request, merge_request.project, current_user, merge_request.assignee)
+    def create_assignee_note(merge_request, old_assignees)
+      SystemNoteService.change_issuable_assignees(
+        merge_request, merge_request.project, current_user, old_assignees)
     end
 
     def create_pipeline_for(merge_request, user)
diff --git a/app/services/merge_requests/update_service.rb b/app/services/merge_requests/update_service.rb
index 8112c2a4299..faaa4d66726 100644
--- a/app/services/merge_requests/update_service.rb
+++ b/app/services/merge_requests/update_service.rb
@@ -24,13 +24,13 @@ module MergeRequests
       update_task_event(merge_request) || update(merge_request)
     end
 
-    # rubocop:disable Metrics/AbcSize
     def handle_changes(merge_request, options)
       old_associations = options.fetch(:old_associations, {})
       old_labels = old_associations.fetch(:labels, [])
       old_mentioned_users = old_associations.fetch(:mentioned_users, [])
+      old_assignees = old_associations.fetch(:assignees, [])
 
-      if has_changes?(merge_request, old_labels: old_labels)
+      if has_changes?(merge_request, old_labels: old_labels, old_assignees: old_assignees)
         todo_service.mark_pending_todos_as_done(merge_request, current_user)
       end
 
@@ -45,15 +45,10 @@ module MergeRequests
                                   merge_request.target_branch)
       end
 
-      if merge_request.previous_changes.include?('assignee_id')
-        reassigned_merge_request_args = [merge_request, current_user]
-
-        old_assignee_id = merge_request.previous_changes['assignee_id'].first
-        reassigned_merge_request_args << User.find(old_assignee_id) if old_assignee_id
-
-        create_assignee_note(merge_request)
-        notification_service.async.reassigned_merge_request(*reassigned_merge_request_args)
-        todo_service.reassigned_merge_request(merge_request, current_user)
+      if merge_request.assignees != old_assignees
+        create_assignee_note(merge_request, old_assignees)
+        notification_service.async.reassigned_merge_request(merge_request, current_user, old_assignees)
+        todo_service.reassigned_issuable(merge_request, current_user, old_assignees)
       end
 
       if merge_request.previous_changes.include?('target_branch') ||
@@ -81,7 +76,6 @@ module MergeRequests
         )
       end
     end
-    # rubocop:enable Metrics/AbcSize
 
     def handle_task_changes(merge_request)
       todo_service.mark_pending_todos_as_done(merge_request, current_user)
diff --git a/app/services/notification_recipient_service.rb b/app/services/notification_recipient_service.rb
index 56f11b31110..760962346fb 100644
--- a/app/services/notification_recipient_service.rb
+++ b/app/services/notification_recipient_service.rb
@@ -247,15 +247,15 @@ module NotificationRecipientService
       attr_reader :target
       attr_reader :current_user
       attr_reader :action
-      attr_reader :previous_assignee
+      attr_reader :previous_assignees
       attr_reader :skip_current_user
 
-      def initialize(target, current_user, action:, custom_action: nil, previous_assignee: nil, skip_current_user: true)
+      def initialize(target, current_user, action:, custom_action: nil, previous_assignees: nil, skip_current_user: true)
         @target = target
         @current_user = current_user
         @action = action
         @custom_action = custom_action
-        @previous_assignee = previous_assignee
+        @previous_assignees = previous_assignees
         @skip_current_user = skip_current_user
       end
 
@@ -270,11 +270,7 @@ module NotificationRecipientService
 
         # Re-assign is considered as a mention of the new assignee
         case custom_action
-        when :reassign_merge_request
-          add_recipients(previous_assignee, :mention, nil)
-          add_recipients(target.assignee, :mention, NotificationReason::ASSIGNED)
-        when :reassign_issue
-          previous_assignees = Array(previous_assignee)
+        when :reassign_merge_request, :reassign_issue
           add_recipients(previous_assignees, :mention, nil)
           add_recipients(target.assignees, :mention, NotificationReason::ASSIGNED)
         end
@@ -287,17 +283,11 @@ module NotificationRecipientService
           # receive them, too.
           add_mentions(current_user, target: target)
 
-          # Add the assigned users, if any
-          assignees = case custom_action
-                      when :new_issue
-                        target.assignees
-                      else
-                        target.assignee
-                      end
-
           # We use the `:participating` notification level in order to match existing legacy behavior as captured
           # in existing specs (notification_service_spec.rb ~ line 507)
-          add_recipients(assignees, :participating, NotificationReason::ASSIGNED) if assignees
+          if target.is_a?(Issuable)
+            add_recipients(target.assignees, :participating, NotificationReason::ASSIGNED)
+          end
 
           add_labels_subscribers
         end
diff --git a/app/services/notification_service.rb b/app/services/notification_service.rb
index 1a65561dd70..8d3b569498f 100644
--- a/app/services/notification_service.rb
+++ b/app/services/notification_service.rb
@@ -95,8 +95,8 @@ class NotificationService
 
   # When we reassign an issue we should send an email to:
   #
-  #  * issue old assignee if their notification level is not Disabled
-  #  * issue new assignee if their notification level is not Disabled
+  #  * issue old assignees if their notification level is not Disabled
+  #  * issue new assignees if their notification level is not Disabled
   #  * users with custom level checked with "reassign issue"
   #
   def reassigned_issue(issue, current_user, previous_assignees = [])
@@ -104,7 +104,7 @@ class NotificationService
       issue,
       current_user,
       action: "reassign",
-      previous_assignee: previous_assignees
+      previous_assignees: previous_assignees
     )
 
     previous_assignee_ids = previous_assignees.map(&:id)
@@ -140,7 +140,7 @@ class NotificationService
   # When create a merge request we should send an email to:
   #
   #  * mr author
-  #  * mr assignee if their notification level is not Disabled
+  #  * mr assignees if their notification level is not Disabled
   #  * project team members with notification level higher then Participating
   #  * watchers of the mr's labels
   #  * users with custom level checked with "new merge request"
@@ -184,23 +184,25 @@ class NotificationService
 
   # When we reassign a merge_request we should send an email to:
   #
-  #  * merge_request old assignee if their notification level is not Disabled
-  #  * merge_request assignee if their notification level is not Disabled
+  #  * merge_request old assignees if their notification level is not Disabled
+  #  * merge_request new assignees if their notification level is not Disabled
   #  * users with custom level checked with "reassign merge request"
   #
-  def reassigned_merge_request(merge_request, current_user, previous_assignee = nil)
+  def reassigned_merge_request(merge_request, current_user, previous_assignees = [])
     recipients = NotificationRecipientService.build_recipients(
       merge_request,
       current_user,
       action: "reassign",
-      previous_assignee: previous_assignee
+      previous_assignees: previous_assignees
     )
 
+    previous_assignee_ids = previous_assignees.map(&:id)
+
     recipients.each do |recipient|
       mailer.reassigned_merge_request_email(
         recipient.user.id,
         merge_request.id,
-        previous_assignee&.id,
+        previous_assignee_ids,
         current_user.id,
         recipient.reason
       ).deliver_later
diff --git a/app/services/system_note_service.rb b/app/services/system_note_service.rb
index acbbb0da929..a39ff76b798 100644
--- a/app/services/system_note_service.rb
+++ b/app/services/system_note_service.rb
@@ -69,7 +69,7 @@ module SystemNoteService
 
   # Called when the assignees of an Issue is changed or removed
   #
-  # issue - Issue object
+  # issuable - Issuable object (responds to assignees)
   # project  - Project owning noteable
   # author   - User performing the change
   # assignees - Users being assigned, or nil
@@ -85,9 +85,9 @@ module SystemNoteService
   #   "assigned to @user1 and @user2"
   #
   # Returns the created Note object
-  def change_issue_assignees(issue, project, author, old_assignees)
-    unassigned_users = old_assignees - issue.assignees
-    added_users = issue.assignees.to_a - old_assignees
+  def change_issuable_assignees(issuable, project, author, old_assignees)
+    unassigned_users = old_assignees - issuable.assignees
+    added_users = issuable.assignees.to_a - old_assignees
 
     text_parts = []
     text_parts << "assigned to #{added_users.map(&:to_reference).to_sentence}" if added_users.any?
@@ -95,7 +95,7 @@ module SystemNoteService
 
     body = text_parts.join(' and ')
 
-    create_note(NoteSummary.new(issue, project, author, body, action: 'assignee'))
+    create_note(NoteSummary.new(issuable, project, author, body, action: 'assignee'))
   end
 
   # Called when the milestone of a Noteable is changed
diff --git a/app/services/todo_service.rb b/app/services/todo_service.rb
index f357dc37fe7..0ea230a44a1 100644
--- a/app/services/todo_service.rb
+++ b/app/services/todo_service.rb
@@ -49,12 +49,12 @@ class TodoService
     todo_users.each(&:update_todos_count_cache)
   end
 
-  # When we reassign an issue we should:
+  # When we reassign an issuable we should:
   #
-  #  * create a pending todo for new assignee if issue is assigned
+  #  * create a pending todo for new assignee if issuable is assigned
   #
-  def reassigned_issue(issue, current_user, old_assignees = [])
-    create_assignment_todo(issue, current_user, old_assignees)
+  def reassigned_issuable(issuable, current_user, old_assignees = [])
+    create_assignment_todo(issuable, current_user, old_assignees)
   end
 
   # When create a merge request we should:
@@ -82,14 +82,6 @@ class TodoService
     mark_pending_todos_as_done(merge_request, current_user)
   end
 
-  # When we reassign a merge request we should:
-  #
-  #  * creates a pending todo for new assignee if merge request is assigned
-  #
-  def reassigned_merge_request(merge_request, current_user)
-    create_assignment_todo(merge_request, current_user)
-  end
-
   # When merge a merge request we should:
   #
   #  * mark all pending todos related to the target for the current user as done
diff --git a/app/views/notify/_reassigned_issuable_email.html.haml b/app/views/notify/_reassigned_issuable_email.html.haml
new file mode 100644
index 00000000000..4ab40ff2659
--- /dev/null
+++ b/app/views/notify/_reassigned_issuable_email.html.haml
@@ -0,0 +1,10 @@
+%p
+  Assignee changed
+  - if previous_assignees.any?
+    from
+    %strong= sanitize_name(previous_assignees.map(&:name).to_sentence)
+  to
+  - if issuable.assignees.any?
+    %strong= sanitize_name(issuable.assignee_list)
+  - else
+    %strong Unassigned
diff --git a/app/views/notify/closed_merge_request_email.text.haml b/app/views/notify/closed_merge_request_email.text.haml
index 1094d584a1c..6e84f9fb355 100644
--- a/app/views/notify/closed_merge_request_email.text.haml
+++ b/app/views/notify/closed_merge_request_email.text.haml
@@ -5,4 +5,4 @@ Merge Request url: #{project_merge_request_url(@merge_request.target_project, @m
 = merge_path_description(@merge_request, 'to')
 
 Author: #{sanitize_name(@merge_request.author_name)}
-Assignee: #{sanitize_name(@merge_request.assignee_name)}
+= assignees_label(@merge_request)
diff --git a/app/views/notify/issue_due_email.html.haml b/app/views/notify/issue_due_email.html.haml
index e81144b8fcb..08bc98ca05c 100644
--- a/app/views/notify/issue_due_email.html.haml
+++ b/app/views/notify/issue_due_email.html.haml
@@ -3,7 +3,7 @@
 
 - if @issue.assignees.any?
   %p
-    Assignee: #{@issue.assignee_list}
+    = assignees_label(@issue)
 %p
   This issue is due on: #{@issue.due_date.to_s(:medium)}
 
diff --git a/app/views/notify/issue_due_email.text.erb b/app/views/notify/issue_due_email.text.erb
index 3c7a57a8a2e..ae50b703fe3 100644
--- a/app/views/notify/issue_due_email.text.erb
+++ b/app/views/notify/issue_due_email.text.erb
@@ -2,6 +2,6 @@ The following issue is due on <%= @issue.due_date %>:
 
 Issue <%= @issue.iid %>: <%= url_for(project_issue_url(@issue.project, @issue)) %>
 Author:    <%= @issue.author_name %>
-Assignee:  <%= @issue.assignee_list %>
+<%= assignees_label(@issue) %>
 
 <%= @issue.description %>
diff --git a/app/views/notify/merge_request_status_email.text.haml b/app/views/notify/merge_request_status_email.text.haml
index b9b9e0c3ad7..e3b24bbd405 100644
--- a/app/views/notify/merge_request_status_email.text.haml
+++ b/app/views/notify/merge_request_status_email.text.haml
@@ -5,4 +5,4 @@ Merge Request url: #{project_merge_request_url(@merge_request.target_project, @m
 = merge_path_description(@merge_request, 'to')
 
 Author: #{sanitize_name(@merge_request.author_name)}
-Assignee: #{sanitize_name(@merge_request.assignee_name)}
+= assignees_label(@merge_request)
diff --git a/app/views/notify/merge_request_unmergeable_email.text.haml b/app/views/notify/merge_request_unmergeable_email.text.haml
index 0c7bf1bb044..e9708a297d7 100644
--- a/app/views/notify/merge_request_unmergeable_email.text.haml
+++ b/app/views/notify/merge_request_unmergeable_email.text.haml
@@ -5,4 +5,4 @@ Merge Request url: #{project_merge_request_url(@merge_request.target_project, @m
 = merge_path_description(@merge_request, 'to')
 
 Author: #{sanitize_name(@merge_request.author_name)}
-Assignee: #{sanitize_name(@merge_request.assignee_name)}
+= assignees_label(@merge_request)
diff --git a/app/views/notify/merged_merge_request_email.text.haml b/app/views/notify/merged_merge_request_email.text.haml
index 045a43cbc84..d623e701a30 100644
--- a/app/views/notify/merged_merge_request_email.text.haml
+++ b/app/views/notify/merged_merge_request_email.text.haml
@@ -5,4 +5,4 @@ Merge Request url: #{project_merge_request_url(@merge_request.target_project, @m
 = merge_path_description(@merge_request, 'to')
 
 Author: #{sanitize_name(@merge_request.author_name)}
-Assignee: #{sanitize_name(@merge_request.assignee_name)}
+= assignees_label(@merge_request)
diff --git a/app/views/notify/new_issue_email.html.haml b/app/views/notify/new_issue_email.html.haml
index e6cdaf85c0d..8aa7939dd0b 100644
--- a/app/views/notify/new_issue_email.html.haml
+++ b/app/views/notify/new_issue_email.html.haml
@@ -4,7 +4,7 @@
 
 - if @issue.assignees.any?
   %p
-    Assignee: #{@issue.assignee_list}
+    = assignees_label(@issue)
 
 - if @issue.description
   %div
diff --git a/app/views/notify/new_issue_email.text.erb b/app/views/notify/new_issue_email.text.erb
index 58a2bcbe5eb..ff258711b48 100644
--- a/app/views/notify/new_issue_email.text.erb
+++ b/app/views/notify/new_issue_email.text.erb
@@ -2,6 +2,6 @@ New Issue was created.
 
 Issue <%= @issue.iid %>: <%= url_for(project_issue_url(@issue.project, @issue)) %>
 Author:    <%= sanitize_name(@issue.author_name) %>
-Assignee:  <%= @issue.assignee_list %>
+<%= assignees_label(@issue) %>
 
 <%= @issue.description %>
diff --git a/app/views/notify/new_mention_in_issue_email.text.erb b/app/views/notify/new_mention_in_issue_email.text.erb
index 173091e4a80..8e95063b40f 100644
--- a/app/views/notify/new_mention_in_issue_email.text.erb
+++ b/app/views/notify/new_mention_in_issue_email.text.erb
@@ -2,6 +2,6 @@ You have been mentioned in an issue.
 
 Issue <%= @issue.iid %>: <%= url_for(project_issue_url(@issue.project, @issue)) %>
 Author:    <%= sanitize_name(@issue.author_name) %>
-Assignee:  <%= sanitize_name(@issue.assignee_list) %>
+<%= assignees_label(@issue) %>
 
 <%= @issue.description %>
diff --git a/app/views/notify/new_mention_in_merge_request_email.text.erb b/app/views/notify/new_mention_in_merge_request_email.text.erb
index 96a4f3f9eac..3c78e257a88 100644
--- a/app/views/notify/new_mention_in_merge_request_email.text.erb
+++ b/app/views/notify/new_mention_in_merge_request_email.text.erb
@@ -4,6 +4,6 @@ You have been mentioned in Merge Request <%= @merge_request.to_reference %>
 
 <%= merge_path_description(@merge_request, 'to') %>
 Author:    <%= sanitize_name(@merge_request.author_name) %>
-Assignee:  <%= sanitize_name(@merge_request.assignee_name) %>
+= assignees_label(@merge_request)
 
 <%= @merge_request.description %>
diff --git a/app/views/notify/new_merge_request_email.html.haml b/app/views/notify/new_merge_request_email.html.haml
index db23447dd39..77d2e65d285 100644
--- a/app/views/notify/new_merge_request_email.html.haml
+++ b/app/views/notify/new_merge_request_email.html.haml
@@ -5,9 +5,9 @@
 %p.details
   != merge_path_description(@merge_request, '&rarr;')
 
-- if @merge_request.assignee_id.present?
+- if @merge_request.assignees.any?
   %p
-    Assignee: #{sanitize_name(@merge_request.assignee_name)}
+    = assignees_label(@merge_request)
 
 = render_if_exists 'notify/merge_request_approvers', presenter: @mr_presenter
 
diff --git a/app/views/notify/new_merge_request_email.text.erb b/app/views/notify/new_merge_request_email.text.erb
index 754f4bca1cd..e6c42f1cf5f 100644
--- a/app/views/notify/new_merge_request_email.text.erb
+++ b/app/views/notify/new_merge_request_email.text.erb
@@ -4,7 +4,7 @@ New Merge Request <%= @merge_request.to_reference %>
 
 <%= merge_path_description(@merge_request, 'to') %>
 Author:    <%= @merge_request.author_name %>
-Assignee:  <%= @merge_request.assignee_name %>
+<%= assignees_label(@merge_request) %>
 <%= render_if_exists 'notify/merge_request_approvers', presenter: @mr_presenter %>
 
 <%= @merge_request.description %>
diff --git a/app/views/notify/reassigned_issue_email.html.haml b/app/views/notify/reassigned_issue_email.html.haml
index 6d25488a7e2..6b088927623 100644
--- a/app/views/notify/reassigned_issue_email.html.haml
+++ b/app/views/notify/reassigned_issue_email.html.haml
@@ -1,10 +1 @@
-%p
-  Assignee changed
-  - if @previous_assignees.any?
-    from
-    %strong= sanitize_name(@previous_assignees.map(&:name).to_sentence)
-  to
-  - if @issue.assignees.any?
-    %strong= @issue.assignee_list
-  - else
-    %strong Unassigned
+= render 'reassigned_issuable_email', issuable: @issue, previous_assignees: @previous_assignees
diff --git a/app/views/notify/reassigned_merge_request_email.html.haml b/app/views/notify/reassigned_merge_request_email.html.haml
index e4f19bc3200..0aefca6b14a 100644
--- a/app/views/notify/reassigned_merge_request_email.html.haml
+++ b/app/views/notify/reassigned_merge_request_email.html.haml
@@ -1,10 +1 @@
-%p
-  Assignee changed
-  - if @previous_assignee
-    from
-    %strong= sanitize_name(@previous_assignee.name)
-  to
-  - if @merge_request.assignee_id
-    %strong= sanitize_name(@merge_request.assignee_name)
-  - else
-    %strong Unassigned
+= render 'reassigned_issuable_email', issuable: @merge_request, previous_assignees: @previous_assignees
diff --git a/app/views/notify/reassigned_merge_request_email.text.erb b/app/views/notify/reassigned_merge_request_email.text.erb
index 96c770b5219..82ec7aa0fa4 100644
--- a/app/views/notify/reassigned_merge_request_email.text.erb
+++ b/app/views/notify/reassigned_merge_request_email.text.erb
@@ -2,5 +2,5 @@ Reassigned Merge Request <%= @merge_request.iid %>
 
 <%= url_for([@merge_request.project.namespace.becomes(Namespace), @merge_request.project, @merge_request, { only_path: false }]) %>
 
-Assignee changed <%= "from #{sanitize_name(@previous_assignee.name)}" if @previous_assignee -%>
- to <%= "#{@merge_request.assignee_id ? sanitize_name(@merge_request.assignee_name) : 'Unassigned'}" %>
+Assignee changed <%= "from #{sanitize_name(@previous_assignees.map(&:name).to_sentence)}" if @previous_assignees.any? -%>
+ to <%= "#{@merge_request.assignees.any? ? @merge_request.assignee_list : 'Unassigned'}" %>
diff --git a/app/views/projects/issues/_issue.html.haml b/app/views/projects/issues/_issue.html.haml
index ce7c7091c93..377b2a6d8d9 100644
--- a/app/views/projects/issues/_issue.html.haml
+++ b/app/views/projects/issues/_issue.html.haml
@@ -46,7 +46,7 @@
               CLOSED
           - if issue.assignees.any?
             %li
-              = render 'shared/issuable/assignees', project: @project, issue: issue
+              = render 'shared/issuable/assignees', project: @project, issuable: issue
 
           = render 'shared/issuable_meta_data', issuable: issue
 
diff --git a/app/views/projects/merge_requests/_merge_request.html.haml b/app/views/projects/merge_requests/_merge_request.html.haml
index b8e0b66e277..47c8e3d73f5 100644
--- a/app/views/projects/merge_requests/_merge_request.html.haml
+++ b/app/views/projects/merge_requests/_merge_request.html.haml
@@ -53,9 +53,9 @@
           %li.issuable-pipeline-broken.d-none.d-sm-inline-block
             = link_to merge_request_path(merge_request), class: "has-tooltip", title: _('Cannot be merged automatically') do
               = icon('exclamation-triangle')
-        - if merge_request.assignee
+        - if merge_request.assignees.any?
           %li
-            = link_to_member(merge_request.source_project, merge_request.assignee, name: false, title: _('Assigned to :name'))
+            = render 'shared/issuable/assignees', project: merge_request.project, issuable: merge_request
         = render_if_exists 'projects/merge_requests/approvals_count', merge_request: merge_request
 
         = render 'shared/issuable_meta_data', issuable: merge_request
diff --git a/app/views/shared/boards/components/sidebar/_assignee.html.haml b/app/views/shared/boards/components/sidebar/_assignee.html.haml
index 1374da9d82c..af6a519a967 100644
--- a/app/views/shared/boards/components/sidebar/_assignee.html.haml
+++ b/app/views/shared/boards/components/sidebar/_assignee.html.haml
@@ -19,7 +19,7 @@
         ":data-name" => "assignee.name",
         ":data-username" => "assignee.username" }
       .dropdown
-        - dropdown_options = issue_assignees_dropdown_options
+        - dropdown_options = assignees_dropdown_options('issue')
         %button.dropdown-menu-toggle.js-user-search.js-author-search.js-multiselect.js-save-user-data.js-issue-board-sidebar{ type: 'button', ref: 'assigneeDropdown', data: board_sidebar_user_data,
           ":data-issuable-id" => "issue.iid" }
           = dropdown_options[:title]
diff --git a/app/views/shared/issuable/_assignees.html.haml b/app/views/shared/issuable/_assignees.html.haml
index ef3d44a9241..24734ed66cf 100644
--- a/app/views/shared/issuable/_assignees.html.haml
+++ b/app/views/shared/issuable/_assignees.html.haml
@@ -1,9 +1,9 @@
 - max_render = 4
-- assignees_rendering_overflow = issue.assignees.size > max_render
+- assignees_rendering_overflow = issuable.assignees.size > max_render
 - render_count = assignees_rendering_overflow ? max_render - 1 : max_render
-- more_assignees_count = issue.assignees.size - render_count
+- more_assignees_count = issuable.assignees.size - render_count
 
-- issue.assignees.take(render_count).each do |assignee| # rubocop: disable CodeReuse/ActiveRecord
+- issuable.assignees.take(render_count).each do |assignee| # rubocop: disable CodeReuse/ActiveRecord
   = link_to_member(@project, assignee, name: false, title: "Assigned to :name")
 
 - if more_assignees_count.positive?
diff --git a/app/views/shared/issuable/_bulk_update_sidebar.html.haml b/app/views/shared/issuable/_bulk_update_sidebar.html.haml
index 909eb738f95..a05a13814ac 100644
--- a/app/views/shared/issuable/_bulk_update_sidebar.html.haml
+++ b/app/views/shared/issuable/_bulk_update_sidebar.html.haml
@@ -21,10 +21,7 @@
         .title
           Assignee
         .filter-item
-          - if type == :issues
-            - field_name = "update[assignee_ids][]"
-          - else
-            - field_name = "update[assignee_id]"
+          - field_name = "update[assignee_ids][]"
           = dropdown_tag("Select assignee", options: { toggle_class: "js-user-search js-update-assignee js-filter-submit js-filter-bulk-update", title: "Assign to", filter: true, dropdown_class: "dropdown-menu-user dropdown-menu-selectable",
             placeholder: "Search authors", data: { first_user: (current_user.username if current_user), null_user: true, current_user: true, project_id: @project.id, field_name: field_name } })
       .block
diff --git a/app/views/shared/issuable/_sidebar_assignees.html.haml b/app/views/shared/issuable/_sidebar_assignees.html.haml
index 1a59055f652..ab01094ed6e 100644
--- a/app/views/shared/issuable/_sidebar_assignees.html.haml
+++ b/app/views/shared/issuable/_sidebar_assignees.html.haml
@@ -1,42 +1,10 @@
 - issuable_type = issuable_sidebar[:type]
 - signed_in = !!issuable_sidebar.dig(:current_user, :id)
-- can_edit_issuable = issuable_sidebar.dig(:current_user, :can_edit)
 
-- if issuable_type == "issue"
-  #js-vue-sidebar-assignees{ data: { field: "#{issuable_type}[assignee_ids]", signed_in: signed_in } }
-    .title.hide-collapsed
-      = _('Assignee')
-      = icon('spinner spin')
-- else
-  - assignee = assignees.first
-  .sidebar-collapsed-icon.sidebar-collapsed-user{ data: { toggle: "tooltip", placement: "left", container: "body", boundary: 'viewport' }, title: (issuable_sidebar.dig(:assignee, :name) || _('Assignee')) }
-    - if issuable_sidebar[:assignee]
-      = link_to_member(@project, assignee, size: 24)
-    - else
-      = icon('user', 'aria-hidden': 'true')
+#js-vue-sidebar-assignees{ data: { field: "#{issuable_type}[assignee_ids]", signed_in: signed_in } }
   .title.hide-collapsed
     = _('Assignee')
-    = icon('spinner spin', class: 'hidden block-loading', 'aria-hidden': 'true')
-    - if can_edit_issuable
-      = link_to _('Edit'), '#', class: 'js-sidebar-dropdown-toggle edit-link float-right'
-    - if !signed_in
-      %a.gutter-toggle.float-right.js-sidebar-toggle{ role: "button", href: "#", "aria-label" => _('Toggle sidebar') }
-        = sidebar_gutter_toggle_icon
-  .value.hide-collapsed
-    - if issuable_sidebar[:assignee]
-      = link_to_member(@project, assignee, size: 32, extra_class: 'bold') do
-        - unless issuable_sidebar[:assignee][:can_merge]
-          %span.float-right.cannot-be-merged{ data: { toggle: 'tooltip', placement: 'left' }, title: _('Not allowed to merge') }
-            = icon('exclamation-triangle', 'aria-hidden': 'true')
-        %span.username
-          @#{issuable_sidebar[:assignee][:username]}
-    - else
-      %span.assign-yourself.no-value
-        = _('No assignee')
-        - if can_edit_issuable
-          \-
-          %a.js-assign-yourself{ href: '#' }
-            = _('assign yourself')
+    = icon('spinner spin')
 
 .selectbox.hide-collapsed
   - if assignees.none?
@@ -59,17 +27,15 @@
       ability_name: issuable_type,
       null_user: true,
       display: 'static' } }
-  - title = _('Select assignee')
 
-  - if issuable_type == "issue"
-    - dropdown_options = issue_assignees_dropdown_options
-    - title = dropdown_options[:title]
-    - options[:toggle_class] += ' js-multiselect js-save-user-data'
-    - data = { field_name: "#{issuable_type}[assignee_ids][]" }
-    - data[:multi_select] = true
-    - data['dropdown-title'] = title
-    - data['dropdown-header'] = dropdown_options[:data][:'dropdown-header']
-    - data['max-select'] = dropdown_options[:data][:'max-select'] if dropdown_options[:data][:'max-select']
-    - options[:data].merge!(data)
+  - dropdown_options = assignees_dropdown_options(issuable_type)
+  - title = dropdown_options[:title]
+  - options[:toggle_class] += ' js-multiselect js-save-user-data'
+  - data = { field_name: "#{issuable_type}[assignee_ids][]" }
+  - data[:multi_select] = true
+  - data['dropdown-title'] = title
+  - data['dropdown-header'] = dropdown_options[:data][:'dropdown-header']
+  - data['max-select'] = dropdown_options[:data][:'max-select'] if dropdown_options[:data][:'max-select']
+  - options[:data].merge!(data)
 
   = dropdown_tag(title, options: options)
diff --git a/app/views/shared/issuable/form/_merge_request_assignee.html.haml b/app/views/shared/issuable/form/_merge_request_assignee.html.haml
deleted file mode 100644
index 05c03dedd91..00000000000
--- a/app/views/shared/issuable/form/_merge_request_assignee.html.haml
+++ /dev/null
@@ -1,31 +0,0 @@
-- merge_request = issuable
-.block.assignee
-  .sidebar-collapsed-icon.sidebar-collapsed-user{ data: { toggle: "tooltip", placement: "left", container: "body" }, title: sidebar_assignee_tooltip_label(issuable) }
-    - if merge_request.assignee
-      = link_to_member(@project, merge_request.assignee, size: 24)
-    - else
-      = icon('user', 'aria-hidden': 'true')
-  .title.hide-collapsed
-    Assignee
-    = icon('spinner spin', class: 'hidden block-loading', 'aria-hidden': 'true')
-    - if can_edit_issuable
-      = link_to 'Edit', '#', class: 'js-sidebar-dropdown-toggle edit-link float-right'
-  .value.hide-collapsed
-    - if merge_request.assignee
-      = link_to_member(@project, merge_request.assignee, size: 32, extra_class: 'bold') do
-        - unless merge_request.can_be_merged_by?(merge_request.assignee)
-          %span.float-right.cannot-be-merged{ data: { toggle: 'tooltip', placement: 'left' }, title: 'Not allowed to merge' }
-            = icon('exclamation-triangle', 'aria-hidden': 'true')
-        %span.username
-          = merge_request.assignee.to_reference
-    - else
-      %span.assign-yourself.no-value
-        No assignee
-        - if can_edit_issuable
-          \-
-          %a.js-assign-yourself{ href: '#' }
-            assign yourself
-
-  .selectbox.hide-collapsed
-    = f.hidden_field 'assignee_id', value: merge_request.assignee_id, id: 'issue_assignee_id'
-    = dropdown_tag('Select assignee', options: { toggle_class: 'js-user-search js-author-search', title: 'Assign to', filter: true, dropdown_class: 'dropdown-menu-user dropdown-menu-selectable dropdown-menu-author', placeholder: 'Search users', data: { first_user: (current_user.username if current_user), current_user: true, project_id: @project&.id, author_id: merge_request.author_id, field_name: 'merge_request[assignee_id]', issue_update: issuable_json_path(merge_request), ability_name: 'merge_request', null_user: true } })
diff --git a/app/views/shared/issuable/form/_metadata.html.haml b/app/views/shared/issuable/form/_metadata.html.haml
index e370dff9526..1e03440a5dc 100644
--- a/app/views/shared/issuable/form/_metadata.html.haml
+++ b/app/views/shared/issuable/form/_metadata.html.haml
@@ -8,11 +8,8 @@
 %hr
 .row
   %div{ class: (has_due_date ? "col-lg-6" : "col-12") }
-    .form-group.row.issue-assignee
-      - if issuable.is_a?(Issue)
-        = render "shared/issuable/form/metadata_issue_assignee", issuable: issuable, form: form, has_due_date: has_due_date
-      - else
-        = render "shared/issuable/form/metadata_merge_request_assignee", issuable: issuable, form: form, has_due_date: has_due_date
+    .form-group.row.merge-request-assignee
+      = render "shared/issuable/form/metadata_issuable_assignee", issuable: issuable, form: form, has_due_date: has_due_date
     .form-group.row.issue-milestone
       = form.label :milestone_id, "Milestone", class: "col-form-label #{has_due_date ? "col-md-2 col-lg-4" : "col-sm-2"}"
       .col-sm-10{ class: ("col-md-8" if has_due_date) }
diff --git a/app/views/shared/issuable/form/_metadata_issuable_assignee.html.haml b/app/views/shared/issuable/form/_metadata_issuable_assignee.html.haml
new file mode 100644
index 00000000000..5336159e762
--- /dev/null
+++ b/app/views/shared/issuable/form/_metadata_issuable_assignee.html.haml
@@ -0,0 +1,11 @@
+= form.label :assignee_id, "Assignee", class: "col-form-label #{has_due_date ? "col-lg-4" : "col-sm-2"}"
+.col-sm-10{ class: ("col-md-8" if has_due_date) }
+  .issuable-form-select-holder.selectbox
+    - issuable.assignees.each do |assignee|
+      = hidden_field_tag "#{issuable.to_ability_name}[assignee_ids][]", assignee.id, id: nil, data: { meta: assignee.name, avatar_url: assignee.avatar_url, name: assignee.name, username: assignee.username }
+
+    - if issuable.assignees.length === 0
+      = hidden_field_tag "#{issuable.to_ability_name}[assignee_ids][]", 0, id: nil, data: { meta: '' }
+
+    = dropdown_tag(users_dropdown_label(issuable.assignees), options: assignees_dropdown_options(issuable.to_ability_name))
+  = link_to 'Assign to me', '#', class: "assign-to-me-link qa-assign-to-me-link #{'hide' if issuable.assignees.include?(current_user)}"
diff --git a/app/views/shared/issuable/form/_metadata_issue_assignee.html.haml b/app/views/shared/issuable/form/_metadata_issue_assignee.html.haml
deleted file mode 100644
index 6d4f9ccd66f..00000000000
--- a/app/views/shared/issuable/form/_metadata_issue_assignee.html.haml
+++ /dev/null
@@ -1,11 +0,0 @@
-= form.label :assignee_ids, "Assignee", class: "col-form-label #{"col-md-2 col-lg-4" if has_due_date}"
-.col-sm-10{ class: ("col-md-8" if has_due_date) }
-  .issuable-form-select-holder.selectbox
-    - issuable.assignees.each do |assignee|
-      = hidden_field_tag "#{issuable.to_ability_name}[assignee_ids][]", assignee.id, id: nil, data: { meta: assignee.name, avatar_url: assignee.avatar_url, name: assignee.name, username: assignee.username }
-
-    - if issuable.assignees.length === 0
-      = hidden_field_tag "#{issuable.to_ability_name}[assignee_ids][]", 0, id: nil, data: { meta: '' }
-
-    = dropdown_tag(users_dropdown_label(issuable.assignees), options: issue_assignees_dropdown_options)
-  = link_to 'Assign to me', '#', class: "assign-to-me-link #{'hide' if issuable.assignees.include?(current_user)}"
diff --git a/db/fixtures/development/10_merge_requests.rb b/db/fixtures/development/10_merge_requests.rb
index 1952f84ed62..43b69470d2c 100644
--- a/db/fixtures/development/10_merge_requests.rb
+++ b/db/fixtures/development/10_merge_requests.rb
@@ -21,7 +21,7 @@ Gitlab::Seeder.quiet do
         title: FFaker::Lorem.sentence(6),
         description: FFaker::Lorem.sentences(3).join(" "),
         milestone: project.milestones.sample,
-        assignee: project.team.users.sample,
+        assignees: [project.team.users.sample],
         label_ids: label_ids
       }
 
diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 2dd3120d3fc..8e0623887a8 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -663,7 +663,11 @@ module API
       expose(:user_notes_count) { |merge_request, options| issuable_metadata(merge_request, options, :user_notes_count) }
       expose(:upvotes)          { |merge_request, options| issuable_metadata(merge_request, options, :upvotes) }
       expose(:downvotes)        { |merge_request, options| issuable_metadata(merge_request, options, :downvotes) }
-      expose :author, :assignee, using: Entities::UserBasic
+      expose :assignee, using: ::API::Entities::UserBasic do |merge_request|
+        merge_request.assignee
+      end
+      expose :author, :assignees, using: Entities::UserBasic
+
       expose :source_project_id, :target_project_id
       expose :labels do |merge_request|
         # Avoids an N+1 query since labels are preloaded
diff --git a/lib/api/merge_requests.rb b/lib/api/merge_requests.rb
index e4b21b7d1c4..1cc0ecc6df8 100644
--- a/lib/api/merge_requests.rb
+++ b/lib/api/merge_requests.rb
@@ -20,6 +20,7 @@ module API
     def self.update_params_at_least_one_of
       %i[
         assignee_id
+        assignee_ids
         description
         labels
         milestone_id
@@ -184,6 +185,7 @@ module API
         params :optional_params do
           optional :description, type: String, desc: 'The description of the merge request'
           optional :assignee_id, type: Integer, desc: 'The ID of a user to assign the merge request'
+          optional :assignee_ids, type: Array[Integer], desc: 'The array of user IDs to assign issue'
           optional :milestone_id, type: Integer, desc: 'The ID of a milestone to assign the merge request'
           optional :labels, type: Array[String], coerce_with: Validations::Types::LabelsList.coerce, desc: 'Comma-separated list of label names'
           optional :remove_source_branch, type: Boolean, desc: 'Remove source branch when merging'
@@ -231,6 +233,7 @@ module API
 
         mr_params = declared_params(include_missing: false)
         mr_params[:force_remove_source_branch] = mr_params.delete(:remove_source_branch)
+        mr_params = convert_parameters_from_legacy_format(mr_params)
 
         merge_request = ::MergeRequests::CreateService.new(user_project, current_user, mr_params).execute
 
@@ -334,6 +337,7 @@ module API
 
         mr_params = declared_params(include_missing: false)
         mr_params[:force_remove_source_branch] = mr_params.delete(:remove_source_branch) if mr_params[:remove_source_branch].present?
+        mr_params = convert_parameters_from_legacy_format(mr_params)
 
         merge_request = ::MergeRequests::UpdateService.new(user_project, current_user, mr_params).execute(merge_request)
 
diff --git a/lib/banzai/reference_parser/merge_request_parser.rb b/lib/banzai/reference_parser/merge_request_parser.rb
index e8147ac591a..d7bf450465e 100644
--- a/lib/banzai/reference_parser/merge_request_parser.rb
+++ b/lib/banzai/reference_parser/merge_request_parser.rb
@@ -10,7 +10,7 @@ module Banzai
           nodes,
           MergeRequest.includes(
             :author,
-            :assignee,
+            :assignees,
             {
               # These associations are primarily used for checking permissions.
               # Eager loading these ensures we don't end up running dozens of
diff --git a/lib/gitlab/hook_data/issuable_builder.rb b/lib/gitlab/hook_data/issuable_builder.rb
index 0803df65632..b8da6731081 100644
--- a/lib/gitlab/hook_data/issuable_builder.rb
+++ b/lib/gitlab/hook_data/issuable_builder.rb
@@ -20,11 +20,7 @@ module Gitlab
           repository: issuable.project.hook_attrs.slice(:name, :url, :description, :homepage)
         }
 
-        if issuable.is_a?(Issue)
-          hook_data[:assignees] = issuable.assignees.map(&:hook_attrs) if issuable.assignees.any?
-        else
-          hook_data[:assignee] = issuable.assignee.hook_attrs if issuable.assignee
-        end
+        hook_data[:assignees] = issuable.assignees.map(&:hook_attrs) if issuable.assignees.any?
 
         hook_data
       end
diff --git a/lib/gitlab/hook_data/merge_request_builder.rb b/lib/gitlab/hook_data/merge_request_builder.rb
index d77b1d04644..a8e993e087e 100644
--- a/lib/gitlab/hook_data/merge_request_builder.rb
+++ b/lib/gitlab/hook_data/merge_request_builder.rb
@@ -34,7 +34,6 @@ module Gitlab
       end
 
       SAFE_HOOK_RELATIONS = %i[
-        assignee
         labels
         total_time_spent
       ].freeze
@@ -51,7 +50,9 @@ module Gitlab
           work_in_progress: merge_request.work_in_progress?,
           total_time_spent: merge_request.total_time_spent,
           human_total_time_spent: merge_request.human_total_time_spent,
-          human_time_estimate: merge_request.human_time_estimate
+          human_time_estimate: merge_request.human_time_estimate,
+          assignee_ids: merge_request.assignee_ids,
+          assignee_id: merge_request.assignee_ids.first # This key is deprecated
         }
 
         merge_request.attributes.with_indifferent_access.slice(*self.class.safe_hook_attributes)
diff --git a/locale/gitlab.pot b/locale/gitlab.pot
index aa116a44254..48917ce445c 100644
--- a/locale/gitlab.pot
+++ b/locale/gitlab.pot
@@ -987,9 +987,6 @@ msgstr ""
 msgid "Assigned Merge Requests"
 msgstr ""
 
-msgid "Assigned to :name"
-msgstr ""
-
 msgid "Assigned to me"
 msgstr ""
 
@@ -5474,9 +5471,6 @@ msgstr ""
 msgid "No activities found"
 msgstr ""
 
-msgid "No assignee"
-msgstr ""
-
 msgid "No branches found"
 msgstr ""
 
@@ -5564,9 +5558,6 @@ msgstr ""
 msgid "None"
 msgstr ""
 
-msgid "Not allowed to merge"
-msgstr ""
-
 msgid "Not available"
 msgstr ""
 
@@ -7277,9 +7268,6 @@ msgstr ""
 msgid "Select an existing Kubernetes cluster or create a new one"
 msgstr ""
 
-msgid "Select assignee"
-msgstr ""
-
 msgid "Select branch/tag"
 msgstr ""
 
@@ -8860,9 +8848,6 @@ msgstr ""
 msgid "Toggle navigation"
 msgstr ""
 
-msgid "Toggle sidebar"
-msgstr ""
-
 msgid "ToggleButton|Toggle Status: OFF"
 msgstr ""
 
@@ -9870,9 +9855,6 @@ msgstr ""
 msgid "among other things"
 msgstr ""
 
-msgid "assign yourself"
-msgstr ""
-
 msgid "attach a new file"
 msgstr ""
 
diff --git a/qa/qa/page/merge_request/new.rb b/qa/qa/page/merge_request/new.rb
index 20d9c336367..56fbf59b9bc 100644
--- a/qa/qa/page/merge_request/new.rb
+++ b/qa/qa/page/merge_request/new.rb
@@ -26,7 +26,7 @@ module QA
           element :issuable_label
         end
 
-        view 'app/views/shared/issuable/form/_metadata_merge_request_assignee.html.haml' do
+        view 'app/views/shared/issuable/form/_metadata_issuable_assignee.html.haml' do
           element :assign_to_me_link
         end
 
diff --git a/spec/controllers/projects/merge_requests_controller_spec.rb b/spec/controllers/projects/merge_requests_controller_spec.rb
index 017162519d8..a125e470522 100644
--- a/spec/controllers/projects/merge_requests_controller_spec.rb
+++ b/spec/controllers/projects/merge_requests_controller_spec.rb
@@ -238,11 +238,11 @@ describe Projects::MergeRequestsController do
         assignee = create(:user)
         project.add_developer(assignee)
 
-        update_merge_request({ assignee_id: assignee.id }, format: :json)
+        update_merge_request({ assignee_ids: [assignee.id] }, format: :json)
+
         body = JSON.parse(response.body)
 
-        expect(body['assignee'].keys)
-          .to match_array(%w(name username avatar_url id state web_url))
+        expect(body['assignees']).to all(include(*%w(name username avatar_url id state web_url)))
       end
     end
 
diff --git a/spec/features/dashboard/issuables_counter_spec.rb b/spec/features/dashboard/issuables_counter_spec.rb
index fbc2e5cc3d3..50b71368e13 100644
--- a/spec/features/dashboard/issuables_counter_spec.rb
+++ b/spec/features/dashboard/issuables_counter_spec.rb
@@ -8,7 +8,7 @@ describe 'Navigation bar counter', :use_clean_rails_memory_store_caching do
 
   before do
     issue.assignees = [user]
-    merge_request.update(assignee: user)
+    merge_request.update(assignees: [user])
     sign_in(user)
   end
 
@@ -33,7 +33,7 @@ describe 'Navigation bar counter', :use_clean_rails_memory_store_caching do
 
     expect_counters('merge_requests', '1')
 
-    merge_request.update(assignee: nil)
+    merge_request.update(assignees: [])
 
     user.invalidate_cache_counts
 
diff --git a/spec/features/dashboard/merge_requests_spec.rb b/spec/features/dashboard/merge_requests_spec.rb
index 4965770605a..0c6713f623c 100644
--- a/spec/features/dashboard/merge_requests_spec.rb
+++ b/spec/features/dashboard/merge_requests_spec.rb
@@ -48,14 +48,14 @@ describe 'Dashboard Merge Requests' do
 
     let!(:assigned_merge_request) do
       create(:merge_request,
-        assignee: current_user,
+        assignees: [current_user],
         source_project: project,
         author: create(:user))
     end
 
     let!(:assigned_merge_request_from_fork) do
       create(:merge_request,
-              source_branch: 'markdown', assignee: current_user,
+              source_branch: 'markdown', assignees: [current_user],
               target_project: public_project, source_project: forked_project,
               author: create(:user))
     end
diff --git a/spec/features/groups/merge_requests_spec.rb b/spec/features/groups/merge_requests_spec.rb
index 54a8016c157..e1bc4eca619 100644
--- a/spec/features/groups/merge_requests_spec.rb
+++ b/spec/features/groups/merge_requests_spec.rb
@@ -38,7 +38,7 @@ describe 'Group merge requests page' do
 
   context 'when merge request assignee to user' do
     before do
-      issuable.update!(assignee: user)
+      issuable.update!(assignees: [user])
 
       visit path
     end
diff --git a/spec/features/issues/form_spec.rb b/spec/features/issues/form_spec.rb
index 26c781350e5..6fa2ad8711f 100644
--- a/spec/features/issues/form_spec.rb
+++ b/spec/features/issues/form_spec.rb
@@ -30,8 +30,8 @@ describe 'New/edit issue', :js do
         # the original method, resulting in infinite recursion when called.
         # This is likely a bug with helper modules included into dynamically generated view classes.
         # To work around this, we have to hold on to and call to the original implementation manually.
-        original_issue_dropdown_options = FormHelper.instance_method(:issue_assignees_dropdown_options)
-        allow_any_instance_of(FormHelper).to receive(:issue_assignees_dropdown_options).and_wrap_original do |original, *args|
+        original_issue_dropdown_options = FormHelper.instance_method(:assignees_dropdown_options)
+        allow_any_instance_of(FormHelper).to receive(:assignees_dropdown_options).and_wrap_original do |original, *args|
           options = original_issue_dropdown_options.bind(original.receiver).call(*args)
           options[:data][:per_page] = 2
 
diff --git a/spec/features/merge_request/user_creates_merge_request_spec.rb b/spec/features/merge_request/user_creates_merge_request_spec.rb
index ea2bb1503bb..bcc11217389 100644
--- a/spec/features/merge_request/user_creates_merge_request_spec.rb
+++ b/spec/features/merge_request/user_creates_merge_request_spec.rb
@@ -68,15 +68,15 @@ describe "User creates a merge request", :js do
         fill_in("Title", with: title)
       end
 
-      click_button("Assignee")
-
       expect(find(".js-assignee-search")["data-project-id"]).to eq(project.id.to_s)
+      find('.js-assignee-search').click
 
       page.within(".dropdown-menu-user") do
         expect(page).to have_content("Unassigned")
                    .and have_content(user.name)
                    .and have_content(project.users.first.name)
       end
+      find('.js-assignee-search').click
 
       click_button("Submit merge request")
 
diff --git a/spec/features/merge_request/user_creates_mr_spec.rb b/spec/features/merge_request/user_creates_mr_spec.rb
index c169a68cd1c..c9dedab048a 100644
--- a/spec/features/merge_request/user_creates_mr_spec.rb
+++ b/spec/features/merge_request/user_creates_mr_spec.rb
@@ -1,11 +1,18 @@
 require 'rails_helper'
 
 describe 'Merge request > User creates MR' do
-  it_behaves_like 'a creatable merge request'
+  include ProjectForksHelper
 
-  context 'from a forked project' do
-    include ProjectForksHelper
+  before do
+    stub_licensed_features(multiple_merge_request_assignees: false)
+  end
 
+  context 'non-fork merge request' do
+    include_context 'merge request create context'
+    it_behaves_like 'a creatable merge request'
+  end
+
+  context 'from a forked project' do
     let(:canonical_project) { create(:project, :public, :repository) }
 
     let(:source_project) do
@@ -15,6 +22,7 @@ describe 'Merge request > User creates MR' do
     end
 
     context 'to canonical project' do
+      include_context 'merge request create context'
       it_behaves_like 'a creatable merge request'
     end
 
@@ -25,6 +33,7 @@ describe 'Merge request > User creates MR' do
           namespace: user.namespace)
       end
 
+      include_context 'merge request create context'
       it_behaves_like 'a creatable merge request'
     end
   end
diff --git a/spec/features/merge_request/user_edits_mr_spec.rb b/spec/features/merge_request/user_edits_mr_spec.rb
index 3152707136c..25979513ead 100644
--- a/spec/features/merge_request/user_edits_mr_spec.rb
+++ b/spec/features/merge_request/user_edits_mr_spec.rb
@@ -1,13 +1,21 @@
-require 'rails_helper'
+require 'spec_helper'
 
 describe 'Merge request > User edits MR' do
   include ProjectForksHelper
 
-  it_behaves_like 'an editable merge request'
+  before do
+    stub_licensed_features(multiple_merge_request_assignees: false)
+  end
+
+  context 'non-fork merge request' do
+    include_context 'merge request edit context'
+    it_behaves_like 'an editable merge request'
+  end
 
   context 'for a forked project' do
-    it_behaves_like 'an editable merge request' do
-      let(:source_project) { fork_project(target_project, nil, repository: true) }
-    end
+    let(:source_project) { fork_project(target_project, nil, repository: true) }
+
+    include_context 'merge request edit context'
+    it_behaves_like 'an editable merge request'
   end
 end
diff --git a/spec/features/merge_requests/user_filters_by_assignees_spec.rb b/spec/features/merge_requests/user_filters_by_assignees_spec.rb
index d6c770c93f1..0cbf1bcae30 100644
--- a/spec/features/merge_requests/user_filters_by_assignees_spec.rb
+++ b/spec/features/merge_requests/user_filters_by_assignees_spec.rb
@@ -7,7 +7,7 @@ describe 'Merge Requests > User filters by assignees', :js do
   let(:user)    { project.creator }
 
   before do
-    create(:merge_request, assignee: user, title: 'Bugfix1', source_project: project, target_project: project, source_branch: 'bugfix1')
+    create(:merge_request, assignees: [user], title: 'Bugfix1', source_project: project, target_project: project, source_branch: 'bugfix1')
     create(:merge_request, title: 'Bugfix2', source_project: project, target_project: project, source_branch: 'bugfix2')
 
     sign_in(user)
diff --git a/spec/features/merge_requests/user_filters_by_multiple_criteria_spec.rb b/spec/features/merge_requests/user_filters_by_multiple_criteria_spec.rb
index 1615899a047..4627931f26a 100644
--- a/spec/features/merge_requests/user_filters_by_multiple_criteria_spec.rb
+++ b/spec/features/merge_requests/user_filters_by_multiple_criteria_spec.rb
@@ -10,7 +10,7 @@ describe 'Merge requests > User filters by multiple criteria', :js do
 
   before do
     sign_in(user)
-    mr = create(:merge_request, title: 'Bugfix2', author: user, assignee: user, source_project: project, target_project: project, milestone: milestone)
+    mr = create(:merge_request, title: 'Bugfix2', author: user, assignees: [user], source_project: project, target_project: project, milestone: milestone)
     mr.labels << wontfix
 
     visit project_merge_requests_path(project)
diff --git a/spec/features/merge_requests/user_lists_merge_requests_spec.rb b/spec/features/merge_requests/user_lists_merge_requests_spec.rb
index c691011b9ca..bd91fae1453 100644
--- a/spec/features/merge_requests/user_lists_merge_requests_spec.rb
+++ b/spec/features/merge_requests/user_lists_merge_requests_spec.rb
@@ -12,7 +12,7 @@ describe 'Merge requests > User lists merge requests' do
                   title: 'fix',
                   source_project: project,
                   source_branch: 'fix',
-                  assignee: user,
+                  assignees: [user],
                   milestone: create(:milestone, project: project, due_date: '2013-12-11'),
                   created_at: 1.minute.ago,
                   updated_at: 1.minute.ago)
@@ -20,7 +20,7 @@ describe 'Merge requests > User lists merge requests' do
            title: 'markdown',
            source_project: project,
            source_branch: 'markdown',
-           assignee: user,
+           assignees: [user],
            milestone: create(:milestone, project: project, due_date: '2013-12-12'),
            created_at: 2.minutes.ago,
            updated_at: 2.minutes.ago)
diff --git a/spec/features/merge_requests/user_mass_updates_spec.rb b/spec/features/merge_requests/user_mass_updates_spec.rb
index e535c7e5811..c2dd105324d 100644
--- a/spec/features/merge_requests/user_mass_updates_spec.rb
+++ b/spec/features/merge_requests/user_mass_updates_spec.rb
@@ -54,8 +54,7 @@ describe 'Merge requests > User mass updates', :js do
 
     describe 'remove assignee' do
       before do
-        merge_request.assignee = user
-        merge_request.save
+        merge_request.assignees = [user]
         visit project_merge_requests_path(project)
       end
 
diff --git a/spec/features/search/user_uses_header_search_field_spec.rb b/spec/features/search/user_uses_header_search_field_spec.rb
index 444de26733f..1cc47cd6bd1 100644
--- a/spec/features/search/user_uses_header_search_field_spec.rb
+++ b/spec/features/search/user_uses_header_search_field_spec.rb
@@ -36,7 +36,7 @@ describe 'User uses header search field' do
     end
 
     context 'when clicking merge requests' do
-      let!(:merge_request) { create(:merge_request, source_project: project, author: user, assignee: user) }
+      let!(:merge_request) { create(:merge_request, source_project: project, author: user, assignees: [user]) }
 
       it 'shows assigned merge requests' do
         find('.search-input-container .dropdown-menu').click_link('Merge requests assigned to me')
@@ -100,7 +100,7 @@ describe 'User uses header search field' do
       end
 
       context 'when clicking merge requests' do
-        let!(:merge_request) { create(:merge_request, source_project: project, author: user, assignee: user) }
+        let!(:merge_request) { create(:merge_request, source_project: project, author: user, assignees: [user]) }
 
         it 'shows assigned merge requests' do
           find('.dropdown-menu').click_link('Merge requests assigned to me')
diff --git a/spec/finders/issues_finder_spec.rb b/spec/finders/issues_finder_spec.rb
index fe53fabe54c..6e6dc334354 100644
--- a/spec/finders/issues_finder_spec.rb
+++ b/spec/finders/issues_finder_spec.rb
@@ -13,60 +13,32 @@ describe IssuesFinder do
         expect(issues).to contain_exactly(issue1, issue2, issue3, issue4)
       end
 
-      context 'filtering by assignee ID' do
-        let(:params) { { assignee_id: user.id } }
+      context 'assignee filtering' do
+        let(:issuables) { issues }
 
-        it 'returns issues assigned to that user' do
-          expect(issues).to contain_exactly(issue1, issue2)
-        end
-      end
-
-      context 'filtering by assignee usernames' do
-        set(:user3) { create(:user) }
-        let(:params) { { assignee_username: [user2.username, user3.username] } }
-
-        before do
-          project2.add_developer(user3)
-
-          issue3.assignees = [user2, user3]
-        end
-
-        it 'returns issues assigned to those users' do
-          expect(issues).to contain_exactly(issue3)
-        end
-      end
-
-      context 'filtering by no assignee' do
-        let(:params) { { assignee_id: 'None' } }
-
-        it 'returns issues not assigned to any assignee' do
-          expect(issues).to contain_exactly(issue4)
-        end
-
-        it 'returns issues not assigned to any assignee' do
-          params[:assignee_id] = 0
-
-          expect(issues).to contain_exactly(issue4)
+        it_behaves_like 'assignee ID filter' do
+          let(:params) { { assignee_id: user.id } }
+          let(:expected_issuables) { [issue1, issue2] }
         end
 
-        it 'returns issues not assigned to any assignee' do
-          params[:assignee_id] = 'none'
+        it_behaves_like 'assignee username filter' do
+          before do
+            project2.add_developer(user3)
+            issue3.assignees = [user2, user3]
+          end
 
-          expect(issues).to contain_exactly(issue4)
+          set(:user3) { create(:user) }
+          let(:params) { { assignee_username: [user2.username, user3.username] } }
+          let(:expected_issuables) { [issue3] }
         end
-      end
 
-      context 'filtering by any assignee' do
-        let(:params) { { assignee_id: 'Any' } }
-
-        it 'returns issues assigned to any assignee' do
-          expect(issues).to contain_exactly(issue1, issue2, issue3)
+        it_behaves_like 'no assignee filter' do
+          set(:user3) { create(:user) }
+          let(:expected_issuables) { [issue4] }
         end
 
-        it 'returns issues assigned to any assignee' do
-          params[:assignee_id] = 'any'
-
-          expect(issues).to contain_exactly(issue1, issue2, issue3)
+        it_behaves_like 'any assignee filter' do
+          let(:expected_issuables) { [issue1, issue2, issue3] }
         end
       end
 
diff --git a/spec/finders/merge_requests_finder_spec.rb b/spec/finders/merge_requests_finder_spec.rb
index f508b9bdb6f..ef002125635 100644
--- a/spec/finders/merge_requests_finder_spec.rb
+++ b/spec/finders/merge_requests_finder_spec.rb
@@ -136,21 +136,50 @@ describe MergeRequestsFinder do
         end
       end
 
-      context 'filtering by group milestone' do
-        let(:group_milestone) { create(:milestone, group: group) }
+      context 'assignee filtering' do
+        let(:issuables) { described_class.new(user, params).execute }
 
-        before do
-          project2.update(namespace: group)
-          merge_request2.update(milestone: group_milestone)
-          merge_request3.update(milestone: group_milestone)
+        it_behaves_like 'assignee ID filter' do
+          let(:params) { { assignee_id: user.id } }
+          let(:expected_issuables) { [merge_request1, merge_request2] }
         end
 
-        it 'returns merge requests assigned to that group milestone' do
-          params = { milestone_title: group_milestone.title }
+        it_behaves_like 'assignee username filter' do
+          before do
+            project2.add_developer(user3)
+            merge_request3.assignees = [user2, user3]
+          end
 
-          merge_requests = described_class.new(user, params).execute
+          set(:user3) { create(:user) }
+          let(:params) { { assignee_username: [user2.username, user3.username] } }
+          let(:expected_issuables) { [merge_request3] }
+        end
+
+        it_behaves_like 'no assignee filter' do
+          set(:user3) { create(:user) }
+          let(:expected_issuables) { [merge_request4, merge_request5] }
+        end
+
+        it_behaves_like 'any assignee filter' do
+          let(:expected_issuables) { [merge_request1, merge_request2, merge_request3] }
+        end
+
+        context 'filtering by group milestone' do
+          let(:group_milestone) { create(:milestone, group: group) }
+
+          before do
+            project2.update(namespace: group)
+            merge_request2.update(milestone: group_milestone)
+            merge_request3.update(milestone: group_milestone)
+          end
+
+          it 'returns merge requests assigned to that group milestone' do
+            params = { milestone_title: group_milestone.title }
 
-          expect(merge_requests).to contain_exactly(merge_request2, merge_request3)
+            merge_requests = described_class.new(user, params).execute
+
+            expect(merge_requests).to contain_exactly(merge_request2, merge_request3)
+          end
         end
       end
 
diff --git a/spec/fixtures/api/schemas/entities/merge_request_basic.json b/spec/fixtures/api/schemas/entities/merge_request_basic.json
index 3006b482d41..88a600398b1 100644
--- a/spec/fixtures/api/schemas/entities/merge_request_basic.json
+++ b/spec/fixtures/api/schemas/entities/merge_request_basic.json
@@ -6,14 +6,14 @@
     "source_branch_exists": { "type": "boolean" },
     "merge_error": { "type": ["string", "null"] },
     "rebase_in_progress": { "type": "boolean" },
-    "assignee_id": { "type": ["integer", "null"] },
     "allow_collaboration": { "type": "boolean"},
     "allow_maintainer_to_push": { "type": "boolean"},
-    "assignee": {
-      "oneOf": [
-        { "type": "null" },
-        { "$ref": "user.json" }
-      ]
+    "assignees": {
+      "type": ["array"],
+      "items": {
+        "type": "object",
+        "$ref": "../public_api/v4/user/basic.json"
+      }
     },
     "milestone": {
       "type": [ "object", "null" ]
diff --git a/spec/fixtures/api/schemas/public_api/v4/merge_request.json b/spec/fixtures/api/schemas/public_api/v4/merge_request.json
index 918f2c4b47d..a423bf70b69 100644
--- a/spec/fixtures/api/schemas/public_api/v4/merge_request.json
+++ b/spec/fixtures/api/schemas/public_api/v4/merge_request.json
@@ -64,6 +64,11 @@
         },
         "additionalProperties": false
       },
+      "assignees": {
+        "items": {
+          "$ref": "./merge_request.json"
+        }
+      },
       "source_project_id": { "type": "integer" },
       "target_project_id": { "type": "integer" },
       "labels": {
diff --git a/spec/javascripts/sidebar/assignees_spec.js b/spec/javascripts/sidebar/assignees_spec.js
index 57b16b12cb0..47fee5d2b21 100644
--- a/spec/javascripts/sidebar/assignees_spec.js
+++ b/spec/javascripts/sidebar/assignees_spec.js
@@ -132,9 +132,94 @@ describe('Assignee component', () => {
         -1,
       );
     });
+
+    it('has correct "cannot merge" tooltip when user cannot merge', () => {
+      const user = Object.assign({}, UsersMock.user, { can_merge: false });
+
+      component = new AssigneeComponent({
+        propsData: {
+          rootPath: 'http://localhost:3000/',
+          users: [user],
+          editable: true,
+          issuableType: 'merge_request',
+        },
+      }).$mount();
+
+      expect(component.mergeNotAllowedTooltipMessage).toEqual('Cannot merge');
+    });
   });
 
   describe('Two or more assignees/users', () => {
+    it('has correct "cannot merge" tooltip when one user can merge', () => {
+      const users = UsersMockHelper.createNumberRandomUsers(3);
+      users[0].can_merge = true;
+      users[1].can_merge = false;
+      users[2].can_merge = false;
+
+      component = new AssigneeComponent({
+        propsData: {
+          rootPath: 'http://localhost:3000/',
+          users,
+          editable: true,
+          issuableType: 'merge_request',
+        },
+      }).$mount();
+
+      expect(component.mergeNotAllowedTooltipMessage).toEqual('1/3 can merge');
+    });
+
+    it('has correct "cannot merge" tooltip when no user can merge', () => {
+      const users = UsersMockHelper.createNumberRandomUsers(2);
+      users[0].can_merge = false;
+      users[1].can_merge = false;
+
+      component = new AssigneeComponent({
+        propsData: {
+          rootPath: 'http://localhost:3000/',
+          users,
+          editable: true,
+          issuableType: 'merge_request',
+        },
+      }).$mount();
+
+      expect(component.mergeNotAllowedTooltipMessage).toEqual('No one can merge');
+    });
+
+    it('has correct "cannot merge" tooltip when more than one user can merge', () => {
+      const users = UsersMockHelper.createNumberRandomUsers(3);
+      users[0].can_merge = false;
+      users[1].can_merge = true;
+      users[2].can_merge = true;
+
+      component = new AssigneeComponent({
+        propsData: {
+          rootPath: 'http://localhost:3000/',
+          users,
+          editable: true,
+          issuableType: 'merge_request',
+        },
+      }).$mount();
+
+      expect(component.mergeNotAllowedTooltipMessage).toEqual('2/3 can merge');
+    });
+
+    it('has no "cannot merge" tooltip when every user can merge', () => {
+      const users = UsersMockHelper.createNumberRandomUsers(2);
+      users[0].can_merge = true;
+      users[1].can_merge = true;
+
+      component = new AssigneeComponent({
+        propsData: {
+          rootPath: 'http://localhost:3000/',
+          users,
+          editable: true,
+          issuableType: 'merge_request',
+        },
+      }).$mount();
+
+      expect(component.mergeNotAllowedTooltipMessage).toEqual(null);
+    });
+
     it('displays two assignee icons when collapsed', () => {
       const users = UsersMockHelper.createNumberRandomUsers(2);
       component = new AssigneeComponent({
diff --git a/spec/lib/gitlab/hook_data/issuable_builder_spec.rb b/spec/lib/gitlab/hook_data/issuable_builder_spec.rb
index 26529c4759d..569d5dcc757 100644
--- a/spec/lib/gitlab/hook_data/issuable_builder_spec.rb
+++ b/spec/lib/gitlab/hook_data/issuable_builder_spec.rb
@@ -97,13 +97,13 @@ describe Gitlab::HookData::IssuableBuilder do
     end
 
     context 'merge_request is assigned' do
-      let(:merge_request) { create(:merge_request, assignee: user) }
+      let(:merge_request) { create(:merge_request, assignees: [user]) }
       let(:data) { described_class.new(merge_request).build(user: user) }
 
       it 'returns correct hook data' do
         expect(data[:object_attributes]['assignee_id']).to eq(user.id)
-        expect(data[:assignee]).to eq(user.hook_attrs)
-        expect(data).not_to have_key(:assignees)
+        expect(data[:assignees].first).to eq(user.hook_attrs)
+        expect(data).not_to have_key(:assignee)
       end
     end
   end
diff --git a/spec/lib/gitlab/hook_data/merge_request_builder_spec.rb b/spec/lib/gitlab/hook_data/merge_request_builder_spec.rb
index 9ce697adbba..39f80f92fa6 100644
--- a/spec/lib/gitlab/hook_data/merge_request_builder_spec.rb
+++ b/spec/lib/gitlab/hook_data/merge_request_builder_spec.rb
@@ -10,6 +10,7 @@ describe Gitlab::HookData::MergeRequestBuilder do
     it 'includes safe attribute' do
       %w[
         assignee_id
+        assignee_ids
         author_id
         created_at
         description
diff --git a/spec/lib/gitlab/import_export/all_models.yml b/spec/lib/gitlab/import_export/all_models.yml
index e418516569a..ed557ffd4e3 100644
--- a/spec/lib/gitlab/import_export/all_models.yml
+++ b/spec/lib/gitlab/import_export/all_models.yml
@@ -102,6 +102,7 @@ merge_requests:
 - merge_request_pipelines
 - merge_request_assignees
 - suggestions
+- assignees
 merge_request_diff:
 - merge_request
 - merge_request_diff_commits
@@ -336,6 +337,9 @@ push_event_payload:
 issue_assignees:
 - issue
 - assignee
+merge_request_assignees:
+- merge_request
+- assignee
 lfs_file_locks:
 - user
 project_badges:
diff --git a/spec/lib/gitlab/import_export/safe_model_attributes.yml b/spec/lib/gitlab/import_export/safe_model_attributes.yml
index d0ed588f05f..06995604a24 100644
--- a/spec/lib/gitlab/import_export/safe_model_attributes.yml
+++ b/spec/lib/gitlab/import_export/safe_model_attributes.yml
@@ -621,3 +621,7 @@ Suggestion:
 - outdated
 - lines_above
 - lines_below
+MergeRequestAssignee:
+- id
+- user_id
+- merge_request_id
diff --git a/spec/lib/gitlab/issuable_metadata_spec.rb b/spec/lib/gitlab/issuable_metadata_spec.rb
index 6ec86163233..916f3876a8e 100644
--- a/spec/lib/gitlab/issuable_metadata_spec.rb
+++ b/spec/lib/gitlab/issuable_metadata_spec.rb
@@ -19,7 +19,7 @@ describe Gitlab::IssuableMetadata do
     let!(:closed_issue) { create(:issue, state: :closed, author: user, project: project) }
     let!(:downvote) { create(:award_emoji, :downvote, awardable: closed_issue) }
     let!(:upvote) { create(:award_emoji, :upvote, awardable: issue) }
-    let!(:merge_request) { create(:merge_request, :simple, author: user, assignee: user, source_project: project, target_project: project, title: "Test") }
+    let!(:merge_request) { create(:merge_request, :simple, author: user, assignees: [user], source_project: project, target_project: project, title: "Test") }
     let!(:closing_issues) { create(:merge_requests_closing_issues, issue: issue, merge_request: merge_request) }
 
     it 'aggregates stats on issues' do
@@ -39,7 +39,7 @@ describe Gitlab::IssuableMetadata do
   end
 
   context 'merge requests' do
-    let!(:merge_request) { create(:merge_request, :simple, author: user, assignee: user, source_project: project, target_project: project, title: "Test") }
+    let!(:merge_request) { create(:merge_request, :simple, author: user, assignees: [user], source_project: project, target_project: project, title: "Test") }
     let!(:merge_request_closed) { create(:merge_request, state: "closed", source_project: project, target_project: project, title: "Closed Test") }
     let!(:downvote) { create(:award_emoji, :downvote, awardable: merge_request) }
     let!(:upvote) { create(:award_emoji, :upvote, awardable: merge_request) }
diff --git a/spec/mailers/notify_spec.rb b/spec/mailers/notify_spec.rb
index 5fa1369c00a..fee1d701e3a 100644
--- a/spec/mailers/notify_spec.rb
+++ b/spec/mailers/notify_spec.rb
@@ -19,7 +19,7 @@ describe Notify do
     create(:merge_request, source_project: project,
                            target_project: project,
                            author: current_user,
-                           assignee: assignee,
+                           assignees: [assignee],
                            description: 'Awesome description')
   end
 
@@ -275,7 +275,7 @@ describe Notify do
 
     context 'for merge requests' do
       describe 'that are new' do
-        subject { described_class.new_merge_request_email(merge_request.assignee_id, merge_request.id) }
+        subject { described_class.new_merge_request_email(merge_request.assignee_ids.first, merge_request.id) }
 
         it_behaves_like 'an assignee email'
         it_behaves_like 'an email starting a new thread with reply-by-email enabled' do
@@ -300,7 +300,7 @@ describe Notify do
         end
 
         context 'when sent with a reason' do
-          subject { described_class.new_merge_request_email(merge_request.assignee_id, merge_request.id, NotificationReason::ASSIGNED) }
+          subject { described_class.new_merge_request_email(merge_request.assignee_ids.first, merge_request.id, NotificationReason::ASSIGNED) }
 
           it_behaves_like 'appearance header and footer enabled'
           it_behaves_like 'appearance header and footer not enabled'
@@ -324,7 +324,7 @@ describe Notify do
 
       describe 'that are reassigned' do
         let(:previous_assignee) { create(:user, name: 'Previous Assignee') }
-        subject { described_class.reassigned_merge_request_email(recipient.id, merge_request.id, previous_assignee.id, current_user.id) }
+        subject { described_class.reassigned_merge_request_email(recipient.id, merge_request.id, [previous_assignee.id], current_user.id) }
 
         it_behaves_like 'a multiple recipients email'
         it_behaves_like 'an answer to an existing thread with reply-by-email enabled' do
@@ -351,7 +351,7 @@ describe Notify do
         end
 
         context 'when sent with a reason' do
-          subject { described_class.reassigned_merge_request_email(recipient.id, merge_request.id, previous_assignee.id, current_user.id, NotificationReason::ASSIGNED) }
+          subject { described_class.reassigned_merge_request_email(recipient.id, merge_request.id, [previous_assignee.id], current_user.id, NotificationReason::ASSIGNED) }
 
           it_behaves_like 'appearance header and footer enabled'
           it_behaves_like 'appearance header and footer not enabled'
@@ -364,11 +364,11 @@ describe Notify do
             text = EmailsHelper.instance_method(:notification_reason_text).bind(self).call(NotificationReason::ASSIGNED)
             is_expected.to have_body_text(text)
 
-            new_subject = described_class.reassigned_merge_request_email(recipient.id, merge_request.id, previous_assignee.id, current_user.id, NotificationReason::MENTIONED)
+            new_subject = described_class.reassigned_merge_request_email(recipient.id, merge_request.id, [previous_assignee.id], current_user.id, NotificationReason::MENTIONED)
             text = EmailsHelper.instance_method(:notification_reason_text).bind(self).call(NotificationReason::MENTIONED)
             expect(new_subject).to have_body_text(text)
 
-            new_subject = described_class.reassigned_merge_request_email(recipient.id, merge_request.id, previous_assignee.id, current_user.id, nil)
+            new_subject = described_class.reassigned_merge_request_email(recipient.id, merge_request.id, [previous_assignee.id], current_user.id, nil)
             text = EmailsHelper.instance_method(:notification_reason_text).bind(self).call(nil)
             expect(new_subject).to have_body_text(text)
           end
@@ -376,7 +376,7 @@ describe Notify do
       end
 
       describe 'that are new with a description' do
-        subject { described_class.new_merge_request_email(merge_request.assignee_id, merge_request.id) }
+        subject { described_class.new_merge_request_email(merge_request.assignee_ids.first, merge_request.id) }
 
         it_behaves_like 'it should show Gmail Actions View Merge request link'
         it_behaves_like "an unsubscribeable thread"
@@ -476,7 +476,7 @@ describe Notify do
                  source_project: project,
                  target_project: project,
                  author: current_user,
-                 assignee: assignee,
+                 assignees: [assignee],
                  description: 'Awesome description')
         end
 
diff --git a/spec/models/ci/pipeline_spec.rb b/spec/models/ci/pipeline_spec.rb
index 83b0f172f03..f3e78630c1b 100644
--- a/spec/models/ci/pipeline_spec.rb
+++ b/spec/models/ci/pipeline_spec.rb
@@ -684,12 +684,12 @@ describe Ci::Pipeline, :mailer do
                source_branch: 'feature',
                target_project: project,
                target_branch: 'master',
-               assignee: assignee,
+               assignees: assignees,
                milestone: milestone,
                labels: labels)
       end
 
-      let(:assignee) { create(:user) }
+      let(:assignees) { create_list(:user, 2) }
       let(:milestone) { create(:milestone, project: project) }
       let(:labels) { create_list(:label, 2) }
 
@@ -710,7 +710,7 @@ describe Ci::Pipeline, :mailer do
             'CI_MERGE_REQUEST_SOURCE_BRANCH_NAME' => merge_request.source_branch.to_s,
             'CI_MERGE_REQUEST_SOURCE_BRANCH_SHA' => pipeline.source_sha.to_s,
             'CI_MERGE_REQUEST_TITLE' => merge_request.title,
-            'CI_MERGE_REQUEST_ASSIGNEES' => assignee.username,
+            'CI_MERGE_REQUEST_ASSIGNEES' => merge_request.assignee_username_list,
             'CI_MERGE_REQUEST_MILESTONE' => milestone.title,
             'CI_MERGE_REQUEST_LABELS' => labels.map(&:title).join(','))
       end
@@ -730,7 +730,7 @@ describe Ci::Pipeline, :mailer do
       end
 
       context 'without assignee' do
-        let(:assignee) { nil }
+        let(:assignees) { [] }
 
         it 'does not expose assignee variable' do
           expect(subject.to_hash.keys).not_to include('CI_MERGE_REQUEST_ASSIGNEES')
diff --git a/spec/models/concerns/deprecated_assignee_spec.rb b/spec/models/concerns/deprecated_assignee_spec.rb
new file mode 100644
index 00000000000..e394de0aa34
--- /dev/null
+++ b/spec/models/concerns/deprecated_assignee_spec.rb
@@ -0,0 +1,160 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe DeprecatedAssignee do
+  let(:user) { create(:user) }
+
+  describe '#assignee_id=' do
+    it 'creates the merge_request_assignees relation' do
+      merge_request = create(:merge_request, assignee_id: user.id)
+
+      merge_request.reload
+
+      expect(merge_request.merge_request_assignees.count).to eq(1)
+    end
+
+    it 'nullifies the assignee_id column' do
+      merge_request = create(:merge_request, assignee_id: user.id)
+
+      merge_request.reload
+
+      expect(merge_request.read_attribute(:assignee_id)).to be_nil
+    end
+
+    context 'when relation already exists' do
+      it 'overwrites existing assignees' do
+        other_user = create(:user)
+        merge_request = create(:merge_request, assignee_id: nil)
+        merge_request.merge_request_assignees.create!(user_id: user.id)
+        merge_request.merge_request_assignees.create!(user_id: other_user.id)
+
+        expect { merge_request.update!(assignee_id: other_user.id) }
+          .to change { merge_request.reload.merge_request_assignees.count }
+          .from(2).to(1)
+      end
+    end
+  end
+
+  describe '#assignee=' do
+    it 'creates the merge_request_assignees relation' do
+      merge_request = create(:merge_request, assignee: user)
+
+      merge_request.reload
+
+      expect(merge_request.merge_request_assignees.count).to eq(1)
+    end
+
+    it 'nullifies the assignee_id column' do
+      merge_request = create(:merge_request, assignee: user)
+
+      merge_request.reload
+
+      expect(merge_request.read_attribute(:assignee_id)).to be_nil
+    end
+
+    context 'when relation already exists' do
+      it 'overwrites existing assignees' do
+        other_user = create(:user)
+        merge_request = create(:merge_request, assignee: nil)
+        merge_request.merge_request_assignees.create!(user_id: user.id)
+        merge_request.merge_request_assignees.create!(user_id: other_user.id)
+
+        expect { merge_request.update!(assignee: other_user) }
+          .to change { merge_request.reload.merge_request_assignees.count }
+          .from(2).to(1)
+      end
+    end
+  end
+
+  describe '#assignee_id' do
+    it 'returns the first assignee ID' do
+      other_user = create(:user)
+      merge_request = create(:merge_request, assignees: [user, other_user])
+
+      merge_request.reload
+
+      expect(merge_request.assignee_id).to eq(merge_request.assignee_ids.first)
+    end
+  end
+
+  describe '#assignees' do
+    context 'when assignee_id exists and there is no relation' do
+      it 'creates the relation' do
+        merge_request = create(:merge_request, assignee_id: nil)
+        merge_request.update_column(:assignee_id, user.id)
+
+        expect { merge_request.assignees }.to change { merge_request.merge_request_assignees.count }.from(0).to(1)
+      end
+
+      it 'nullifies the assignee_id' do
+        merge_request = create(:merge_request, assignee_id: nil)
+        merge_request.update_column(:assignee_id, user.id)
+
+        expect { merge_request.assignees }
+          .to change { merge_request.read_attribute(:assignee_id) }
+          .from(user.id).to(nil)
+      end
+    end
+
+    context 'when DB is read-only' do
+      before do
+        allow(Gitlab::Database).to receive(:read_only?) { true }
+      end
+
+      it 'returns a users relation' do
+        merge_request = create(:merge_request, assignee_id: user.id)
+
+        expect(merge_request.assignees).to be_a(ActiveRecord::Relation)
+        expect(merge_request.assignees).to eq([user])
+      end
+
+      it 'returns an empty relation if no assignee_id is set' do
+        merge_request = create(:merge_request, assignee_id: nil)
+
+        expect(merge_request.assignees).to be_a(ActiveRecord::Relation)
+        expect(merge_request.assignees).to eq([])
+      end
+    end
+  end
+
+  describe '#assignee_ids' do
+    context 'when assignee_id exists and there is no relation' do
+      it 'creates the relation' do
+        merge_request = create(:merge_request, assignee_id: nil)
+        merge_request.update_column(:assignee_id, user.id)
+
+        expect { merge_request.assignee_ids }.to change { merge_request.merge_request_assignees.count }.from(0).to(1)
+      end
+
+      it 'nullifies the assignee_id' do
+        merge_request = create(:merge_request, assignee_id: nil)
+        merge_request.update_column(:assignee_id, user.id)
+
+        expect { merge_request.assignee_ids }
+          .to change { merge_request.read_attribute(:assignee_id) }
+          .from(user.id).to(nil)
+      end
+    end
+
+    context 'when DB is read-only' do
+      before do
+        allow(Gitlab::Database).to receive(:read_only?) { true }
+      end
+
+      it 'returns a list of user IDs' do
+        merge_request = create(:merge_request, assignee_id: user.id)
+
+        expect(merge_request.assignee_ids).to be_a(Array)
+        expect(merge_request.assignee_ids).to eq([user.id])
+      end
+
+      it 'returns an empty relation if no assignee_id is set' do
+        merge_request = create(:merge_request, assignee_id: nil)
+
+        expect(merge_request.assignee_ids).to be_a(Array)
+        expect(merge_request.assignee_ids).to eq([])
+      end
+    end
+  end
+end
diff --git a/spec/models/concerns/issuable_spec.rb b/spec/models/concerns/issuable_spec.rb
index 27ed298ae08..64f02978d79 100644
--- a/spec/models/concerns/issuable_spec.rb
+++ b/spec/models/concerns/issuable_spec.rb
@@ -502,8 +502,8 @@ describe Issuable do
       let(:user2) { create(:user) }
 
       before do
-        merge_request.update(assignee: user)
-        merge_request.update(assignee: user2)
+        merge_request.update(assignees: [user])
+        merge_request.update(assignees: [user, user2])
         expect(Gitlab::HookData::IssuableBuilder)
           .to receive(:new).with(merge_request).and_return(builder)
       end
@@ -512,8 +512,7 @@ describe Issuable do
         expect(builder).to receive(:build).with(
           user: user,
           changes: hash_including(
-            'assignee_id' => [user.id, user2.id],
-            'assignee' => [user.hook_attrs, user2.hook_attrs]
+            'assignees' => [[user.hook_attrs], [user.hook_attrs, user2.hook_attrs]]
           ))
 
         merge_request.to_hook_data(user, old_associations: { assignees: [user] })
diff --git a/spec/models/event_spec.rb b/spec/models/event_spec.rb
index d192fe70506..e91b5c4c86f 100644
--- a/spec/models/event_spec.rb
+++ b/spec/models/event_spec.rb
@@ -263,7 +263,7 @@ describe Event do
 
     context 'merge request diff note event' do
       let(:project) { create(:project, :public) }
-      let(:merge_request) { create(:merge_request, source_project: project, author: author, assignee: assignee) }
+      let(:merge_request) { create(:merge_request, source_project: project, author: author, assignees: [assignee]) }
       let(:note_on_merge_request) { create(:legacy_diff_note_on_merge_request, noteable: merge_request, project: project) }
       let(:target) { note_on_merge_request }
 
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index 6f34ef9c1bc..f61857ea5ff 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -13,7 +13,7 @@ describe MergeRequest do
     it { is_expected.to belong_to(:target_project).class_name('Project') }
     it { is_expected.to belong_to(:source_project).class_name('Project') }
     it { is_expected.to belong_to(:merge_user).class_name("User") }
-    it { is_expected.to belong_to(:assignee) }
+    it { is_expected.to have_many(:assignees).through(:merge_request_assignees) }
     it { is_expected.to have_many(:merge_request_diffs) }
 
     context 'for forks' do
@@ -181,31 +181,6 @@ describe MergeRequest do
         expect(MergeRequest::Metrics.count).to eq(1)
       end
     end
-
-    describe '#refresh_merge_request_assignees' do
-      set(:user) { create(:user) }
-
-      it 'creates merge request assignees relation upon MR creation' do
-        merge_request = create(:merge_request, assignee: nil)
-
-        expect(merge_request.merge_request_assignees).to be_empty
-
-        expect { merge_request.update!(assignee: user) }
-          .to change { merge_request.reload.merge_request_assignees.count }
-          .from(0).to(1)
-      end
-
-      it 'updates merge request assignees relation upon MR assignee change' do
-        another_user = create(:user)
-        merge_request = create(:merge_request, assignee: user)
-
-        expect { merge_request.update!(assignee: another_user) }
-          .to change { merge_request.reload.merge_request_assignees.first.assignee }
-          .from(user).to(another_user)
-
-        expect(merge_request.merge_request_assignees.count).to eq(1)
-      end
-    end
   end
 
   describe 'respond to' do
@@ -337,34 +312,18 @@ describe MergeRequest do
   describe '#card_attributes' do
     it 'includes the author name' do
       allow(subject).to receive(:author).and_return(double(name: 'Robert'))
-      allow(subject).to receive(:assignee).and_return(nil)
+      allow(subject).to receive(:assignees).and_return([])
 
       expect(subject.card_attributes)
-        .to eq({ 'Author' => 'Robert', 'Assignee' => nil })
+        .to eq({ 'Author' => 'Robert', 'Assignee' => "" })
     end
 
-    it 'includes the assignee name' do
+    it 'includes the assignees name' do
       allow(subject).to receive(:author).and_return(double(name: 'Robert'))
-      allow(subject).to receive(:assignee).and_return(double(name: 'Douwe'))
+      allow(subject).to receive(:assignees).and_return([double(name: 'Douwe'), double(name: 'Robert')])
 
       expect(subject.card_attributes)
-        .to eq({ 'Author' => 'Robert', 'Assignee' => 'Douwe' })
-    end
-  end
-
-  describe '#assignee_ids' do
-    it 'returns an array of the assigned user id' do
-      subject.assignee_id = 123
-
-      expect(subject.assignee_ids).to eq([123])
-    end
-  end
-
-  describe '#assignee_ids=' do
-    it 'sets assignee_id to the last id in the array' do
-      subject.assignee_ids = [123, 456]
-
-      expect(subject.assignee_id).to eq(456)
+        .to eq({ 'Author' => 'Robert', 'Assignee' => 'Douwe and Robert' })
     end
   end
 
@@ -372,7 +331,7 @@ describe MergeRequest do
     let(:user) { create(:user) }
 
     it 'returns true for a user that is assigned to a merge request' do
-      subject.assignee = user
+      subject.assignees = [user]
 
       expect(subject.assignee_or_author?(user)).to eq(true)
     end
@@ -1949,15 +1908,14 @@ describe MergeRequest do
     it 'updates when assignees change' do
       user1 = create(:user)
       user2 = create(:user)
-      mr = create(:merge_request, assignee: user1)
+      mr = create(:merge_request, assignees: [user1])
       mr.project.add_developer(user1)
       mr.project.add_developer(user2)
 
       expect(user1.assigned_open_merge_requests_count).to eq(1)
       expect(user2.assigned_open_merge_requests_count).to eq(0)
 
-      mr.assignee = user2
-      mr.save
+      mr.assignees = [user2]
 
       expect(user1.assigned_open_merge_requests_count).to eq(0)
       expect(user2.assigned_open_merge_requests_count).to eq(1)
diff --git a/spec/models/user_spec.rb b/spec/models/user_spec.rb
index a45a2737b13..d1338e34bb8 100644
--- a/spec/models/user_spec.rb
+++ b/spec/models/user_spec.rb
@@ -2816,9 +2816,9 @@ describe User do
       project = create(:project, :public)
       archived_project = create(:project, :public, :archived)
 
-      create(:merge_request, source_project: project, author: user, assignee: user)
-      create(:merge_request, :closed, source_project: project, author: user, assignee: user)
-      create(:merge_request, source_project: archived_project, author: user, assignee: user)
+      create(:merge_request, source_project: project, author: user, assignees: [user])
+      create(:merge_request, :closed, source_project: project, author: user, assignees: [user])
+      create(:merge_request, source_project: archived_project, author: user, assignees: [user])
 
       expect(user.assigned_open_merge_requests_count(force: true)).to eq 1
     end
diff --git a/spec/requests/api/events_spec.rb b/spec/requests/api/events_spec.rb
index 0ac23505de7..065b16c6221 100644
--- a/spec/requests/api/events_spec.rb
+++ b/spec/requests/api/events_spec.rb
@@ -270,8 +270,8 @@ describe API::Events do
     end
 
     context 'when exists some events' do
-      let(:merge_request1) { create(:merge_request, :closed, author: user, assignee: user, source_project: private_project, title: 'Test') }
-      let(:merge_request2) { create(:merge_request, :closed, author: user, assignee: user, source_project: private_project, title: 'Test') }
+      let(:merge_request1) { create(:merge_request, :closed, author: user, assignees: [user], source_project: private_project, title: 'Test') }
+      let(:merge_request2) { create(:merge_request, :closed, author: user, assignees: [user], source_project: private_project, title: 'Test') }
 
       before do
         create_event(merge_request1)
diff --git a/spec/requests/api/merge_requests_spec.rb b/spec/requests/api/merge_requests_spec.rb
index 7ffa365c651..45818edbf68 100644
--- a/spec/requests/api/merge_requests_spec.rb
+++ b/spec/requests/api/merge_requests_spec.rb
@@ -5,14 +5,15 @@ describe API::MergeRequests do
 
   let(:base_time)   { Time.now }
   set(:user)        { create(:user) }
+  set(:user2)       { create(:user) }
   set(:admin)       { create(:user, :admin) }
   let(:project)     { create(:project, :public, :repository, creator: user, namespace: user.namespace, only_allow_merge_if_pipeline_succeeds: false) }
   let(:milestone)   { create(:milestone, title: '1.0.0', project: project) }
   let(:milestone1)  { create(:milestone, title: '0.9', project: project) }
-  let!(:merge_request) { create(:merge_request, :simple, milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: "Test", created_at: base_time) }
-  let!(:merge_request_closed) { create(:merge_request, state: "closed", milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: "Closed test", created_at: base_time + 1.second) }
-  let!(:merge_request_merged) { create(:merge_request, state: "merged", author: user, assignee: user, source_project: project, target_project: project, title: "Merged test", created_at: base_time + 2.seconds, merge_commit_sha: '9999999999999999999999999999999999999999') }
-  let!(:merge_request_locked) { create(:merge_request, state: "locked", milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: "Locked test", created_at: base_time + 1.second) }
+  let!(:merge_request) { create(:merge_request, :simple, milestone: milestone1, author: user, assignees: [user], source_project: project, target_project: project, title: "Test", created_at: base_time) }
+  let!(:merge_request_closed) { create(:merge_request, state: "closed", milestone: milestone1, author: user, assignees: [user], source_project: project, target_project: project, title: "Closed test", created_at: base_time + 1.second) }
+  let!(:merge_request_merged) { create(:merge_request, state: "merged", author: user, assignees: [user], source_project: project, target_project: project, title: "Merged test", created_at: base_time + 2.seconds, merge_commit_sha: '9999999999999999999999999999999999999999') }
+  let!(:merge_request_locked) { create(:merge_request, state: "locked", milestone: milestone1, author: user, assignees: [user], source_project: project, target_project: project, title: "Locked test", created_at: base_time + 1.second) }
   let!(:note)       { create(:note_on_merge_request, author: user, project: project, noteable: merge_request, note: "a comment on a MR") }
   let!(:note2)      { create(:note_on_merge_request, author: user, project: project, noteable: merge_request, note: "another comment on a MR") }
   let(:label) { create(:label, title: 'label', color: '#FFAABB', project: project) }
@@ -20,6 +21,9 @@ describe API::MergeRequests do
 
   before do
     project.add_reporter(user)
+    project.add_reporter(user2)
+
+    stub_licensed_features(multiple_merge_request_assignees: false)
   end
 
   shared_context 'with labels' do
@@ -45,9 +49,9 @@ describe API::MergeRequests do
           get api(endpoint_path, user)
         end
 
-        create(:merge_request, state: 'closed', milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: 'Test', created_at: base_time)
+        create(:merge_request, state: 'closed', milestone: milestone1, author: user, assignees: [user], source_project: project, target_project: project, title: 'Test', created_at: base_time)
 
-        merge_request = create(:merge_request, milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: 'Test', created_at: base_time)
+        merge_request = create(:merge_request, milestone: milestone1, author: user, assignees: [user], source_project: project, target_project: project, title: 'Test', created_at: base_time)
 
         merge_request.metrics.update!(merged_by: user,
                                       latest_closed_by: user,
@@ -333,7 +337,7 @@ describe API::MergeRequests do
                    state: 'closed',
                    milestone: milestone1,
                    author: user,
-                   assignee: user,
+                   assignees: [user],
                    source_project: project,
                    target_project: project,
                    title: "Test",
@@ -451,7 +455,7 @@ describe API::MergeRequests do
 
     context 'when authenticated' do
       let!(:project2) { create(:project, :public, namespace: user.namespace) }
-      let!(:merge_request2) { create(:merge_request, :simple, author: user, assignee: user, source_project: project2, target_project: project2) }
+      let!(:merge_request2) { create(:merge_request, :simple, author: user, assignees: [user], source_project: project2, target_project: project2) }
       let(:user2) { create(:user) }
 
       it 'returns an array of all merge requests except unauthorized ones' do
@@ -494,7 +498,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests created by current user if no scope is given' do
-        merge_request3 = create(:merge_request, :simple, author: user2, assignee: user, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user2, assignees: [user], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user2)
 
@@ -502,7 +506,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests authored by the given user' do
-        merge_request3 = create(:merge_request, :simple, author: user2, assignee: user, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user2, assignees: [user], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user), params: { author_id: user2.id, scope: :all }
 
@@ -510,7 +514,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests assigned to the given user' do
-        merge_request3 = create(:merge_request, :simple, author: user, assignee: user2, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user, assignees: [user2], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user), params: { assignee_id: user2.id, scope: :all }
 
@@ -535,7 +539,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests assigned to me' do
-        merge_request3 = create(:merge_request, :simple, author: user, assignee: user2, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user, assignees: [user2], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user2), params: { scope: 'assigned_to_me' }
 
@@ -543,7 +547,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests assigned to me (kebab-case)' do
-        merge_request3 = create(:merge_request, :simple, author: user, assignee: user2, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user, assignees: [user2], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user2), params: { scope: 'assigned-to-me' }
 
@@ -551,7 +555,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests created by me' do
-        merge_request3 = create(:merge_request, :simple, author: user2, assignee: user, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user2, assignees: [user], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user2), params: { scope: 'created_by_me' }
 
@@ -559,7 +563,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests created by me (kebab-case)' do
-        merge_request3 = create(:merge_request, :simple, author: user2, assignee: user, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user2, assignees: [user], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user2), params: { scope: 'created-by-me' }
 
@@ -567,7 +571,7 @@ describe API::MergeRequests do
       end
 
       it 'returns merge requests reacted by the authenticated user by the given emoji' do
-        merge_request3 = create(:merge_request, :simple, author: user, assignee: user, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user, assignees: [user], source_project: project2, target_project: project2, source_branch: 'other-branch')
         award_emoji = create(:award_emoji, awardable: merge_request3, user: user2, name: 'star')
 
         get api('/merge_requests', user2), params: { my_reaction_emoji: award_emoji.name, scope: 'all' }
@@ -700,7 +704,7 @@ describe API::MergeRequests do
         get api("/projects/#{project.id}/merge_requests", user)
       end.count
 
-      create(:merge_request, author: user, assignee: user, source_project: project, target_project: project, created_at: base_time)
+      create(:merge_request, author: user, assignees: [user], source_project: project, target_project: project, created_at: base_time)
 
       expect do
         get api("/projects/#{project.id}/merge_requests", user)
@@ -730,7 +734,7 @@ describe API::MergeRequests do
 
   describe "GET /projects/:id/merge_requests/:merge_request_iid" do
     it 'matches json schema' do
-      merge_request = create(:merge_request, :with_test_reports, milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: "Test", created_at: base_time)
+      merge_request = create(:merge_request, :with_test_reports, milestone: milestone1, author: user, assignees: [user], source_project: project, target_project: project, title: "Test", created_at: base_time)
       get api("/projects/#{project.id}/merge_requests/#{merge_request.iid}", user)
 
       expect(response).to have_gitlab_http_status(200)
@@ -851,7 +855,7 @@ describe API::MergeRequests do
     end
 
     context 'Work in Progress' do
-      let!(:merge_request_wip) { create(:merge_request, author: user, assignee: user, source_project: project, target_project: project, title: "WIP: Test", created_at: base_time + 1.second) }
+      let!(:merge_request_wip) { create(:merge_request, author: user, assignees: [user], source_project: project, target_project: project, title: "WIP: Test", created_at: base_time + 1.second) }
 
       it "returns merge request" do
         get api("/projects/#{project.id}/merge_requests/#{merge_request_wip.iid}", user)
@@ -867,7 +871,7 @@ describe API::MergeRequests do
 
         merge_request_overflow = create(:merge_request, :simple,
                                         author: user,
-                                        assignee: user,
+                                        assignees: [user],
                                         source_project: project,
                                         source_branch: 'expand-collapse-files',
                                         target_project: project,
@@ -1005,6 +1009,71 @@ describe API::MergeRequests do
   end
 
   describe 'POST /projects/:id/merge_requests' do
+    context 'support for deprecated assignee_id' do
+      let(:params) do
+        {
+          title: 'Test merge request',
+          source_branch: 'feature_conflict',
+          target_branch: 'master',
+          author_id: user.id,
+          assignee_id: user2.id
+        }
+      end
+
+      it 'creates a new merge request' do
+        post api("/projects/#{project.id}/merge_requests", user), params: params
+
+        expect(response).to have_gitlab_http_status(201)
+        expect(json_response['title']).to eq('Test merge request')
+        expect(json_response['assignee']['name']).to eq(user2.name)
+        expect(json_response['assignees'].first['name']).to eq(user2.name)
+      end
+
+      it 'creates a new merge request when assignee_id is empty' do
+        params[:assignee_id] = ''
+
+        post api("/projects/#{project.id}/merge_requests", user), params: params
+
+        expect(response).to have_gitlab_http_status(201)
+        expect(json_response['title']).to eq('Test merge request')
+        expect(json_response['assignee']).to be_nil
+      end
+
+      it 'filters assignee_id of unauthorized user' do
+        private_project = create(:project, :private, :repository)
+        another_user = create(:user)
+        private_project.add_maintainer(user)
+        params[:assignee_id] = another_user.id
+
+        post api("/projects/#{private_project.id}/merge_requests", user), params: params
+
+        expect(response).to have_gitlab_http_status(201)
+        expect(json_response['assignee']).to be_nil
+      end
+    end
+
+    context 'single assignee restrictions' do
+      let(:params) do
+        {
+          title: 'Test merge request',
+          source_branch: 'feature_conflict',
+          target_branch: 'master',
+          author_id: user.id,
+          assignee_ids: [user.id, user2.id]
+        }
+      end
+
+      it 'creates a new project merge request with no more than one assignee' do
+        post api("/projects/#{project.id}/merge_requests", user), params: params
+
+        expect(response).to have_gitlab_http_status(201)
+        expect(json_response['title']).to eq('Test merge request')
+        expect(json_response['assignees'].count).to eq(1)
+        expect(json_response['assignees'].first['name']).to eq(user.name)
+        expect(json_response.dig('assignee', 'name')).to eq(user.name)
+      end
+    end
+
     context 'between branches projects' do
       context 'different labels' do
         let(:params) do
@@ -1574,6 +1643,19 @@ describe API::MergeRequests do
       expect(json_response['force_remove_source_branch']).to be_truthy
     end
 
+    it 'filters assignee_id of unauthorized user' do
+      private_project = create(:project, :private, :repository)
+      mr = create(:merge_request, source_project: private_project, target_project: private_project)
+      another_user = create(:user)
+      private_project.add_maintainer(user)
+      params = { assignee_id: another_user.id }
+
+      put api("/projects/#{private_project.id}/merge_requests/#{mr.iid}", user), params: params
+
+      expect(response).to have_gitlab_http_status(200)
+      expect(json_response['assignee']).to be_nil
+    end
+
     context 'when updating labels' do
       it 'allows special label names' do
         put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}", user),
@@ -1728,7 +1810,7 @@ describe API::MergeRequests do
       issue = create(:issue, project: jira_project)
       description = "Closes #{ext_issue.to_reference(jira_project)}\ncloses #{issue.to_reference}"
       merge_request = create(:merge_request,
-        :simple, author: user, assignee: user, source_project: jira_project, description: description)
+        :simple, author: user, assignees: [user], source_project: jira_project, description: description)
 
       get api("/projects/#{jira_project.id}/merge_requests/#{merge_request.iid}/closes_issues", user)
 
diff --git a/spec/services/issuable/bulk_update_service_spec.rb b/spec/services/issuable/bulk_update_service_spec.rb
index ca366cdf1df..363b7266940 100644
--- a/spec/services/issuable/bulk_update_service_spec.rb
+++ b/spec/services/issuable/bulk_update_service_spec.rb
@@ -76,14 +76,14 @@ describe Issuable::BulkUpdateService do
   end
 
   describe 'updating merge request assignee' do
-    let(:merge_request) { create(:merge_request, target_project: project, source_project: project, assignee: user) }
+    let(:merge_request) { create(:merge_request, target_project: project, source_project: project, assignees: [user]) }
 
     context 'when the new assignee ID is a valid user' do
       it 'succeeds' do
         new_assignee = create(:user)
         project.add_developer(new_assignee)
 
-        result = bulk_update(merge_request, assignee_id: new_assignee.id)
+        result = bulk_update(merge_request, assignee_ids: [user.id, new_assignee.id])
 
         expect(result[:success]).to be_truthy
         expect(result[:count]).to eq(1)
@@ -93,22 +93,22 @@ describe Issuable::BulkUpdateService do
         assignee = create(:user)
         project.add_developer(assignee)
 
-        expect { bulk_update(merge_request, assignee_id: assignee.id) }
-          .to change { merge_request.reload.assignee }.from(user).to(assignee)
+        expect { bulk_update(merge_request, assignee_ids: [assignee.id]) }
+          .to change { merge_request.reload.assignee_ids }.from([user.id]).to([assignee.id])
       end
     end
 
     context "when the new assignee ID is #{IssuableFinder::NONE}" do
       it 'unassigns the issues' do
-        expect { bulk_update(merge_request, assignee_id: IssuableFinder::NONE) }
-          .to change { merge_request.reload.assignee }.to(nil)
+        expect { bulk_update(merge_request, assignee_ids: [IssuableFinder::NONE]) }
+          .to change { merge_request.reload.assignee_ids }.to([])
       end
     end
 
     context 'when the new assignee ID is not present' do
       it 'does not unassign' do
-        expect { bulk_update(merge_request, assignee_id: nil) }
-          .not_to change { merge_request.reload.assignee }
+        expect { bulk_update(merge_request, assignee_ids: []) }
+          .not_to change { merge_request.reload.assignee_ids }
       end
     end
   end
diff --git a/spec/services/issuable/destroy_service_spec.rb b/spec/services/issuable/destroy_service_spec.rb
index 8ccbba7fa58..15d1bb73ca3 100644
--- a/spec/services/issuable/destroy_service_spec.rb
+++ b/spec/services/issuable/destroy_service_spec.rb
@@ -34,7 +34,7 @@ describe Issuable::DestroyService do
     end
 
     context 'when issuable is a merge request' do
-      let!(:merge_request) { create(:merge_request, target_project: project, source_project: project, author: user, assignee: user) }
+      let!(:merge_request) { create(:merge_request, target_project: project, source_project: project, author: user, assignees: [user]) }
 
       it 'destroys the merge request' do
         expect { service.execute(merge_request) }.to change { project.merge_requests.count }.by(-1)
diff --git a/spec/services/members/destroy_service_spec.rb b/spec/services/members/destroy_service_spec.rb
index d37ca13ebd2..91bf4dccd77 100644
--- a/spec/services/members/destroy_service_spec.rb
+++ b/spec/services/members/destroy_service_spec.rb
@@ -43,9 +43,9 @@ describe Members::DestroyService do
   shared_examples 'a service destroying a member with access' do
     it_behaves_like 'a service destroying a member'
 
-    it 'invalidates cached counts for todos and assigned issues and merge requests', :aggregate_failures do
+    it 'invalidates cached counts for assigned issues and merge requests', :aggregate_failures do
       create(:issue, project: group_project, assignees: [member_user])
-      create(:merge_request, source_project: group_project, assignee: member_user)
+      create(:merge_request, source_project: group_project, assignees: [member_user])
       create(:todo, :pending, project: group_project, user: member_user)
       create(:todo, :done, project: group_project, user: member_user)
 
diff --git a/spec/services/merge_requests/close_service_spec.rb b/spec/services/merge_requests/close_service_spec.rb
index 433ffbd97f0..706bcea8199 100644
--- a/spec/services/merge_requests/close_service_spec.rb
+++ b/spec/services/merge_requests/close_service_spec.rb
@@ -4,7 +4,7 @@ describe MergeRequests::CloseService do
   let(:user) { create(:user) }
   let(:user2) { create(:user) }
   let(:guest) { create(:user) }
-  let(:merge_request) { create(:merge_request, assignee: user2, author: create(:user)) }
+  let(:merge_request) { create(:merge_request, assignees: [user2], author: create(:user)) }
   let(:project) { merge_request.project }
   let!(:todo) { create(:todo, :assigned, user: user, project: project, target: merge_request, author: user2) }
 
diff --git a/spec/services/merge_requests/create_from_issue_service_spec.rb b/spec/services/merge_requests/create_from_issue_service_spec.rb
index 393299cce00..20bf1cbb8b6 100644
--- a/spec/services/merge_requests/create_from_issue_service_spec.rb
+++ b/spec/services/merge_requests/create_from_issue_service_spec.rb
@@ -118,7 +118,7 @@ describe MergeRequests::CreateFromIssueService do
 
       result = service.execute
 
-      expect(result[:merge_request].assignee).to eq(user)
+      expect(result[:merge_request].assignees).to eq([user])
     end
 
     context 'when ref branch is set' do
diff --git a/spec/services/merge_requests/create_service_spec.rb b/spec/services/merge_requests/create_service_spec.rb
index dc5d1cf2f04..30271e04c8e 100644
--- a/spec/services/merge_requests/create_service_spec.rb
+++ b/spec/services/merge_requests/create_service_spec.rb
@@ -32,7 +32,7 @@ describe MergeRequests::CreateService do
         expect(merge_request).to be_valid
         expect(merge_request.work_in_progress?).to be(false)
         expect(merge_request.title).to eq('Awesome merge_request')
-        expect(merge_request.assignee).to be_nil
+        expect(merge_request.assignees).to be_empty
         expect(merge_request.merge_params['force_remove_source_branch']).to eq('1')
       end
 
@@ -73,7 +73,7 @@ describe MergeRequests::CreateService do
               description: "well this is not done yet\n/wip",
               source_branch: 'feature',
               target_branch: 'master',
-              assignee: assignee
+              assignees: [assignee]
             }
           end
 
@@ -89,7 +89,7 @@ describe MergeRequests::CreateService do
               description: "well this is not done yet\n/wip",
               source_branch: 'feature',
               target_branch: 'master',
-              assignee: assignee
+              assignees: [assignee]
             }
           end
 
@@ -106,11 +106,11 @@ describe MergeRequests::CreateService do
             description: 'please fix',
             source_branch: 'feature',
             target_branch: 'master',
-            assignee: assignee
+            assignees: [assignee]
           }
         end
 
-        it { expect(merge_request.assignee).to eq assignee }
+        it { expect(merge_request.assignees).to eq([assignee]) }
 
         it 'creates a todo for new assignee' do
           attributes = {
@@ -301,7 +301,7 @@ describe MergeRequests::CreateService do
 
         let(:opts) do
           {
-            assignee_id: create(:user).id,
+            assignee_ids: create(:user).id,
             milestone_id: 1,
             title: 'Title',
             description: %(/assign @#{assignee.username}\n/milestone %"#{milestone.name}"),
@@ -317,7 +317,7 @@ describe MergeRequests::CreateService do
 
         it 'assigns and sets milestone to issuable from command' do
           expect(merge_request).to be_persisted
-          expect(merge_request.assignee).to eq(assignee)
+          expect(merge_request.assignees).to eq([assignee])
           expect(merge_request.milestone).to eq(milestone)
         end
       end
@@ -332,28 +332,28 @@ describe MergeRequests::CreateService do
         end
 
         it 'removes assignee_id when user id is invalid' do
-          opts = { title: 'Title', description: 'Description', assignee_id: -1 }
+          opts = { title: 'Title', description: 'Description', assignee_ids: [-1] }
 
           merge_request = described_class.new(project, user, opts).execute
 
-          expect(merge_request.assignee_id).to be_nil
+          expect(merge_request.assignee_ids).to be_empty
         end
 
         it 'removes assignee_id when user id is 0' do
-          opts = { title: 'Title', description: 'Description', assignee_id: 0 }
+          opts = { title: 'Title', description: 'Description', assignee_ids: [0] }
 
           merge_request = described_class.new(project, user, opts).execute
 
-          expect(merge_request.assignee_id).to be_nil
+          expect(merge_request.assignee_ids).to be_empty
         end
 
         it 'saves assignee when user id is valid' do
           project.add_maintainer(assignee)
-          opts = { title: 'Title', description: 'Description', assignee_id: assignee.id }
+          opts = { title: 'Title', description: 'Description', assignee_ids: [assignee.id] }
 
           merge_request = described_class.new(project, user, opts).execute
 
-          expect(merge_request.assignee).to eq(assignee)
+          expect(merge_request.assignees).to eq([assignee])
         end
 
         context 'when assignee is set' do
@@ -361,7 +361,7 @@ describe MergeRequests::CreateService do
             {
               title: 'Title',
               description: 'Description',
-              assignee_id: assignee.id,
+              assignee_ids: [assignee.id],
               source_branch: 'feature',
               target_branch: 'master'
             }
@@ -387,7 +387,7 @@ describe MergeRequests::CreateService do
           levels.each do |level|
             it "removes not authorized assignee when project is #{Gitlab::VisibilityLevel.level_name(level)}" do
               project.update(visibility_level: level)
-              opts = { title: 'Title', description: 'Description', assignee_id: assignee.id }
+              opts = { title: 'Title', description: 'Description', assignee_ids: [assignee.id] }
 
               merge_request = described_class.new(project, user, opts).execute
 
diff --git a/spec/services/merge_requests/ff_merge_service_spec.rb b/spec/services/merge_requests/ff_merge_service_spec.rb
index 1430e12a07e..a87d8b8752c 100644
--- a/spec/services/merge_requests/ff_merge_service_spec.rb
+++ b/spec/services/merge_requests/ff_merge_service_spec.rb
@@ -7,7 +7,7 @@ describe MergeRequests::FfMergeService do
     create(:merge_request,
            source_branch: 'flatten-dir',
            target_branch: 'improve/awesome',
-           assignee: user2,
+           assignees: [user2],
            author: create(:user))
   end
   let(:project) { merge_request.project }
diff --git a/spec/services/merge_requests/merge_service_spec.rb b/spec/services/merge_requests/merge_service_spec.rb
index 887ec17171e..b0b3273e3dc 100644
--- a/spec/services/merge_requests/merge_service_spec.rb
+++ b/spec/services/merge_requests/merge_service_spec.rb
@@ -3,7 +3,7 @@ require 'spec_helper'
 describe MergeRequests::MergeService do
   set(:user) { create(:user) }
   set(:user2) { create(:user) }
-  let(:merge_request) { create(:merge_request, :simple, author: user2, assignee: user2) }
+  let(:merge_request) { create(:merge_request, :simple, author: user2, assignees: [user2]) }
   let(:project) { merge_request.project }
 
   before do
@@ -111,7 +111,7 @@ describe MergeRequests::MergeService do
     end
 
     context 'closes related todos' do
-      let(:merge_request) { create(:merge_request, assignee: user, author: user) }
+      let(:merge_request) { create(:merge_request, assignees: [user], author: user) }
       let(:project) { merge_request.project }
       let(:service) { described_class.new(project, user, commit_message: 'Awesome message') }
       let!(:todo) do
diff --git a/spec/services/merge_requests/merge_to_ref_service_spec.rb b/spec/services/merge_requests/merge_to_ref_service_spec.rb
index a3b48abae26..24d09c1fd00 100644
--- a/spec/services/merge_requests/merge_to_ref_service_spec.rb
+++ b/spec/services/merge_requests/merge_to_ref_service_spec.rb
@@ -149,7 +149,7 @@ describe MergeRequests::MergeToRefService do
     end
 
     context 'does not close related todos' do
-      let(:merge_request) { create(:merge_request, assignee: user, author: user) }
+      let(:merge_request) { create(:merge_request, assignees: [user], author: user) }
       let(:project) { merge_request.project }
       let!(:todo) do
         create(:todo, :assigned,
diff --git a/spec/services/merge_requests/post_merge_service_spec.rb b/spec/services/merge_requests/post_merge_service_spec.rb
index 5ad6f5528f9..2cebefee5d6 100644
--- a/spec/services/merge_requests/post_merge_service_spec.rb
+++ b/spec/services/merge_requests/post_merge_service_spec.rb
@@ -2,7 +2,7 @@ require 'spec_helper'
 
 describe MergeRequests::PostMergeService do
   let(:user) { create(:user) }
-  let(:merge_request) { create(:merge_request, assignee: user) }
+  let(:merge_request) { create(:merge_request, assignees: [user]) }
   let(:project) { merge_request.project }
 
   before do
diff --git a/spec/services/merge_requests/reopen_service_spec.rb b/spec/services/merge_requests/reopen_service_spec.rb
index 21e71509ed6..8b6db1ce33e 100644
--- a/spec/services/merge_requests/reopen_service_spec.rb
+++ b/spec/services/merge_requests/reopen_service_spec.rb
@@ -4,7 +4,7 @@ describe MergeRequests::ReopenService do
   let(:user) { create(:user) }
   let(:user2) { create(:user) }
   let(:guest) { create(:user) }
-  let(:merge_request) { create(:merge_request, :closed, assignee: user2, author: create(:user)) }
+  let(:merge_request) { create(:merge_request, :closed, assignees: [user2], author: create(:user)) }
   let(:project) { merge_request.project }
 
   before do
diff --git a/spec/services/merge_requests/update_service_spec.rb b/spec/services/merge_requests/update_service_spec.rb
index 8e367db031c..0525899ebfa 100644
--- a/spec/services/merge_requests/update_service_spec.rb
+++ b/spec/services/merge_requests/update_service_spec.rb
@@ -13,7 +13,7 @@ describe MergeRequests::UpdateService, :mailer do
   let(:merge_request) do
     create(:merge_request, :simple, title: 'Old title',
                                     description: "FYI #{user2.to_reference}",
-                                    assignee_id: user3.id,
+                                    assignee_ids: [user3.id],
                                     source_project: project,
                                     author: create(:user))
   end
@@ -48,7 +48,7 @@ describe MergeRequests::UpdateService, :mailer do
         {
           title: 'New title',
           description: 'Also please fix',
-          assignee_id: user2.id,
+          assignee_ids: [user.id],
           state_event: 'close',
           label_ids: [label.id],
           target_branch: 'target',
@@ -71,7 +71,7 @@ describe MergeRequests::UpdateService, :mailer do
       it 'matches base expectations' do
         expect(@merge_request).to be_valid
         expect(@merge_request.title).to eq('New title')
-        expect(@merge_request.assignee).to eq(user2)
+        expect(@merge_request.assignees).to match_array([user])
         expect(@merge_request).to be_closed
         expect(@merge_request.labels.count).to eq(1)
         expect(@merge_request.labels.first.title).to eq(label.name)
@@ -106,7 +106,7 @@ describe MergeRequests::UpdateService, :mailer do
         note = find_note('assigned to')
 
         expect(note).not_to be_nil
-        expect(note.note).to include "assigned to #{user2.to_reference}"
+        expect(note.note).to include "assigned to #{user.to_reference} and unassigned #{user3.to_reference}"
       end
 
       it 'creates a resource label event' do
@@ -293,7 +293,7 @@ describe MergeRequests::UpdateService, :mailer do
 
       context 'when is reassigned' do
         before do
-          update_merge_request({ assignee: user2 })
+          update_merge_request({ assignee_ids: [user2.id] })
         end
 
         it 'marks previous assignee pending todos as done' do
@@ -387,7 +387,7 @@ describe MergeRequests::UpdateService, :mailer do
 
       context 'when the assignee changes' do
         it 'updates open merge request counter for assignees when merge request is reassigned' do
-          update_merge_request(assignee_id: user2.id)
+          update_merge_request(assignee_ids: [user2.id])
 
           expect(user3.assigned_open_merge_requests_count).to eq 0
           expect(user2.assigned_open_merge_requests_count).to eq 1
@@ -541,36 +541,36 @@ describe MergeRequests::UpdateService, :mailer do
       end
     end
 
-    context 'updating asssignee_id' do
+    context 'updating asssignee_ids' do
       it 'does not update assignee when assignee_id is invalid' do
-        merge_request.update(assignee_id: user.id)
+        merge_request.update(assignee_ids: [user.id])
 
-        update_merge_request(assignee_id: -1)
+        update_merge_request(assignee_ids: [-1])
 
-        expect(merge_request.reload.assignee).to eq(user)
+        expect(merge_request.reload.assignees).to eq([user])
       end
 
       it 'unassigns assignee when user id is 0' do
-        merge_request.update(assignee_id: user.id)
+        merge_request.update(assignee_ids: [user.id])
 
-        update_merge_request(assignee_id: 0)
+        update_merge_request(assignee_ids: [0])
 
-        expect(merge_request.assignee_id).to be_nil
+        expect(merge_request.assignee_ids).to be_empty
       end
 
       it 'saves assignee when user id is valid' do
-        update_merge_request(assignee_id: user.id)
+        update_merge_request(assignee_ids: [user.id])
 
-        expect(merge_request.assignee_id).to eq(user.id)
+        expect(merge_request.assignee_ids).to eq([user.id])
       end
 
       it 'does not update assignee_id when user cannot read issue' do
-        non_member        = create(:user)
-        original_assignee = merge_request.assignee
+        non_member = create(:user)
+        original_assignees = merge_request.assignees
 
-        update_merge_request(assignee_id: non_member.id)
+        update_merge_request(assignee_ids: [non_member.id])
 
-        expect(merge_request.assignee_id).to eq(original_assignee.id)
+        expect(merge_request.reload.assignees).to eq(original_assignees)
       end
 
       context "when issuable feature is private" do
@@ -583,7 +583,7 @@ describe MergeRequests::UpdateService, :mailer do
             feature_visibility_attr = :"#{merge_request.model_name.plural}_access_level"
             project.project_feature.update_attribute(feature_visibility_attr, ProjectFeature::PRIVATE)
 
-            expect { update_merge_request(assignee_id: assignee) }.not_to change { merge_request.assignee }
+            expect { update_merge_request(assignee_ids: [assignee]) }.not_to change { merge_request.reload.assignees }
           end
         end
       end
@@ -619,7 +619,7 @@ describe MergeRequests::UpdateService, :mailer do
       end
 
       it 'is allowed by a user that can push to the source and can update the merge request' do
-        merge_request.update!(assignee: user)
+        merge_request.update!(assignees: [user])
         source_project.add_developer(user)
 
         update_merge_request(allow_collaboration: true, title: 'Updated title')
diff --git a/spec/services/notification_service_spec.rb b/spec/services/notification_service_spec.rb
index 9ba4a11104a..14c73852e65 100644
--- a/spec/services/notification_service_spec.rb
+++ b/spec/services/notification_service_spec.rb
@@ -125,11 +125,7 @@ describe NotificationService, :mailer do
 
   shared_examples 'participating by assignee notification' do
     it 'emails the participant' do
-      if issuable.is_a?(Issue)
-        issuable.assignees << participant
-      else
-        issuable.update_attribute(:assignee, participant)
-      end
+      issuable.assignees << participant
 
       notification_trigger
 
@@ -620,13 +616,13 @@ describe NotificationService, :mailer do
     context "merge request diff note" do
       let(:project) { create(:project, :repository) }
       let(:user) { create(:user) }
-      let(:merge_request) { create(:merge_request, source_project: project, assignee: user, author: create(:user)) }
+      let(:merge_request) { create(:merge_request, source_project: project, assignees: [user], author: create(:user)) }
       let(:note) { create(:diff_note_on_merge_request, project: project, noteable: merge_request) }
 
       before do
         build_team(note.project)
         project.add_maintainer(merge_request.author)
-        project.add_maintainer(merge_request.assignee)
+        merge_request.assignees.each { |assignee| project.add_maintainer(assignee) }
       end
 
       describe '#new_note' do
@@ -637,7 +633,7 @@ describe NotificationService, :mailer do
           notification.new_note(note)
 
           expect(SentNotification.last(3).map(&:recipient).map(&:id))
-            .to contain_exactly(merge_request.assignee.id, merge_request.author.id, @u_watcher.id)
+            .to contain_exactly(*merge_request.assignees.pluck(:id), merge_request.author.id, @u_watcher.id)
           expect(SentNotification.last.in_reply_to_discussion_id).to eq(note.discussion_id)
         end
       end
@@ -1223,11 +1219,12 @@ describe NotificationService, :mailer do
     let(:group) { create(:group) }
     let(:project) { create(:project, :public, :repository, namespace: group) }
     let(:another_project) { create(:project, :public, namespace: group) }
-    let(:merge_request) { create :merge_request, source_project: project, assignee: create(:user), description: 'cc @participant' }
+    let(:assignee) { create(:user) }
+    let(:merge_request) { create :merge_request, source_project: project, assignees: [assignee], description: 'cc @participant' }
 
     before do
       project.add_maintainer(merge_request.author)
-      project.add_maintainer(merge_request.assignee)
+      merge_request.assignees.each { |assignee| project.add_maintainer(assignee) }
       build_team(merge_request.target_project)
       add_users_with_subscription(merge_request.target_project, merge_request)
       update_custom_notification(:new_merge_request, @u_guest_custom, resource: project)
@@ -1239,7 +1236,7 @@ describe NotificationService, :mailer do
       it do
         notification.new_merge_request(merge_request, @u_disabled)
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(@u_watcher)
         should_email(@watcher_and_subscriber)
         should_email(@u_participant_mentioned)
@@ -1254,9 +1251,11 @@ describe NotificationService, :mailer do
       it 'adds "assigned" reason for assignee, if any' do
         notification.new_merge_request(merge_request, @u_disabled)
 
-        email = find_email_for(merge_request.assignee)
+        merge_request.assignees.each do |assignee|
+          email = find_email_for(assignee)
 
-        expect(email).to have_header('X-GitLab-NotificationReason', NotificationReason::ASSIGNED)
+          expect(email).to have_header('X-GitLab-NotificationReason', NotificationReason::ASSIGNED)
+        end
       end
 
       it "emails any mentioned users with the mention level" do
@@ -1347,9 +1346,9 @@ describe NotificationService, :mailer do
       end
 
       it do
-        notification.reassigned_merge_request(merge_request, current_user, merge_request.author)
+        notification.reassigned_merge_request(merge_request, current_user, [assignee])
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(merge_request.author)
         should_email(@u_watcher)
         should_email(@u_participant_mentioned)
@@ -1365,17 +1364,19 @@ describe NotificationService, :mailer do
       end
 
       it 'adds "assigned" reason for new assignee' do
-        notification.reassigned_merge_request(merge_request, current_user, merge_request.author)
+        notification.reassigned_merge_request(merge_request, current_user, [assignee])
 
-        email = find_email_for(merge_request.assignee)
+        merge_request.assignees.each do |assignee|
+          email = find_email_for(assignee)
 
-        expect(email).to have_header('X-GitLab-NotificationReason', NotificationReason::ASSIGNED)
+          expect(email).to have_header('X-GitLab-NotificationReason', NotificationReason::ASSIGNED)
+        end
       end
 
       it_behaves_like 'participating notifications' do
         let(:participant) { create(:user, username: 'user-participant') }
         let(:issuable) { merge_request }
-        let(:notification_trigger) { notification.reassigned_merge_request(merge_request, current_user, merge_request.author) }
+        let(:notification_trigger) { notification.reassigned_merge_request(merge_request, current_user, [assignee]) }
       end
     end
 
@@ -1388,7 +1389,7 @@ describe NotificationService, :mailer do
       it do
         notification.push_to_merge_request(merge_request, @u_disabled)
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(@u_guest_custom)
         should_email(@u_custom_global)
         should_email(@u_participant_mentioned)
@@ -1430,7 +1431,7 @@ describe NotificationService, :mailer do
         should_email(subscriber_1_to_group_label_2)
         should_email(subscriber_2_to_group_label_2)
         should_email(subscriber_to_label_2)
-        should_not_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_not_email(assignee) }
         should_not_email(merge_request.author)
         should_not_email(@u_watcher)
         should_not_email(@u_participant_mentioned)
@@ -1499,7 +1500,7 @@ describe NotificationService, :mailer do
       it do
         notification.close_mr(merge_request, @u_disabled)
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(@u_watcher)
         should_email(@u_guest_watcher)
         should_email(@u_guest_custom)
@@ -1529,7 +1530,7 @@ describe NotificationService, :mailer do
       it do
         notification.merge_mr(merge_request, @u_disabled)
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(@u_watcher)
         should_email(@u_guest_watcher)
         should_email(@u_guest_custom)
@@ -1581,7 +1582,7 @@ describe NotificationService, :mailer do
       it do
         notification.reopen_mr(merge_request, @u_disabled)
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(@u_watcher)
         should_email(@u_participant_mentioned)
         should_email(@subscriber)
@@ -1606,7 +1607,7 @@ describe NotificationService, :mailer do
       it do
         notification.resolve_all_discussions(merge_request, @u_disabled)
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(@u_watcher)
         should_email(@u_participant_mentioned)
         should_email(@subscriber)
@@ -1850,8 +1851,8 @@ describe NotificationService, :mailer do
     let(:guest) { create(:user) }
     let(:developer) { create(:user) }
     let(:assignee) { create(:user) }
-    let(:merge_request) { create(:merge_request, source_project: private_project, assignee: assignee) }
-    let(:merge_request1) { create(:merge_request, source_project: private_project, assignee: assignee, description: "cc @#{guest.username}") }
+    let(:merge_request) { create(:merge_request, source_project: private_project, assignees: [assignee]) }
+    let(:merge_request1) { create(:merge_request, source_project: private_project, assignees: [assignee], description: "cc @#{guest.username}") }
     let(:note) { create(:note, noteable: merge_request, project: private_project) }
 
     before do
diff --git a/spec/services/quick_actions/interpret_service_spec.rb b/spec/services/quick_actions/interpret_service_spec.rb
index c7e5cca324f..c450f89c3cb 100644
--- a/spec/services/quick_actions/interpret_service_spec.rb
+++ b/spec/services/quick_actions/interpret_service_spec.rb
@@ -16,7 +16,9 @@ describe QuickActions::InterpretService do
   let(:service) { described_class.new(project, developer) }
 
   before do
-    stub_licensed_features(multiple_issue_assignees: false)
+    stub_licensed_features(multiple_issue_assignees: false,
+                           multiple_merge_request_assignees: false)
+
     project.add_developer(developer)
   end
 
diff --git a/spec/services/system_note_service_spec.rb b/spec/services/system_note_service_spec.rb
index 13d7d795703..51c5a803dbd 100644
--- a/spec/services/system_note_service_spec.rb
+++ b/spec/services/system_note_service_spec.rb
@@ -166,8 +166,8 @@ describe SystemNoteService do
     end
   end
 
-  describe '.change_issue_assignees' do
-    subject { described_class.change_issue_assignees(noteable, project, author, [assignee]) }
+  describe '.change_issuable_assignees' do
+    subject { described_class.change_issuable_assignees(noteable, project, author, [assignee]) }
 
     let(:assignee) { create(:user) }
     let(:assignee1) { create(:user) }
@@ -180,7 +180,7 @@ describe SystemNoteService do
 
     def build_note(old_assignees, new_assignees)
       issue.assignees = new_assignees
-      described_class.change_issue_assignees(issue, project, author, old_assignees).note
+      described_class.change_issuable_assignees(issue, project, author, old_assignees).note
     end
 
     it_behaves_like 'a note with overridable created_at'
diff --git a/spec/services/todo_service_spec.rb b/spec/services/todo_service_spec.rb
index 8631f3f9a33..89411b2e908 100644
--- a/spec/services/todo_service_spec.rb
+++ b/spec/services/todo_service_spec.rb
@@ -272,28 +272,6 @@ describe TodoService do
       end
     end
 
-    describe '#reassigned_issue' do
-      it 'creates a pending todo for new assignee' do
-        unassigned_issue.assignees << john_doe
-        service.reassigned_issue(unassigned_issue, author)
-
-        should_create_todo(user: john_doe, target: unassigned_issue, action: Todo::ASSIGNED)
-      end
-
-      it 'does not create a todo if unassigned' do
-        issue.assignees.destroy_all # rubocop: disable DestroyAll
-
-        should_not_create_any_todo { service.reassigned_issue(issue, author) }
-      end
-
-      it 'creates a todo if new assignee is the current user' do
-        unassigned_issue.assignees << john_doe
-        service.reassigned_issue(unassigned_issue, john_doe)
-
-        should_create_todo(user: john_doe, target: unassigned_issue, author: john_doe, action: Todo::ASSIGNED)
-      end
-    end
-
     describe '#mark_pending_todos_as_done' do
       it 'marks related pending todos to the target for the user as done' do
         first_todo = create(:todo, :assigned, user: john_doe, project: project, target: issue, author: author)
@@ -504,10 +482,60 @@ describe TodoService do
     end
   end
 
+  describe '#reassigned_issuable' do
+    shared_examples 'reassigned issuable' do
+      it 'creates a pending todo for new assignee' do
+        issuable_unassigned.assignees = [john_doe]
+        service.reassigned_issuable(issuable_unassigned, author)
+
+        should_create_todo(user: john_doe, target: issuable_unassigned, action: Todo::ASSIGNED)
+      end
+
+      it 'does not create a todo if unassigned' do
+        issuable_assigned.assignees = []
+
+        should_not_create_any_todo { service.reassigned_issuable(issuable_assigned, author) }
+      end
+
+      it 'creates a todo if new assignee is the current user' do
+        issuable_assigned.assignees = [john_doe]
+        service.reassigned_issuable(issuable_assigned, john_doe)
+
+        should_create_todo(user: john_doe, target: issuable_assigned, author: john_doe, action: Todo::ASSIGNED)
+      end
+
+      it 'does not create a todo for guests' do
+        service.reassigned_issuable(issuable_assigned, author)
+        should_not_create_todo(user: guest, target: issuable_assigned, action: Todo::MENTIONED)
+      end
+
+      it 'does not create a directly addressed todo for guests' do
+        service.reassigned_issuable(addressed_issuable_assigned, author)
+        should_not_create_todo(user: guest, target: addressed_issuable_assigned, action: Todo::DIRECTLY_ADDRESSED)
+      end
+    end
+
+    context 'issuable is a merge request' do
+      it_behaves_like 'reassigned issuable' do
+        let(:issuable_assigned) { create(:merge_request, source_project: project, author: author, assignees: [john_doe], description: "- [ ] Task 1\n- [ ] Task 2 #{mentions}") }
+        let(:addressed_issuable_assigned) { create(:merge_request, source_project: project, author: author, assignees: [john_doe], description: "#{directly_addressed}\n- [ ] Task 1\n- [ ] Task 2") }
+        let(:issuable_unassigned) { create(:merge_request, source_project: project, author: author, assignees: []) }
+      end
+    end
+
+    context 'issuable is an issue' do
+      it_behaves_like 'reassigned issuable' do
+        let(:issuable_assigned) { create(:issue, project: project, author: author, assignees: [john_doe], description: "- [ ] Task 1\n- [ ] Task 2 #{mentions}") }
+        let(:addressed_issuable_assigned) { create(:issue, project: project, author: author, assignees: [john_doe], description: "#{directly_addressed}\n- [ ] Task 1\n- [ ] Task 2") }
+        let(:issuable_unassigned) { create(:issue, project: project, author: author, assignees: []) }
+      end
+    end
+  end
+
   describe 'Merge Requests' do
-    let(:mr_assigned) { create(:merge_request, source_project: project, author: author, assignee: john_doe, description: "- [ ] Task 1\n- [ ] Task 2 #{mentions}") }
-    let(:addressed_mr_assigned) { create(:merge_request, source_project: project, author: author, assignee: john_doe, description: "#{directly_addressed}\n- [ ] Task 1\n- [ ] Task 2") }
-    let(:mr_unassigned) { create(:merge_request, source_project: project, author: author, assignee: nil) }
+    let(:mr_assigned) { create(:merge_request, source_project: project, author: author, assignees: [john_doe], description: "- [ ] Task 1\n- [ ] Task 2 #{mentions}") }
+    let(:addressed_mr_assigned) { create(:merge_request, source_project: project, author: author, assignees: [john_doe], description: "#{directly_addressed}\n- [ ] Task 1\n- [ ] Task 2") }
+    let(:mr_unassigned) { create(:merge_request, source_project: project, author: author, assignees: []) }
 
     describe '#new_merge_request' do
       it 'creates a pending todo if assigned' do
@@ -659,38 +687,6 @@ describe TodoService do
       end
     end
 
-    describe '#reassigned_merge_request' do
-      it 'creates a pending todo for new assignee' do
-        mr_unassigned.update_attribute(:assignee, john_doe)
-        service.reassigned_merge_request(mr_unassigned, author)
-
-        should_create_todo(user: john_doe, target: mr_unassigned, action: Todo::ASSIGNED)
-      end
-
-      it 'does not create a todo if unassigned' do
-        mr_assigned.update_attribute(:assignee, nil)
-
-        should_not_create_any_todo { service.reassigned_merge_request(mr_assigned, author) }
-      end
-
-      it 'creates a todo if new assignee is the current user' do
-        mr_assigned.update_attribute(:assignee, john_doe)
-        service.reassigned_merge_request(mr_assigned, john_doe)
-
-        should_create_todo(user: john_doe, target: mr_assigned, author: john_doe, action: Todo::ASSIGNED)
-      end
-
-      it 'does not create a todo for guests' do
-        service.reassigned_merge_request(mr_assigned, author)
-        should_not_create_todo(user: guest, target: mr_assigned, action: Todo::MENTIONED)
-      end
-
-      it 'does not create a directly addressed todo for guests' do
-        service.reassigned_merge_request(addressed_mr_assigned, author)
-        should_not_create_todo(user: guest, target: addressed_mr_assigned, action: Todo::DIRECTLY_ADDRESSED)
-      end
-    end
-
     describe '#merge_merge_request' do
       it 'marks related pending todos to the target for the user as done' do
         first_todo = create(:todo, :assigned, user: john_doe, project: project, target: mr_assigned, author: author)
diff --git a/spec/services/users/destroy_service_spec.rb b/spec/services/users/destroy_service_spec.rb
index 83f1495a1c6..450e76d5f58 100644
--- a/spec/services/users/destroy_service_spec.rb
+++ b/spec/services/users/destroy_service_spec.rb
@@ -78,7 +78,7 @@ describe Users::DestroyService do
       end
 
       context "for an merge request the user was assigned to" do
-        let!(:merge_request) { create(:merge_request, source_project: project, assignee: user) }
+        let!(:merge_request) { create(:merge_request, source_project: project, assignees: [user]) }
 
         before do
           service.execute(user)
@@ -91,7 +91,7 @@ describe Users::DestroyService do
         it 'migrates the merge request so that it is "Unassigned"' do
           migrated_merge_request = MergeRequest.find_by_id(merge_request.id)
 
-          expect(migrated_merge_request.assignee).to be_nil
+          expect(migrated_merge_request.assignees).to be_empty
         end
       end
     end
diff --git a/spec/support/shared_contexts/finders/merge_requests_finder_shared_contexts.rb b/spec/support/shared_contexts/finders/merge_requests_finder_shared_contexts.rb
index 4df80b4168a..ab6687f1d07 100644
--- a/spec/support/shared_contexts/finders/merge_requests_finder_shared_contexts.rb
+++ b/spec/support/shared_contexts/finders/merge_requests_finder_shared_contexts.rb
@@ -46,9 +46,9 @@ RSpec.shared_context 'MergeRequestsFinder multiple projects with merge requests
     allow_gitaly_n_plus_1 { create(:project, group: subgroup) }
   end
 
-  let!(:merge_request1) { create(:merge_request, author: user, source_project: project2, target_project: project1, target_branch: 'merged-target') }
-  let!(:merge_request2) { create(:merge_request, :conflict, author: user, source_project: project2, target_project: project1, state: 'closed') }
-  let!(:merge_request3) { create(:merge_request, :simple, author: user, source_project: project2, target_project: project2, state: 'locked', title: 'thing WIP thing') }
+  let!(:merge_request1) { create(:merge_request, assignees: [user], author: user, source_project: project2, target_project: project1, target_branch: 'merged-target') }
+  let!(:merge_request2) { create(:merge_request, :conflict, assignees: [user], author: user, source_project: project2, target_project: project1, state: 'closed') }
+  let!(:merge_request3) { create(:merge_request, :simple, author: user, assignees: [user2], source_project: project2, target_project: project2, state: 'locked', title: 'thing WIP thing') }
   let!(:merge_request4) { create(:merge_request, :simple, author: user, source_project: project3, target_project: project3, title: 'WIP thing') }
   let!(:merge_request5) { create(:merge_request, :simple, author: user, source_project: project4, target_project: project4, title: '[WIP]') }
 
diff --git a/spec/support/shared_contexts/merge_request_create.rb b/spec/support/shared_contexts/merge_request_create.rb
new file mode 100644
index 00000000000..529f481c2b6
--- /dev/null
+++ b/spec/support/shared_contexts/merge_request_create.rb
@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+shared_context 'merge request create context' do
+  let(:user)        { create(:user) }
+  let(:user2)       { create(:user) }
+  let(:target_project) { create(:project, :public, :repository) }
+  let(:source_project) { target_project }
+  let!(:milestone)   { create(:milestone, project: target_project) }
+  let!(:label)       { create(:label, project: target_project) }
+  let!(:label2)      { create(:label, project: target_project) }
+
+  before do
+    source_project.add_maintainer(user)
+    target_project.add_maintainer(user)
+    target_project.add_maintainer(user2)
+
+    sign_in(user)
+    visit project_new_merge_request_path(target_project,
+                                         merge_request: {
+                                           source_project_id: source_project.id,
+                                           target_project_id: target_project.id,
+                                           source_branch: 'fix',
+                                           target_branch: 'master'
+                                         })
+  end
+end
diff --git a/spec/support/shared_contexts/merge_request_edit.rb b/spec/support/shared_contexts/merge_request_edit.rb
new file mode 100644
index 00000000000..c84510ff47d
--- /dev/null
+++ b/spec/support/shared_contexts/merge_request_edit.rb
@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+shared_context 'merge request edit context' do
+  let(:user)        { create(:user) }
+  let(:user2)       { create(:user) }
+  let!(:milestone)   { create(:milestone, project: target_project) }
+  let!(:label)       { create(:label, project: target_project) }
+  let!(:label2)      { create(:label, project: target_project) }
+  let(:target_project) { create(:project, :public, :repository) }
+  let(:source_project) { target_project }
+  let(:merge_request) do
+    create(:merge_request,
+           source_project: source_project,
+           target_project: target_project,
+           source_branch: 'fix',
+           target_branch: 'master')
+  end
+
+  before do
+    source_project.add_maintainer(user)
+    target_project.add_maintainer(user)
+    target_project.add_maintainer(user2)
+
+    sign_in(user)
+    visit edit_project_merge_request_path(target_project, merge_request)
+  end
+end
diff --git a/spec/support/shared_examples/features/creatable_merge_request_shared_examples.rb b/spec/support/shared_examples/features/creatable_merge_request_shared_examples.rb
index 7038a366144..ec1b1754cf0 100644
--- a/spec/support/shared_examples/features/creatable_merge_request_shared_examples.rb
+++ b/spec/support/shared_examples/features/creatable_merge_request_shared_examples.rb
@@ -1,42 +1,17 @@
 RSpec.shared_examples 'a creatable merge request' do
   include WaitForRequests
 
-  let(:user)        { create(:user) }
-  let(:user2)       { create(:user) }
-  let(:target_project) { create(:project, :public, :repository) }
-  let(:source_project) { target_project }
-  let!(:milestone)   { create(:milestone, project: target_project) }
-  let!(:label)       { create(:label, project: target_project) }
-  let!(:label2)      { create(:label, project: target_project) }
-
-  before do
-    source_project.add_maintainer(user)
-    target_project.add_maintainer(user)
-    target_project.add_maintainer(user2)
-
-    sign_in(user)
-    visit project_new_merge_request_path(
-      target_project,
-      merge_request: {
-        source_project_id: source_project.id,
-        target_project_id: target_project.id,
-        source_branch: 'fix',
-        target_branch: 'master'
-      })
-  end
-
   it 'creates new merge request', :js do
-    click_button 'Assignee'
+    find('.js-assignee-search').click
     page.within '.dropdown-menu-user' do
       click_link user2.name
     end
-    expect(find('input[name="merge_request[assignee_id]"]', visible: false).value).to match(user2.id.to_s)
+    expect(find('input[name="merge_request[assignee_ids][]"]', visible: false).value).to match(user2.id.to_s)
     page.within '.js-assignee-search' do
       expect(page).to have_content user2.name
     end
-
     click_link 'Assign to me'
-    expect(find('input[name="merge_request[assignee_id]"]', visible: false).value).to match(user.id.to_s)
+    expect(find('input[name="merge_request[assignee_ids][]"]', visible: false).value).to match(user.id.to_s)
     page.within '.js-assignee-search' do
       expect(page).to have_content user.name
     end
diff --git a/spec/support/shared_examples/features/editable_merge_request_shared_examples.rb b/spec/support/shared_examples/features/editable_merge_request_shared_examples.rb
index eef0327c9a6..a6121fcc50a 100644
--- a/spec/support/shared_examples/features/editable_merge_request_shared_examples.rb
+++ b/spec/support/shared_examples/features/editable_merge_request_shared_examples.rb
@@ -1,34 +1,10 @@
 RSpec.shared_examples 'an editable merge request' do
-  let(:user)        { create(:user) }
-  let(:user2)       { create(:user) }
-  let!(:milestone)   { create(:milestone, project: target_project) }
-  let!(:label)       { create(:label, project: target_project) }
-  let!(:label2)      { create(:label, project: target_project) }
-  let(:target_project) { create(:project, :public, :repository) }
-  let(:source_project) { target_project }
-  let(:merge_request) do
-    create(:merge_request,
-      source_project: source_project,
-      target_project: target_project,
-      source_branch: 'fix',
-      target_branch: 'master')
-  end
-
-  before do
-    source_project.add_maintainer(user)
-    target_project.add_maintainer(user)
-    target_project.add_maintainer(user2)
-
-    sign_in(user)
-    visit edit_project_merge_request_path(target_project, merge_request)
-  end
-
   it 'updates merge request', :js do
-    click_button 'Assignee'
+    find('.js-assignee-search').click
     page.within '.dropdown-menu-user' do
       click_link user.name
     end
-    expect(find('input[name="merge_request[assignee_id]"]', visible: false).value).to match(user.id.to_s)
+    expect(find('input[name="merge_request[assignee_ids][]"]', visible: false).value).to match(user.id.to_s)
     page.within '.js-assignee-search' do
       expect(page).to have_content user.name
     end
diff --git a/spec/support/shared_examples/features/multiple_assignees_mr_shared_examples.rb b/spec/support/shared_examples/features/multiple_assignees_mr_shared_examples.rb
new file mode 100644
index 00000000000..bab7963f06f
--- /dev/null
+++ b/spec/support/shared_examples/features/multiple_assignees_mr_shared_examples.rb
@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+shared_examples 'multiple assignees merge request' do |action, save_button_title|
+  it "#{action} a MR with multiple assignees", :js do
+    find('.js-assignee-search').click
+    page.within '.dropdown-menu-user' do
+      click_link user.name
+      click_link user2.name
+    end
+
+    # Extra click needed in order to toggle the dropdown
+    find('.js-assignee-search').click
+
+    expect(all('input[name="merge_request[assignee_ids][]"]', visible: false).map(&:value))
+      .to match_array([user.id.to_s, user2.id.to_s])
+
+    page.within '.js-assignee-search' do
+      expect(page).to have_content "#{user2.name} + 1 more"
+    end
+
+    click_button save_button_title
+
+    page.within '.issuable-sidebar' do
+      page.within '.assignee' do
+        expect(page).to have_content '2 Assignees'
+
+        click_link 'Edit'
+
+        expect(page).to have_content user.name
+        expect(page).to have_content user2.name
+      end
+    end
+
+    page.within '.dropdown-menu-user' do
+      click_link user.name
+    end
+
+    page.within '.issuable-sidebar' do
+      page.within '.assignee' do
+        # Closing dropdown to persist
+        click_link 'Edit'
+
+        expect(page).to have_content user2.name
+      end
+    end
+  end
+end
diff --git a/spec/support/shared_examples/finders/assignees_filter_spec.rb b/spec/support/shared_examples/finders/assignees_filter_spec.rb
new file mode 100644
index 00000000000..782a2d97746
--- /dev/null
+++ b/spec/support/shared_examples/finders/assignees_filter_spec.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+shared_examples 'assignee ID filter' do
+  it 'returns issuables assigned to that user' do
+    expect(issuables).to contain_exactly(*expected_issuables)
+  end
+end
+
+shared_examples 'assignee username filter' do
+  it 'returns issuables assigned to those users' do
+    expect(issuables).to contain_exactly(*expected_issuables)
+  end
+end
+
+shared_examples 'no assignee filter' do
+  let(:params) { { assignee_id: 'None' } }
+
+  it 'returns issuables not assigned to any assignee' do
+    expect(issuables).to contain_exactly(*expected_issuables)
+  end
+
+  it 'returns issuables not assigned to any assignee' do
+    params[:assignee_id] = 0
+
+    expect(issuables).to contain_exactly(*expected_issuables)
+  end
+
+  it 'returns issuables not assigned to any assignee' do
+    params[:assignee_id] = 'none'
+
+    expect(issuables).to contain_exactly(*expected_issuables)
+  end
+end
+
+shared_examples 'any assignee filter' do
+  context '' do
+    let(:params) { { assignee_id: 'Any' } }
+
+    it 'returns issuables assigned to any assignee' do
+      expect(issuables).to contain_exactly(*expected_issuables)
+    end
+
+    it 'returns issuables assigned to any assignee' do
+      params[:assignee_id] = 'any'
+
+      expect(issuables).to contain_exactly(*expected_issuables)
+    end
+  end
+end
diff --git a/spec/views/projects/merge_requests/edit.html.haml_spec.rb b/spec/views/projects/merge_requests/edit.html.haml_spec.rb
index c13eab30054..529afa03f9c 100644
--- a/spec/views/projects/merge_requests/edit.html.haml_spec.rb
+++ b/spec/views/projects/merge_requests/edit.html.haml_spec.rb
@@ -17,7 +17,7 @@ describe 'projects/merge_requests/edit.html.haml' do
       source_project: forked_project,
       target_project: project,
       author: user,
-      assignee: user,
+      assignees: [user],
       milestone: milestone)
   end
 
@@ -40,7 +40,7 @@ describe 'projects/merge_requests/edit.html.haml' do
 
       expect(rendered).to have_field('merge_request[title]')
       expect(rendered).to have_field('merge_request[description]')
-      expect(rendered).to have_selector('#merge_request_assignee_id', visible: false)
+      expect(rendered).to have_selector('input[name="merge_request[label_ids][]"]', visible: false)
       expect(rendered).to have_selector('#merge_request_milestone_id', visible: false)
       expect(rendered).not_to have_selector('#merge_request_target_branch', visible: false)
     end
@@ -52,7 +52,7 @@ describe 'projects/merge_requests/edit.html.haml' do
 
       expect(rendered).to have_field('merge_request[title]')
       expect(rendered).to have_field('merge_request[description]')
-      expect(rendered).to have_selector('#merge_request_assignee_id', visible: false)
+      expect(rendered).to have_selector('input[name="merge_request[label_ids][]"]', visible: false)
       expect(rendered).to have_selector('#merge_request_milestone_id', visible: false)
       expect(rendered).to have_selector('#merge_request_target_branch', visible: false)
     end
diff --git a/spec/views/projects/merge_requests/show.html.haml_spec.rb b/spec/views/projects/merge_requests/show.html.haml_spec.rb
index d9bda1a3414..23cb319a202 100644
--- a/spec/views/projects/merge_requests/show.html.haml_spec.rb
+++ b/spec/views/projects/merge_requests/show.html.haml_spec.rb
@@ -53,19 +53,6 @@ describe 'projects/merge_requests/show.html.haml' do
         expect(rendered).not_to have_css('.cannot-be-merged')
       end
     end
-
-    context 'when assignee is not allowed to merge' do
-      it 'shows a warning icon' do
-        reporter = create(:user)
-        project.add_reporter(reporter)
-        closed_merge_request.update(assignee_id: reporter.id)
-        assign(:issuable_sidebar, serialize_issuable_sidebar(user, project, closed_merge_request))
-
-        render
-
-        expect(rendered).to have_css('.cannot-be-merged')
-      end
-    end
   end
 
   context 'when the merge request is closed' do
-- 
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