[CE] Support multiple assignees for merge requestsosw-multi-assignees-merge-requests

Backports https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10161
(code out of ee/ folder).

60 files changed, 355 insertions, 372 deletions

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_issue_assignee.html.haml b/app/views/shared/issuable/form/_metadata_issuable_assignee.html.haml
index 6d4f9ccd66f..5336159e762 100644
--- a/app/views/shared/issuable/form/_metadata_issue_assignee.html.haml
+++ b/app/views/shared/issuable/form/_metadata_issuable_assignee.html.haml
@@ -1,4 +1,4 @@
-= form.label :assignee_ids, "Assignee", class: "col-form-label #{"col-md-2 col-lg-4" if has_due_date}"
+= 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|
@@ -7,5 +7,5 @@
     - 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)}"
+    = 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)}"