Backport of multiple_assignees_feature [ci skip]

99 files changed, 2328 insertions, 475 deletions

diff --git a/app/assets/javascripts/blob/target_branch_dropdown.js b/app/assets/javascripts/blob/target_branch_dropdown.js
index 216f069ef71..d52d69b1274 100644
--- a/app/assets/javascripts/blob/target_branch_dropdown.js
+++ b/app/assets/javascripts/blob/target_branch_dropdown.js
@@ -37,8 +37,8 @@ class TargetBranchDropDown {
         }
         return SELECT_ITEM_MSG;
       },
-      clicked(item, el, e) {
-        e.preventDefault();
+      clicked(options) {
+        options.e.preventDefault();
         self.onClick.call(self);
       },
       fieldName: self.fieldName,
diff --git a/app/assets/javascripts/blob/template_selector.js b/app/assets/javascripts/blob/template_selector.js
index d7c1c32efbd..888883163c5 100644
--- a/app/assets/javascripts/blob/template_selector.js
+++ b/app/assets/javascripts/blob/template_selector.js
@@ -24,7 +24,7 @@ export default class TemplateSelector {
       search: {
         fields: ['name'],
       },
-      clicked: (item, el, e) => this.fetchFileTemplate(item, el, e),
+      clicked: options => this.fetchFileTemplate(options),
       text: item => item.name,
     });
   }
@@ -51,7 +51,10 @@ export default class TemplateSelector {
     return this.$dropdownContainer.removeClass('hidden');
   }
 
-  fetchFileTemplate(item, el, e) {
+  fetchFileTemplate(options) {
+    const { e } = options;
+    const item = options.selectedObj;
+
     e.preventDefault();
     return this.requestFile(item);
   }
diff --git a/app/assets/javascripts/boards/boards_bundle.js b/app/assets/javascripts/boards/boards_bundle.js
index b6dee8177d2..dbbfb6de3ea 100644
--- a/app/assets/javascripts/boards/boards_bundle.js
+++ b/app/assets/javascripts/boards/boards_bundle.js
@@ -11,7 +11,7 @@ require('./models/issue');
 require('./models/label');
 require('./models/list');
 require('./models/milestone');
-require('./models/user');
+require('./models/assignee');
 require('./stores/boards_store');
 require('./stores/modal_store');
 require('./services/board_service');
diff --git a/app/assets/javascripts/boards/components/board_new_issue.js b/app/assets/javascripts/boards/components/board_new_issue.js
index 0fa85b6fe14..1ce95b62138 100644
--- a/app/assets/javascripts/boards/components/board_new_issue.js
+++ b/app/assets/javascripts/boards/components/board_new_issue.js
@@ -26,6 +26,7 @@ export default {
         title: this.title,
         labels,
         subscribed: true,
+        assignees: [],
       });
 
       this.list.newIssue(issue)
diff --git a/app/assets/javascripts/boards/components/board_sidebar.js b/app/assets/javascripts/boards/components/board_sidebar.js
index f0066d4ec5d..317cef9f227 100644
--- a/app/assets/javascripts/boards/components/board_sidebar.js
+++ b/app/assets/javascripts/boards/components/board_sidebar.js
@@ -3,8 +3,13 @@
 /* global MilestoneSelect */
 /* global LabelsSelect */
 /* global Sidebar */
+/* global Flash */
 
 import Vue from 'vue';
+import eventHub from '../../sidebar/event_hub';
+
+import AssigneeTitle from '../../sidebar/components/assignees/assignee_title';
+import Assignees from '../../sidebar/components/assignees/assignees';
 
 require('./sidebar/remove_issue');
 
@@ -22,6 +27,7 @@ gl.issueBoards.BoardSidebar = Vue.extend({
       detail: Store.detail,
       issue: {},
       list: {},
+      loadingAssignees: false,
     };
   },
   computed: {
@@ -43,6 +49,10 @@ gl.issueBoards.BoardSidebar = Vue.extend({
 
         this.issue = this.detail.issue;
         this.list = this.detail.list;
+
+        this.$nextTick(() => {
+          this.endpoint = this.$refs.assigneeDropdown.dataset.issueUpdate;
+        });
       },
       deep: true
     },
@@ -53,12 +63,57 @@ gl.issueBoards.BoardSidebar = Vue.extend({
           $('.right-sidebar').getNiceScroll().resize();
         });
       }
-    }
+
+      this.issue = this.detail.issue;
+      this.list = this.detail.list;
+    },
+    deep: true
   },
   methods: {
     closeSidebar () {
       this.detail.issue = {};
-    }
+    },
+    assignSelf () {
+      // Notify gl dropdown that we are now assigning to current user
+      this.$refs.assigneeBlock.dispatchEvent(new Event('assignYourself'));
+
+      this.addAssignee(this.currentUser);
+      this.saveAssignees();
+    },
+    removeAssignee (a) {
+      gl.issueBoards.BoardsStore.detail.issue.removeAssignee(a);
+    },
+    addAssignee (a) {
+      gl.issueBoards.BoardsStore.detail.issue.addAssignee(a);
+    },
+    removeAllAssignees () {
+      gl.issueBoards.BoardsStore.detail.issue.removeAllAssignees();
+    },
+    saveAssignees () {
+      this.loadingAssignees = true;
+
+      gl.issueBoards.BoardsStore.detail.issue.update(this.endpoint)
+        .then(() => {
+          this.loadingAssignees = false;
+        })
+        .catch(() => {
+          this.loadingAssignees = false;
+          return new Flash('An error occurred while saving assignees');
+        });
+    },
+  },
+  created () {
+    // Get events from glDropdown
+    eventHub.$on('sidebar.removeAssignee', this.removeAssignee);
+    eventHub.$on('sidebar.addAssignee', this.addAssignee);
+    eventHub.$on('sidebar.removeAllAssignees', this.removeAllAssignees);
+    eventHub.$on('sidebar.saveAssignees', this.saveAssignees);
+  },
+  beforeDestroy() {
+    eventHub.$off('sidebar.removeAssignee', this.removeAssignee);
+    eventHub.$off('sidebar.addAssignee', this.addAssignee);
+    eventHub.$off('sidebar.removeAllAssignees', this.removeAllAssignees);
+    eventHub.$off('sidebar.saveAssignees', this.saveAssignees);
   },
   mounted () {
     new IssuableContext(this.currentUser);
@@ -70,5 +125,7 @@ gl.issueBoards.BoardSidebar = Vue.extend({
   },
   components: {
     removeBtn: gl.issueBoards.RemoveIssueBtn,
+    'assignee-title': AssigneeTitle,
+    assignees: Assignees,
   },
 });
diff --git a/app/assets/javascripts/boards/components/issue_card_inner.js b/app/assets/javascripts/boards/components/issue_card_inner.js
index fc154ee7b8b..2f06d186c50 100644
--- a/app/assets/javascripts/boards/components/issue_card_inner.js
+++ b/app/assets/javascripts/boards/components/issue_card_inner.js
@@ -31,18 +31,36 @@ gl.issueBoards.IssueCardInner = Vue.extend({
       default: false,
     },
   },
+  data() {
+    return {
+      limitBeforeCounter: 3,
+      maxRender: 4,
+      maxCounter: 99,
+    };
+  },
   computed: {
-    cardUrl() {
-      return `${this.issueLinkBase}/${this.issue.id}`;
+    numberOverLimit() {
+      return this.issue.assignees.length - this.limitBeforeCounter;
     },
-    assigneeUrl() {
-      return `${this.rootPath}${this.issue.assignee.username}`;
+    assigneeCounterTooltip() {
+      return `${this.assigneeCounterLabel} more`;
+    },
+    assigneeCounterLabel() {
+      if (this.numberOverLimit > this.maxCounter) {
+        return `${this.maxCounter}+`;
+      }
+
+      return `+${this.numberOverLimit}`;
     },
-    assigneeUrlTitle() {
-      return `Assigned to ${this.issue.assignee.name}`;
+    shouldRenderCounter() {
+      if (this.issue.assignees.length <= this.maxRender) {
+        return false;
+      }
+
+      return this.issue.assignees.length > this.numberOverLimit;
     },
-    avatarUrlTitle() {
-      return `Avatar for ${this.issue.assignee.name}`;
+    cardUrl() {
+      return `${this.issueLinkBase}/${this.issue.id}`;
     },
     issueId() {
       return `#${this.issue.id}`;
@@ -52,6 +70,28 @@ gl.issueBoards.IssueCardInner = Vue.extend({
     },
   },
   methods: {
+    isIndexLessThanlimit(index) {
+      return index < this.limitBeforeCounter;
+    },
+    shouldRenderAssignee(index) {
+      // Eg. maxRender is 4,
+      // Render up to all 4 assignees if there are only 4 assigness
+      // Otherwise render up to the limitBeforeCounter
+      if (this.issue.assignees.length <= this.maxRender) {
+        return index < this.maxRender;
+      }
+
+      return index < this.limitBeforeCounter;
+    },
+    assigneeUrl(assignee) {
+      return `${this.rootPath}${assignee.username}`;
+    },
+    assigneeUrlTitle(assignee) {
+      return `Assigned to ${assignee.name}`;
+    },
+    avatarUrlTitle(assignee) {
+      return `Avatar for ${assignee.name}`;
+    },
     showLabel(label) {
       if (!this.list) return true;
 
@@ -105,25 +145,39 @@ gl.issueBoards.IssueCardInner = Vue.extend({
             {{ issueId }}
           </span>
         </h4>
-        <a
-          class="card-assignee has-tooltip js-no-trigger"
-          :href="assigneeUrl"
-          :title="assigneeUrlTitle"
-          v-if="issue.assignee"
-          data-container="body"
-        >
-          <img
-            class="avatar avatar-inline s20 js-no-trigger"
-            :src="issue.assignee.avatar"
-            width="20"
-            height="20"
-            :alt="avatarUrlTitle"
-          />
-        </a>
+        <div class="card-assignee">
+          <a
+            class="has-tooltip js-no-trigger"
+            :href="assigneeUrl(assignee)"
+            :title="assigneeUrlTitle(assignee)"
+            v-for="(assignee, index) in issue.assignees"
+            v-if="shouldRenderAssignee(index)"
+            data-container="body"
+            data-placement="bottom"
+          >
+            <img
+              class="avatar avatar-inline s20"
+              :src="assignee.avatarUrl"
+              width="20"
+              height="20"
+              :alt="avatarUrlTitle(assignee)"
+            />
+          </a>
+          <span
+            class="avatar-counter has-tooltip"
+            :title="assigneeCounterTooltip"
+            v-if="shouldRenderCounter"
+          >
+           {{ assigneeCounterLabel }}
+          </span>
+        </div>
       </div>
-      <div class="card-footer" v-if="showLabelFooter">
+      <div
+        class="card-footer"
+        v-if="showLabelFooter"
+      >
         <button
-          class="label color-label has-tooltip js-no-trigger"
+          class="label color-label has-tooltip"
           v-for="label in issue.labels"
           type="button"
           v-if="showLabel(label)"
diff --git a/app/assets/javascripts/boards/components/new_list_dropdown.js b/app/assets/javascripts/boards/components/new_list_dropdown.js
index 7e3bb79af1d..f29b6caa1ac 100644
--- a/app/assets/javascripts/boards/components/new_list_dropdown.js
+++ b/app/assets/javascripts/boards/components/new_list_dropdown.js
@@ -52,7 +52,9 @@ gl.issueBoards.newListDropdownInit = () => {
       filterable: true,
       selectable: true,
       multiSelect: true,
-      clicked (label, $el, e) {
+      clicked (options) {
+        const { e } = options;
+        const label = options.selectedObj;
         e.preventDefault();
 
         if (!Store.findList('title', label.title)) {
diff --git a/app/assets/javascripts/boards/models/user.js b/app/assets/javascripts/boards/models/assignee.js
index 8e9de4d4cbb..ee0f4e608c9 100644
--- a/app/assets/javascripts/boards/models/user.js
+++ b/app/assets/javascripts/boards/models/assignee.js
@@ -1,12 +1,12 @@
 /* eslint-disable no-unused-vars */
 
-class ListUser {
+class ListAssignee {
   constructor(user) {
     this.id = user.id;
     this.name = user.name;
     this.username = user.username;
-    this.avatar = user.avatar_url;
+    this.avatarUrl = user.avatar_url;
   }
 }
 
-window.ListUser = ListUser;
+window.ListAssignee = ListAssignee;
diff --git a/app/assets/javascripts/boards/models/issue.js b/app/assets/javascripts/boards/models/issue.js
index d6175069e37..76ebd4c9eab 100644
--- a/app/assets/javascripts/boards/models/issue.js
+++ b/app/assets/javascripts/boards/models/issue.js
@@ -1,7 +1,7 @@
 /* eslint-disable no-unused-vars, space-before-function-paren, arrow-body-style, arrow-parens, comma-dangle, max-len */
 /* global ListLabel */
 /* global ListMilestone */
-/* global ListUser */
+/* global ListAssignee */
 
 import Vue from 'vue';
 
@@ -14,14 +14,10 @@ class ListIssue {
     this.dueDate = obj.due_date;
     this.subscribed = obj.subscribed;
     this.labels = [];
+    this.assignees = [];
     this.selected = false;
-    this.assignee = false;
     this.position = obj.relative_position || Infinity;
 
-    if (obj.assignee) {
-      this.assignee = new ListUser(obj.assignee);
-    }
-
     if (obj.milestone) {
       this.milestone = new ListMilestone(obj.milestone);
     }
@@ -29,6 +25,8 @@ class ListIssue {
     obj.labels.forEach((label) => {
       this.labels.push(new ListLabel(label));
     });
+
+    this.assignees = obj.assignees.map(a => new ListAssignee(a));
   }
 
   addLabel (label) {
@@ -51,6 +49,26 @@ class ListIssue {
     labels.forEach(this.removeLabel.bind(this));
   }
 
+  addAssignee (assignee) {
+    if (!this.findAssignee(assignee)) {
+      this.assignees.push(new ListAssignee(assignee));
+    }
+  }
+
+  findAssignee (findAssignee) {
+    return this.assignees.filter(assignee => assignee.id === findAssignee.id)[0];
+  }
+
+  removeAssignee (removeAssignee) {
+    if (removeAssignee) {
+      this.assignees = this.assignees.filter(assignee => assignee.id !== removeAssignee.id);
+    }
+  }
+
+  removeAllAssignees () {
+    this.assignees = [];
+  }
+
   getLists () {
     return gl.issueBoards.BoardsStore.state.lists.filter(list => list.findIssue(this.id));
   }
@@ -60,7 +78,7 @@ class ListIssue {
       issue: {
         milestone_id: this.milestone ? this.milestone.id : null,
         due_date: this.dueDate,
-        assignee_id: this.assignee ? this.assignee.id : null,
+        assignee_ids: this.assignees.length > 0 ? this.assignees.map((u) => u.id) : [0],
         label_ids: this.labels.map((label) => label.id)
       }
     };
diff --git a/app/assets/javascripts/gl_dropdown.js b/app/assets/javascripts/gl_dropdown.js
index a03f1202a6d..0c9eb84f0eb 100644
--- a/app/assets/javascripts/gl_dropdown.js
+++ b/app/assets/javascripts/gl_dropdown.js
@@ -255,7 +255,8 @@ GitLabDropdown = (function() {
               }
             };
           // Remote data
-          })(this)
+          })(this),
+          instance: this,
         });
       }
     }
@@ -269,6 +270,7 @@ GitLabDropdown = (function() {
         remote: this.options.filterRemote,
         query: this.options.data,
         keys: searchFields,
+        instance: this,
         elements: (function(_this) {
           return function() {
             selector = '.dropdown-content li:not(' + NON_SELECTABLE_CLASSES + ')';
@@ -343,21 +345,26 @@ GitLabDropdown = (function() {
       }
       this.dropdown.on("click", selector, function(e) {
         var $el, selected, selectedObj, isMarking;
-        $el = $(this);
+        $el = $(e.currentTarget);
         selected = self.rowClicked($el);
         selectedObj = selected ? selected[0] : null;
         isMarking = selected ? selected[1] : null;
-        if (self.options.clicked) {
-          self.options.clicked(selectedObj, $el, e, isMarking);
+        if (this.options.clicked) {
+          this.options.clicked.call(this, {
+            selectedObj,
+            $el,
+            e,
+            isMarking,
+          });
         }
 
         // Update label right after all modifications in dropdown has been done
-        if (self.options.toggleLabel) {
-          self.updateLabel(selectedObj, $el, self);
+        if (this.options.toggleLabel) {
+          this.updateLabel(selectedObj, $el, this);
         }
 
         $el.trigger('blur');
-      });
+      }.bind(this));
     }
   }
 
@@ -439,15 +446,34 @@ GitLabDropdown = (function() {
     }
   };
 
+  GitLabDropdown.prototype.filteredFullData = function() {
+    return this.fullData.filter(r => typeof r === 'object'
+      && !Object.prototype.hasOwnProperty.call(r, 'beforeDivider')
+      && !Object.prototype.hasOwnProperty.call(r, 'header')
+    );
+  };
+
   GitLabDropdown.prototype.opened = function(e) {
     var contentHtml;
     this.resetRows();
     this.addArrowKeyEvent();
 
+    const dropdownToggle = this.dropdown.find('.dropdown-menu-toggle');
+    const hasFilterBulkUpdate = dropdownToggle.hasClass('js-filter-bulk-update');
+    const hasMultiSelect = dropdownToggle.hasClass('js-multiselect');
+
     // Makes indeterminate items effective
-    if (this.fullData && this.dropdown.find('.dropdown-menu-toggle').hasClass('js-filter-bulk-update')) {
+    if (this.fullData && hasFilterBulkUpdate) {
       this.parseData(this.fullData);
     }
+
+    // Process the data to make sure rendered data
+    // matches the correct layout
+    if (this.fullData && hasMultiSelect && this.options.processData) {
+      const inputValue = this.filterInput.val();
+      this.options.processData.call(this.options, inputValue, this.filteredFullData(), this.parseData.bind(this));
+    }
+
     contentHtml = $('.dropdown-content', this.dropdown).html();
     if (this.remote && contentHtml === "") {
       this.remote.execute();
@@ -709,6 +735,11 @@ GitLabDropdown = (function() {
     if (this.options.inputId != null) {
       $input.attr('id', this.options.inputId);
     }
+
+    if (this.options.inputMeta) {
+      $input.attr('data-meta', selectedObject[this.options.inputMeta]);
+    }
+
     return this.dropdown.before($input);
   };
 
@@ -829,7 +860,14 @@ GitLabDropdown = (function() {
     if (instance == null) {
       instance = null;
     }
-    return $(this.el).find(".dropdown-toggle-text").text(this.options.toggleLabel(selected, el, instance));
+
+    let toggleText = this.options.toggleLabel(selected, el, instance);
+    if (this.options.updateLabel) {
+      // Option to override the dropdown label text
+      toggleText = this.options.updateLabel;
+    }
+
+    return $(this.el).find(".dropdown-toggle-text").text(toggleText);
   };
 
   GitLabDropdown.prototype.clearField = function(field, isInput) {
diff --git a/app/assets/javascripts/issuable/issuable_bundle.js b/app/assets/javascripts/issuable/issuable_bundle.js
deleted file mode 100644
index e927cc0077c..00000000000
--- a/app/assets/javascripts/issuable/issuable_bundle.js
+++ /dev/null
@@ -1 +0,0 @@
-require('./time_tracking/time_tracking_bundle');
diff --git a/app/assets/javascripts/issuable/time_tracking/components/no_tracking_pane.js b/app/assets/javascripts/issuable/time_tracking/components/no_tracking_pane.js
deleted file mode 100644
index b081adf5e64..00000000000
--- a/app/assets/javascripts/issuable/time_tracking/components/no_tracking_pane.js
+++ /dev/null
@@ -1,12 +0,0 @@
-import Vue from 'vue';
-
-(() => {
-  Vue.component('time-tracking-no-tracking-pane', {
-    name: 'time-tracking-no-tracking-pane',
-    template: `
-      <div class='time-tracking-no-tracking-pane'>
-        <span class='no-value'>No estimate or time spent</span>
-      </div>
-    `,
-  });
-})();
diff --git a/app/assets/javascripts/issuable/time_tracking/time_tracking_bundle.js b/app/assets/javascripts/issuable/time_tracking/time_tracking_bundle.js
deleted file mode 100644
index 1689a69e1ed..00000000000
--- a/app/assets/javascripts/issuable/time_tracking/time_tracking_bundle.js
+++ /dev/null
@@ -1,66 +0,0 @@
-import Vue from 'vue';
-import VueResource from 'vue-resource';
-
-require('./components/time_tracker');
-require('../../smart_interval');
-require('../../subbable_resource');
-
-Vue.use(VueResource);
-
-(() => {
-  /* This Vue instance represents what will become the parent instance for the
-    * sidebar. It will be responsible for managing `issuable` state and propagating
-    * changes to sidebar components. We will want to create a separate service to
-    * interface with the server at that point.
-   */
-
-  class IssuableTimeTracking {
-    constructor(issuableJSON) {
-      const parsedIssuable = JSON.parse(issuableJSON);
-      return this.initComponent(parsedIssuable);
-    }
-
-    initComponent(parsedIssuable) {
-      this.parentInstance = new Vue({
-        el: '#issuable-time-tracker',
-        data: {
-          issuable: parsedIssuable,
-        },
-        methods: {
-          fetchIssuable() {
-            return gl.IssuableResource.get.call(gl.IssuableResource, {
-              type: 'GET',
-              url: gl.IssuableResource.endpoint,
-            });
-          },
-          updateState(data) {
-            this.issuable = data;
-          },
-          subscribeToUpdates() {
-            gl.IssuableResource.subscribe(data => this.updateState(data));
-          },
-          listenForSlashCommands() {
-            $(document).on('ajax:success', '.gfm-form', (e, data) => {
-              const subscribedCommands = ['spend_time', 'time_estimate'];
-              const changedCommands = data.commands_changes
-                ? Object.keys(data.commands_changes)
-                : [];
-              if (changedCommands && _.intersection(subscribedCommands, changedCommands).length) {
-                this.fetchIssuable();
-              }
-            });
-          },
-        },
-        created() {
-          this.fetchIssuable();
-        },
-        mounted() {
-          this.subscribeToUpdates();
-          this.listenForSlashCommands();
-        },
-      });
-    }
-  }
-
-  gl.IssuableTimeTracking = IssuableTimeTracking;
-})(window.gl || (window.gl = {}));
diff --git a/app/assets/javascripts/issue_status_select.js b/app/assets/javascripts/issue_status_select.js
index b2cfd3ef2a3..56cb536dcde 100644
--- a/app/assets/javascripts/issue_status_select.js
+++ b/app/assets/javascripts/issue_status_select.js
@@ -19,8 +19,8 @@
               return label;
             };
           })(this),
-          clicked: function(item, $el, e) {
-            return e.preventDefault();
+          clicked: function(options) {
+            return options.e.preventDefault();
           },
           id: function(obj, el) {
             return $(el).data("id");
diff --git a/app/assets/javascripts/issues_bulk_assignment.js b/app/assets/javascripts/issues_bulk_assignment.js
index e0ebd36a65c..fee3429e2b8 100644
--- a/app/assets/javascripts/issues_bulk_assignment.js
+++ b/app/assets/javascripts/issues_bulk_assignment.js
@@ -88,7 +88,10 @@
       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(),
           subscription_event: this.form.find('input[name="update[subscription_event]"]').val(),
diff --git a/app/assets/javascripts/labels_select.js b/app/assets/javascripts/labels_select.js
index 9a60f5464df..ac5ce84e31b 100644
--- a/app/assets/javascripts/labels_select.js
+++ b/app/assets/javascripts/labels_select.js
@@ -330,7 +330,10 @@
           },
           multiSelect: $dropdown.hasClass('js-multiselect'),
           vue: $dropdown.hasClass('js-issue-board-sidebar'),
-          clicked: function(label, $el, e, isMarking) {
+          clicked: function(options) {
+            const { $el, e, isMarking } = options;
+            const label = options.selectedObj;
+
             var isIssueIndex, isMRIndex, page, boardsModel;
             var fadeOutLoader = () => {
               $loading.fadeOut();
@@ -352,7 +355,7 @@
 
             if ($dropdown.hasClass('js-filter-bulk-update')) {
               _this.enableBulkLabelDropdown();
-              _this.setDropdownData($dropdown, isMarking, this.id(label));
+              _this.setDropdownData($dropdown, isMarking, label.id);
               return;
             }
 
diff --git a/app/assets/javascripts/main.js b/app/assets/javascripts/main.js
index be3c2c9fbb1..1b0d5fc92e3 100644
--- a/app/assets/javascripts/main.js
+++ b/app/assets/javascripts/main.js
@@ -158,7 +158,6 @@ import './single_file_diff';
 import './smart_interval';
 import './snippets_list';
 import './star';
-import './subbable_resource';
 import './subscription';
 import './subscription_select';
 import './syntax_highlight';
diff --git a/app/assets/javascripts/members.js b/app/assets/javascripts/members.js
index e3f367a11eb..bfc4e551168 100644
--- a/app/assets/javascripts/members.js
+++ b/app/assets/javascripts/members.js
@@ -31,8 +31,27 @@
           toggleLabel(selected, $el) {
             return $el.text();
           },
+<<<<<<< HEAD
           clicked: (selected, $link) => {
             this.formSubmit(null, $link);
+=======
+          clicked: (options) => {
+            const $link = options.$el;
+
+            if (!$link.data('revert')) {
+              this.formSubmit(null, $link);
+            } else {
+              const { $memberListItem, $toggle, $dateInput } = this.getMemberListItems($link);
+
+              $toggle.disable();
+              $dateInput.disable();
+
+              this.overrideLdap($memberListItem, $link.data('endpoint'), false).fail(() => {
+                $toggle.enable();
+                $dateInput.enable();
+              });
+            }
+>>>>>>> b0a2435... Merge branch 'multiple_assignees_review_upstream' into ee_master
           },
         });
       });
diff --git a/app/assets/javascripts/milestone_select.js b/app/assets/javascripts/milestone_select.js
index bebd0aa357e..0ca7aedb5bd 100644
--- a/app/assets/javascripts/milestone_select.js
+++ b/app/assets/javascripts/milestone_select.js
@@ -121,7 +121,30 @@
             return $value.css('display', '');
           },
           vue: $dropdown.hasClass('js-issue-board-sidebar'),
+<<<<<<< HEAD
           clicked: function(selected, $el, e) {
+=======
+          hideRow: function(milestone) {
+            if ($('html').hasClass('issue-boards-page') && !$dropdown.hasClass('js-issue-board-sidebar') &&
+              !$dropdown.closest('.add-issues-modal').length && gl.issueBoards.BoardsStore.state.currentBoard.milestone) {
+              return milestone !== gl.issueBoards.BoardsStore.state.currentBoard.milestone.title;
+            }
+
+            return false;
+          },
+          isSelectable: function() {
+            if ($('html').hasClass('issue-boards-page') && !$dropdown.hasClass('js-issue-board-sidebar') &&
+              !$dropdown.closest('.add-issues-modal').length && gl.issueBoards.BoardsStore.state.currentBoard.milestone_id) {
+              return false;
+            }
+
+            return true;
+          },
+          clicked: function(options) {
+            const { $el, e } = options;
+            let selected = options.selectedObj;
+
+>>>>>>> b0a2435... Merge branch 'multiple_assignees_review_upstream' into ee_master
             var data, isIssueIndex, isMRIndex, page, boardsStore;
             page = $('body').data('page');
             isIssueIndex = page === 'projects:issues:index';
diff --git a/app/assets/javascripts/namespace_select.js b/app/assets/javascripts/namespace_select.js
index b98e6121967..36bc1257cef 100644
--- a/app/assets/javascripts/namespace_select.js
+++ b/app/assets/javascripts/namespace_select.js
@@ -58,7 +58,8 @@
       });
     }
 
-    NamespaceSelect.prototype.onSelectItem = function(item, el, e) {
+    NamespaceSelect.prototype.onSelectItem = function(options) {
+      const { e } = options;
       return e.preventDefault();
     };
 
diff --git a/app/assets/javascripts/project.js b/app/assets/javascripts/project.js
index f944fcc5a58..738e710deb9 100644
--- a/app/assets/javascripts/project.js
+++ b/app/assets/javascripts/project.js
@@ -112,7 +112,8 @@ import Cookies from 'js-cookie';
           toggleLabel: function(obj, $el) {
             return $el.text().trim();
           },
-          clicked: function(selected, $el, e) {
+          clicked: function(options) {
+            const { e } = options;
             e.preventDefault();
             if ($('input[name="ref"]').length) {
               var $form = $dropdown.closest('form');
diff --git a/app/assets/javascripts/protected_branches/protected_branch_access_dropdown.js b/app/assets/javascripts/protected_branches/protected_branch_access_dropdown.js
index e7fff57ff45..1266d70f073 100644
--- a/app/assets/javascripts/protected_branches/protected_branch_access_dropdown.js
+++ b/app/assets/javascripts/protected_branches/protected_branch_access_dropdown.js
@@ -19,7 +19,10 @@
             return 'Select';
           }
         },
-        clicked(item, $el, e) {
+        clicked(opts) {
+          const { $el, e } = opts;
+          const item = opts.selectedObj;
+
           e.preventDefault();
           onSelect();
         }
diff --git a/app/assets/javascripts/protected_branches/protected_branch_dropdown.js b/app/assets/javascripts/protected_branches/protected_branch_dropdown.js
index 1d4bb8a13d6..bc6110fcd4e 100644
--- a/app/assets/javascripts/protected_branches/protected_branch_dropdown.js
+++ b/app/assets/javascripts/protected_branches/protected_branch_dropdown.js
@@ -35,7 +35,8 @@ class ProtectedBranchDropdown {
         return _.escape(protectedBranch.id);
       },
       onFilter: this.toggleCreateNewButton.bind(this),
-      clicked: (item, $el, e) => {
+      clicked: (options) => {
+        const { $el, e } = options;
         e.preventDefault();
         this.onSelect();
       }
diff --git a/app/assets/javascripts/sidebar/components/assignees/assignee_title.js b/app/assets/javascripts/sidebar/components/assignees/assignee_title.js
new file mode 100644
index 00000000000..a9ad3708514
--- /dev/null
+++ b/app/assets/javascripts/sidebar/components/assignees/assignee_title.js
@@ -0,0 +1,41 @@
+export default {
+  name: 'AssigneeTitle',
+  props: {
+    loading: {
+      type: Boolean,
+      required: false,
+      default: false,
+    },
+    numberOfAssignees: {
+      type: Number,
+      required: true,
+    },
+    editable: {
+      type: Boolean,
+      required: true,
+    },
+  },
+  computed: {
+    assigneeTitle() {
+      const assignees = this.numberOfAssignees;
+      return assignees > 1 ? `${assignees} Assignees` : 'Assignee';
+    },
+  },
+  template: `
+    <div class="title hide-collapsed">
+      {{assigneeTitle}}
+      <i
+        v-if="loading"
+        aria-hidden="true"
+        class="fa fa-spinner fa-spin block-loading"
+      />
+      <a
+        v-if="editable"
+        class="edit-link pull-right"
+        href="#"
+      >
+        Edit
+      </a>
+    </div>
+  `,
+};
diff --git a/app/assets/javascripts/sidebar/components/assignees/assignees.js b/app/assets/javascripts/sidebar/components/assignees/assignees.js
new file mode 100644
index 00000000000..88d7650f40a
--- /dev/null
+++ b/app/assets/javascripts/sidebar/components/assignees/assignees.js
@@ -0,0 +1,224 @@
+export default {
+  name: 'Assignees',
+  data() {
+    return {
+      defaultRenderCount: 5,
+      defaultMaxCounter: 99,
+      showLess: true,
+    };
+  },
+  props: {
+    rootPath: {
+      type: String,
+      required: true,
+    },
+    users: {
+      type: Array,
+      required: true,
+    },
+    editable: {
+      type: Boolean,
+      required: true,
+    },
+  },
+  computed: {
+    firstUser() {
+      return this.users[0];
+    },
+    hasMoreThanTwoAssignees() {
+      return this.users.length > 2;
+    },
+    hasMoreThanOneAssignee() {
+      return this.users.length > 1;
+    },
+    hasAssignees() {
+      return this.users.length > 0;
+    },
+    hasNoUsers() {
+      return !this.users.length;
+    },
+    hasOneUser() {
+      return this.users.length === 1;
+    },
+    renderShowMoreSection() {
+      return this.users.length > this.defaultRenderCount;
+    },
+    numberOfHiddenAssignees() {
+      return this.users.length - this.defaultRenderCount;
+    },
+    isHiddenAssignees() {
+      return this.numberOfHiddenAssignees > 0;
+    },
+    hiddenAssigneesLabel() {
+      return `+ ${this.numberOfHiddenAssignees} more`;
+    },
+    collapsedTooltipTitle() {
+      const maxRender = Math.min(this.defaultRenderCount, this.users.length);
+      const renderUsers = this.users.slice(0, maxRender);
+      const names = renderUsers.map(u => u.name);
+
+      if (this.users.length > maxRender) {
+        names.push(`+ ${this.users.length - maxRender} more`);
+      }
+
+      return names.join(', ');
+    },
+    sidebarAvatarCounter() {
+      let counter = `+${this.users.length - 1}`;
+
+      if (this.users.length > this.defaultMaxCounter) {
+        counter = `${this.defaultMaxCounter}+`;
+      }
+
+      return counter;
+    },
+  },
+  methods: {
+    assignSelf() {
+      this.$emit('assign-self');
+    },
+    toggleShowLess() {
+      this.showLess = !this.showLess;
+    },
+    renderAssignee(index) {
+      return !this.showLess || (index < this.defaultRenderCount && this.showLess);
+    },
+    avatarUrl(user) {
+      return user.avatarUrl || user.avatar_url;
+    },
+    assigneeUrl(user) {
+      return `${this.rootPath}${user.username}`;
+    },
+    assigneeAlt(user) {
+      return `${user.name}'s avatar`;
+    },
+    assigneeUsername(user) {
+      return `@${user.username}`;
+    },
+    shouldRenderCollapsedAssignee(index) {
+      const firstTwo = this.users.length <= 2 && index <= 2;
+
+      return index === 0 || firstTwo;
+    },
+  },
+  template: `
+    <div>
+      <div
+        class="sidebar-collapsed-icon sidebar-collapsed-user"
+        :class="{ 'multiple-users': hasMoreThanOneAssignee, 'has-tooltip': hasAssignees }"
+        data-container="body"
+        data-placement="left"
+        :title="collapsedTooltipTitle"
+      >
+        <i
+          v-if="hasNoUsers"
+          aria-label="No Assignee"
+          class="fa fa-user"
+        />
+        <button
+          type="button"
+          class="btn-link"
+          v-for="(user, index) in users"
+          v-if="shouldRenderCollapsedAssignee(index)"
+        >
+          <img
+            width="24"
+            class="avatar avatar-inline s24"
+            :alt="assigneeAlt(user)"
+            :src="avatarUrl(user)"
+          />
+          <span class="author">
+            {{ user.name }}
+          </span>
+        </button>
+        <button
+          v-if="hasMoreThanTwoAssignees"
+          class="btn-link"
+          type="button"
+        >
+          <span
+            class="avatar-counter sidebar-avatar-counter"
+          >
+            {{ sidebarAvatarCounter }}
+          </span>
+        </button>
+      </div>
+      <div class="value hide-collapsed">
+        <template v-if="hasNoUsers">
+          <span class="assign-yourself no-value">
+            No assignee
+            <template v-if="editable">
+             -
+              <button
+                type="button"
+                class="btn-link"
+                @click="assignSelf"
+              >
+                assign yourself
+              </button>
+            </template>
+          </span>
+        </template>
+        <template v-else-if="hasOneUser">
+          <a
+            class="author_link bold"
+            :href="assigneeUrl(firstUser)"
+          >
+            <img
+              width="32"
+              class="avatar avatar-inline s32"
+              :alt="assigneeAlt(firstUser)"
+              :src="avatarUrl(firstUser)"
+            />
+            <span class="author">
+              {{ firstUser.name }}
+            </span>
+            <span class="username">
+              {{ assigneeUsername(firstUser) }}
+            </span>
+          </a>
+        </template>
+        <template v-else>
+          <div class="user-list">
+            <div
+              class="user-item"
+              v-for="(user, index) in users"
+              v-if="renderAssignee(index)"
+            >
+              <a
+                class="user-link has-tooltip"
+                data-placement="bottom"
+                :href="assigneeUrl(user)"
+                :data-title="user.name"
+              >
+                <img
+                  width="32"
+                  class="avatar avatar-inline s32"
+                  :alt="assigneeAlt(user)"
+                  :src="avatarUrl(user)"
+                />
+              </a>
+            </div>
+          </div>
+          <div
+            v-if="renderShowMoreSection"
+            class="user-list-more"
+          >
+            <button
+              type="button"
+              class="btn-link"
+              @click="toggleShowLess"
+            >
+              <template v-if="showLess">
+                {{ hiddenAssigneesLabel }}
+              </template>
+              <template v-else>
+                - show less
+              </template>
+            </button>
+          </div>
+        </template>
+      </div>
+    </div>
+  `,
+};
diff --git a/app/assets/javascripts/sidebar/components/assignees/sidebar_assignees.js b/app/assets/javascripts/sidebar/components/assignees/sidebar_assignees.js
new file mode 100644
index 00000000000..1488a66c695
--- /dev/null
+++ b/app/assets/javascripts/sidebar/components/assignees/sidebar_assignees.js
@@ -0,0 +1,84 @@
+/* global Flash */
+
+import AssigneeTitle from './assignee_title';
+import Assignees from './assignees';
+
+import Store from '../../stores/sidebar_store';
+import Mediator from '../../sidebar_mediator';
+
+import eventHub from '../../event_hub';
+
+export default {
+  name: 'SidebarAssignees',
+  data() {
+    return {
+      mediator: new Mediator(),
+      store: new Store(),
+      loading: false,
+      field: '',
+    };
+  },
+  components: {
+    'assignee-title': AssigneeTitle,
+    assignees: Assignees,
+  },
+  methods: {
+    assignSelf() {
+      // Notify gl dropdown that we are now assigning to current user
+      this.$el.parentElement.dispatchEvent(new Event('assignYourself'));
+
+      this.mediator.assignYourself();
+      this.saveAssignees();
+    },
+    saveAssignees() {
+      this.loading = true;
+
+      function setLoadingFalse() {
+        this.loading = false;
+      }
+
+      this.mediator.saveAssignees(this.field)
+        .then(setLoadingFalse.bind(this))
+        .catch(() => {
+          setLoadingFalse();
+          return new Flash('Error occurred when saving assignees');
+        });
+    },
+  },
+  created() {
+    this.removeAssignee = this.store.removeAssignee.bind(this.store);
+    this.addAssignee = this.store.addAssignee.bind(this.store);
+    this.removeAllAssignees = this.store.removeAllAssignees.bind(this.store);
+
+    // Get events from glDropdown
+    eventHub.$on('sidebar.removeAssignee', this.removeAssignee);
+    eventHub.$on('sidebar.addAssignee', this.addAssignee);
+    eventHub.$on('sidebar.removeAllAssignees', this.removeAllAssignees);
+    eventHub.$on('sidebar.saveAssignees', this.saveAssignees);
+  },
+  beforeDestroy() {
+    eventHub.$off('sidebar.removeAssignee', this.removeAssignee);
+    eventHub.$off('sidebar.addAssignee', this.addAssignee);
+    eventHub.$off('sidebar.removeAllAssignees', this.removeAllAssignees);
+    eventHub.$off('sidebar.saveAssignees', this.saveAssignees);
+  },
+  beforeMount() {
+    this.field = this.$el.dataset.field;
+  },
+  template: `
+    <div>
+      <assignee-title
+        :number-of-assignees="store.assignees.length"
+        :loading="loading"
+        :editable="store.editable"
+      />
+      <assignees
+        class="value"
+        :root-path="store.rootPath"
+        :users="store.assignees"
+        :editable="store.editable"
+        @assign-self="assignSelf"
+      />
+    </div>
+  `,
+};
diff --git a/app/assets/javascripts/sidebar/components/time_tracking/collapsed_state.js b/app/assets/javascripts/sidebar/components/time_tracking/collapsed_state.js
new file mode 100644
index 00000000000..0da265053bd
--- /dev/null
+++ b/app/assets/javascripts/sidebar/components/time_tracking/collapsed_state.js
@@ -0,0 +1,97 @@
+import stopwatchSvg from 'icons/_icon_stopwatch.svg';
+
+import '../../../lib/utils/pretty_time';
+
+export default {
+  name: 'time-tracking-collapsed-state',
+  props: {
+    showComparisonState: {
+      type: Boolean,
+      required: true,
+    },
+    showSpentOnlyState: {
+      type: Boolean,
+      required: true,
+    },
+    showEstimateOnlyState: {
+      type: Boolean,
+      required: true,
+    },
+    showNoTimeTrackingState: {
+      type: Boolean,
+      required: true,
+    },
+    timeSpentHumanReadable: {
+      type: String,
+      required: false,
+      default: '',
+    },
+    timeEstimateHumanReadable: {
+      type: String,
+      required: false,
+      default: '',
+    },
+  },
+  computed: {
+    timeSpent() {
+      return this.abbreviateTime(this.timeSpentHumanReadable);
+    },
+    timeEstimate() {
+      return this.abbreviateTime(this.timeEstimateHumanReadable);
+    },
+    divClass() {
+      if (this.showComparisonState) {
+        return 'compare';
+      } else if (this.showEstimateOnlyState) {
+        return 'estimate-only';
+      } else if (this.showSpentOnlyState) {
+        return 'spend-only';
+      } else if (this.showNoTimeTrackingState) {
+        return 'no-tracking';
+      }
+
+      return '';
+    },
+    spanClass() {
+      if (this.showComparisonState) {
+        return '';
+      } else if (this.showEstimateOnlyState || this.showSpentOnlyState) {
+        return 'bold';
+      } else if (this.showNoTimeTrackingState) {
+        return 'no-value';
+      }
+
+      return '';
+    },
+    text() {
+      if (this.showComparisonState) {
+        return `${this.timeSpent} / ${this.timeEstimate}`;
+      } else if (this.showEstimateOnlyState) {
+        return `-- / ${this.timeEstimate}`;
+      } else if (this.showSpentOnlyState) {
+        return `${this.timeSpent} / --`;
+      } else if (this.showNoTimeTrackingState) {
+        return 'None';
+      }
+
+      return '';
+    },
+  },
+  methods: {
+    abbreviateTime(timeStr) {
+      return gl.utils.prettyTime.abbreviateTime(timeStr);
+    },
+  },
+  template: `
+    <div class="sidebar-collapsed-icon">
+      ${stopwatchSvg}
+      <div class="time-tracking-collapsed-summary">
+        <div :class="divClass">
+          <span :class="spanClass">
+            {{ text }}
+          </span>
+        </div>
+      </div>
+    </div>
+    `,
+};
diff --git a/app/assets/javascripts/sidebar/components/time_tracking/comparison_pane.js b/app/assets/javascripts/sidebar/components/time_tracking/comparison_pane.js
new file mode 100644
index 00000000000..40f5c89c5bb
--- /dev/null
+++ b/app/assets/javascripts/sidebar/components/time_tracking/comparison_pane.js
@@ -0,0 +1,98 @@
+import '../../../lib/utils/pretty_time';
+
+const prettyTime = gl.utils.prettyTime;
+
+export default {
+  name: 'time-tracking-comparison-pane',
+  props: {
+    timeSpent: {
+      type: Number,
+      required: true,
+    },
+    timeEstimate: {
+      type: Number,
+      required: true,
+    },
+    timeSpentHumanReadable: {
+      type: String,
+      required: true,
+    },
+    timeEstimateHumanReadable: {
+      type: String,
+      required: true,
+    },
+  },
+  computed: {
+    parsedRemaining() {
+      const diffSeconds = this.timeEstimate - this.timeSpent;
+      return prettyTime.parseSeconds(diffSeconds);
+    },
+    timeRemainingHumanReadable() {
+      return prettyTime.stringifyTime(this.parsedRemaining);
+    },
+    timeRemainingTooltip() {
+      const prefix = this.timeRemainingMinutes < 0 ? 'Over by' : 'Time remaining:';
+      return `${prefix} ${this.timeRemainingHumanReadable}`;
+    },
+    /* Diff values for comparison meter */
+    timeRemainingMinutes() {
+      return this.timeEstimate - this.timeSpent;
+    },
+    timeRemainingPercent() {
+      return `${Math.floor((this.timeSpent / this.timeEstimate) * 100)}%`;
+    },
+    timeRemainingStatusClass() {
+      return this.timeEstimate >= this.timeSpent ? 'within_estimate' : 'over_estimate';
+    },
+    /* Parsed time values */
+    parsedEstimate() {
+      return prettyTime.parseSeconds(this.timeEstimate);
+    },
+    parsedSpent() {
+      return prettyTime.parseSeconds(this.timeSpent);
+    },
+  },
+  template: `
+    <div class="time-tracking-comparison-pane">
+      <div
+        class="compare-meter"
+        data-toggle="tooltip"
+        data-placement="top"
+        role="timeRemainingDisplay"
+        :aria-valuenow="timeRemainingTooltip"
+        :title="timeRemainingTooltip"
+        :data-original-title="timeRemainingTooltip"
+        :class="timeRemainingStatusClass"
+      >
+        <div
+          class="meter-container"
+          role="timeSpentPercent"
+          :aria-valuenow="timeRemainingPercent"
+        >
+          <div
+            :style="{ width: timeRemainingPercent }"
+            class="meter-fill"
+          />
+        </div>
+        <div class="compare-display-container">
+          <div class="compare-display pull-left">
+            <span class="compare-label">
+              Spent
+            </span>
+            <span class="compare-value spent">
+              {{ timeSpentHumanReadable }}
+            </span>
+          </div>
+          <div class="compare-display estimated pull-right">
+            <span class="compare-label">
+              Est
+            </span>
+            <span class="compare-value">
+              {{ timeEstimateHumanReadable }}
+            </span>
+          </div>
+        </div>
+      </div>
+    </div>
+  `,
+};
diff --git a/app/assets/javascripts/sidebar/components/time_tracking/estimate_only_pane.js b/app/assets/javascripts/sidebar/components/time_tracking/estimate_only_pane.js
new file mode 100644
index 00000000000..ad1b9179db0
--- /dev/null
+++ b/app/assets/javascripts/sidebar/components/time_tracking/estimate_only_pane.js
@@ -0,0 +1,17 @@
+export default {
+  name: 'time-tracking-estimate-only-pane',
+  props: {
+    timeEstimateHumanReadable: {
+      type: String,
+      required: true,
+    },
+  },
+  template: `
+    <div class="time-tracking-estimate-only-pane">
+      <span class="bold">
+        Estimated:
+      </span>
+      {{ timeEstimateHumanReadable }}
+    </div>
+  `,
+};
diff --git a/app/assets/javascripts/sidebar/components/time_tracking/help_state.js b/app/assets/javascripts/sidebar/components/time_tracking/help_state.js
new file mode 100644
index 00000000000..b2a77462fe0
--- /dev/null
+++ b/app/assets/javascripts/sidebar/components/time_tracking/help_state.js
@@ -0,0 +1,44 @@
+export default {
+  name: 'time-tracking-help-state',
+  props: {
+    rootPath: {
+      type: String,
+      required: true,
+    },
+  },
+  computed: {
+    href() {
+      return `${this.rootPath}help/workflow/time_tracking.md`;
+    },
+  },
+  template: `
+    <div class="time-tracking-help-state">
+      <div class="time-tracking-info">
+        <h4>
+          Track time with slash commands
+        </h4>
+        <p>
+          Slash commands can be used in the issues description and comment boxes.
+        </p>
+        <p>
+          <code>
+            /estimate
+          </code>
+          will update the estimated time with the latest command.
+        </p>
+        <p>
+          <code>
+            /spend
+          </code>
+          will update the sum of the time spent.
+        </p>
+        <a
+          class="btn btn-default learn-more-button"
+          :href="href"
+        >
+          Learn more
+        </a>
+      </div>
+    </div>
+  `,
+};
diff --git a/app/assets/javascripts/sidebar/components/time_tracking/no_tracking_pane.js b/app/assets/javascripts/sidebar/components/time_tracking/no_tracking_pane.js
new file mode 100644
index 00000000000..d1dd1dcdd27
--- /dev/null
+++ b/app/assets/javascripts/sidebar/components/time_tracking/no_tracking_pane.js
@@ -0,0 +1,10 @@
+export default {
+  name: 'time-tracking-no-tracking-pane',
+  template: `
+    <div class="time-tracking-no-tracking-pane">
+      <span class="no-value">
+        No estimate or time spent
+      </span>
+    </div>
+  `,
+};
diff --git a/app/assets/javascripts/sidebar/components/time_tracking/sidebar_time_tracking.js b/app/assets/javascripts/sidebar/components/time_tracking/sidebar_time_tracking.js
new file mode 100644
index 00000000000..e2dba1fb0c2
--- /dev/null
+++ b/app/assets/javascripts/sidebar/components/time_tracking/sidebar_time_tracking.js
@@ -0,0 +1,45 @@
+import '~/smart_interval';
+
+import timeTracker from './time_tracker';
+
+import Store from '../../stores/sidebar_store';
+import Mediator from '../../sidebar_mediator';
+
+export default {
+  data() {
+    return {
+      mediator: new Mediator(),
+      store: new Store(),
+    };
+  },
+  components: {
+    'issuable-time-tracker': timeTracker,
+  },
+  methods: {
+    listenForSlashCommands() {
+      $(document).on('ajax:success', '.gfm-form', (e, data) => {
+        const subscribedCommands = ['spend_time', 'time_estimate'];
+        const changedCommands = data.commands_changes
+          ? Object.keys(data.commands_changes)
+          : [];
+        if (changedCommands && _.intersection(subscribedCommands, changedCommands).length) {
+          this.mediator.fetch();
+        }
+      });
+    },
+  },
+  mounted() {
+    this.listenForSlashCommands();
+  },
+  template: `
+    <div class="block">
+      <issuable-time-tracker
+        :time_estimate="store.timeEstimate"
+        :time_spent="store.totalTimeSpent"
+        :human_time_estimate="store.humanTimeEstimate"
+        :human_time_spent="store.humanTotalTimeSpent"
+        :rootPath="store.rootPath"
+      />
+    </div>
+  `,
+};
diff --git a/app/assets/javascripts/sidebar/components/time_tracking/spent_only_pane.js b/app/assets/javascripts/sidebar/components/time_tracking/spent_only_pane.js
new file mode 100644
index 00000000000..bf987562647
--- /dev/null
+++ b/app/assets/javascripts/sidebar/components/time_tracking/spent_only_pane.js
@@ -0,0 +1,15 @@
+export default {
+  name: 'time-tracking-spent-only-pane',
+  props: {
+    timeSpentHumanReadable: {
+      type: String,
+      required: true,
+    },
+  },
+  template: `
+    <div class="time-tracking-spend-only-pane">
+      <span class="bold">Spent:</span>
+      {{ timeSpentHumanReadable }}
+    </div>
+  `,
+};
diff --git a/app/assets/javascripts/sidebar/components/time_tracking/time_tracker.js b/app/assets/javascripts/sidebar/components/time_tracking/time_tracker.js
new file mode 100644
index 00000000000..ed0d71a4f79
--- /dev/null
+++ b/app/assets/javascripts/sidebar/components/time_tracking/time_tracker.js
@@ -0,0 +1,163 @@
+import timeTrackingHelpState from './help_state';
+import timeTrackingCollapsedState from './collapsed_state';
+import timeTrackingSpentOnlyPane from './spent_only_pane';
+import timeTrackingNoTrackingPane from './no_tracking_pane';
+import timeTrackingEstimateOnlyPane from './estimate_only_pane';
+import timeTrackingComparisonPane from './comparison_pane';
+
+import eventHub from '../../event_hub';
+
+export default {
+  name: 'issuable-time-tracker',
+  props: {
+    time_estimate: {
+      type: Number,
+      required: true,
+    },
+    time_spent: {
+      type: Number,
+      required: true,
+    },
+    human_time_estimate: {
+      type: String,
+      required: false,
+      default: '',
+    },
+    human_time_spent: {
+      type: String,
+      required: false,
+      default: '',
+    },
+    rootPath: {
+      type: String,
+      required: true,
+    },
+  },
+  data() {
+    return {
+      showHelp: false,
+    };
+  },
+  components: {
+    'time-tracking-collapsed-state': timeTrackingCollapsedState,
+    'time-tracking-estimate-only-pane': timeTrackingEstimateOnlyPane,
+    'time-tracking-spent-only-pane': timeTrackingSpentOnlyPane,
+    'time-tracking-no-tracking-pane': timeTrackingNoTrackingPane,
+    'time-tracking-comparison-pane': timeTrackingComparisonPane,
+    'time-tracking-help-state': timeTrackingHelpState,
+  },
+  computed: {
+    timeSpent() {
+      return this.time_spent;
+    },
+    timeEstimate() {
+      return this.time_estimate;
+    },
+    timeEstimateHumanReadable() {
+      return this.human_time_estimate;
+    },
+    timeSpentHumanReadable() {
+      return this.human_time_spent;
+    },
+    hasTimeSpent() {
+      return !!this.timeSpent;
+    },
+    hasTimeEstimate() {
+      return !!this.timeEstimate;
+    },
+    showComparisonState() {
+      return this.hasTimeEstimate && this.hasTimeSpent;
+    },
+    showEstimateOnlyState() {
+      return this.hasTimeEstimate && !this.hasTimeSpent;
+    },
+    showSpentOnlyState() {
+      return this.hasTimeSpent && !this.hasTimeEstimate;
+    },
+    showNoTimeTrackingState() {
+      return !this.hasTimeEstimate && !this.hasTimeSpent;
+    },
+    showHelpState() {
+      return !!this.showHelp;
+    },
+  },
+  methods: {
+    toggleHelpState(show) {
+      this.showHelp = show;
+    },
+    update(data) {
+      this.time_estimate = data.time_estimate;
+      this.time_spent = data.time_spent;
+      this.human_time_estimate = data.human_time_estimate;
+      this.human_time_spent = data.human_time_spent;
+    },
+  },
+  created() {
+    eventHub.$on('timeTracker:updateData', this.update);
+  },
+  template: `
+    <div
+      class="time_tracker time-tracking-component-wrap"
+      v-cloak
+    >
+      <time-tracking-collapsed-state
+        :show-comparison-state="showComparisonState"
+        :show-no-time-tracking-state="showNoTimeTrackingState"
+        :show-help-state="showHelpState"
+        :show-spent-only-state="showSpentOnlyState"
+        :show-estimate-only-state="showEstimateOnlyState"
+        :time-spent-human-readable="timeSpentHumanReadable"
+        :time-estimate-human-readable="timeEstimateHumanReadable"
+      />
+      <div class="title hide-collapsed">
+        Time tracking
+        <div
+          class="help-button pull-right"
+          v-if="!showHelpState"
+          @click="toggleHelpState(true)"
+        >
+            <i
+              class="fa fa-question-circle"
+              aria-hidden="true"
+            />
+        </div>
+        <div
+          class="close-help-button pull-right"
+          v-if="showHelpState"
+          @click="toggleHelpState(false)"
+        >
+          <i
+            class="fa fa-close"
+            aria-hidden="true"
+          />
+        </div>
+      </div>
+      <div class="time-tracking-content hide-collapsed">
+        <time-tracking-estimate-only-pane
+          v-if="showEstimateOnlyState"
+          :time-estimate-human-readable="timeEstimateHumanReadable"
+        />
+        <time-tracking-spent-only-pane
+          v-if="showSpentOnlyState"
+          :time-spent-human-readable="timeSpentHumanReadable"
+        />
+        <time-tracking-no-tracking-pane
+          v-if="showNoTimeTrackingState"
+        />
+        <time-tracking-comparison-pane
+          v-if="showComparisonState"
+          :time-estimate="timeEstimate"
+          :time-spent="timeSpent"
+          :time-spent-human-readable="timeSpentHumanReadable"
+          :time-estimate-human-readable="timeEstimateHumanReadable"
+        />
+        <transition name="help-state-toggle">
+          <time-tracking-help-state
+            v-if="showHelpState"
+            :rootPath="rootPath"
+          />
+        </transition>
+      </div>
+    </div>
+  `,
+};
diff --git a/app/assets/javascripts/sidebar/event_hub.js b/app/assets/javascripts/sidebar/event_hub.js
new file mode 100644
index 00000000000..0948c2e5352
--- /dev/null
+++ b/app/assets/javascripts/sidebar/event_hub.js
@@ -0,0 +1,3 @@
+import Vue from 'vue';
+
+export default new Vue();
diff --git a/app/assets/javascripts/sidebar/services/sidebar_service.js b/app/assets/javascripts/sidebar/services/sidebar_service.js
new file mode 100644
index 00000000000..5a82d01dc41
--- /dev/null
+++ b/app/assets/javascripts/sidebar/services/sidebar_service.js
@@ -0,0 +1,28 @@
+import Vue from 'vue';
+import VueResource from 'vue-resource';
+
+Vue.use(VueResource);
+
+export default class SidebarService {
+  constructor(endpoint) {
+    if (!SidebarService.singleton) {
+      this.endpoint = endpoint;
+
+      SidebarService.singleton = this;
+    }
+
+    return SidebarService.singleton;
+  }
+
+  get() {
+    return Vue.http.get(this.endpoint);
+  }
+
+  update(key, data) {
+    return Vue.http.put(this.endpoint, {
+      [key]: data,
+    }, {
+      emulateJSON: true,
+    });
+  }
+}
diff --git a/app/assets/javascripts/sidebar/sidebar_bundle.js b/app/assets/javascripts/sidebar/sidebar_bundle.js
new file mode 100644
index 00000000000..2ce53c2ed30
--- /dev/null
+++ b/app/assets/javascripts/sidebar/sidebar_bundle.js
@@ -0,0 +1,21 @@
+import Vue from 'vue';
+import sidebarTimeTracking from './components/time_tracking/sidebar_time_tracking';
+import sidebarAssignees from './components/assignees/sidebar_assignees';
+
+import Mediator from './sidebar_mediator';
+
+document.addEventListener('DOMContentLoaded', () => {
+  const mediator = new Mediator(gl.sidebarOptions);
+  mediator.fetch();
+
+  const sidebarAssigneesEl = document.querySelector('#js-vue-sidebar-assignees');
+
+  // Only create the sidebarAssignees vue app if it is found in the DOM
+  // We currently do not use sidebarAssignees for the MR page
+  if (sidebarAssigneesEl) {
+    new Vue(sidebarAssignees).$mount(sidebarAssigneesEl);
+  }
+
+  new Vue(sidebarTimeTracking).$mount('#issuable-time-tracker');
+});
+
diff --git a/app/assets/javascripts/sidebar/sidebar_mediator.js b/app/assets/javascripts/sidebar/sidebar_mediator.js
new file mode 100644
index 00000000000..c13f3391f0d
--- /dev/null
+++ b/app/assets/javascripts/sidebar/sidebar_mediator.js
@@ -0,0 +1,38 @@
+/* global Flash */
+
+import Service from './services/sidebar_service';
+import Store from './stores/sidebar_store';
+
+export default class SidebarMediator {
+  constructor(options) {
+    if (!SidebarMediator.singleton) {
+      this.store = new Store(options);
+      this.service = new Service(options.endpoint);
+      SidebarMediator.singleton = this;
+    }
+
+    return SidebarMediator.singleton;
+  }
+
+  assignYourself() {
+    this.store.addAssignee(this.store.currentUser);
+  }
+
+  saveAssignees(field) {
+    const selected = this.store.assignees.map(u => u.id);
+
+    // If there are no ids, that means we have to unassign (which is id = 0)
+    // And it only accepts an array, hence [0]
+    return this.service.update(field, selected.length === 0 ? [0] : selected);
+  }
+
+  fetch() {
+    this.service.get()
+      .then((response) => {
+        const data = response.json();
+        this.store.processAssigneeData(data);
+        this.store.processTimeTrackingData(data);
+      })
+      .catch(() => new Flash('Error occured when fetching sidebar data'));
+  }
+}
diff --git a/app/assets/javascripts/sidebar/stores/sidebar_store.js b/app/assets/javascripts/sidebar/stores/sidebar_store.js
new file mode 100644
index 00000000000..94408c4d715
--- /dev/null
+++ b/app/assets/javascripts/sidebar/stores/sidebar_store.js
@@ -0,0 +1,52 @@
+export default class SidebarStore {
+  constructor(store) {
+    if (!SidebarStore.singleton) {
+      const { currentUser, rootPath, editable } = store;
+      this.currentUser = currentUser;
+      this.rootPath = rootPath;
+      this.editable = editable;
+      this.timeEstimate = 0;
+      this.totalTimeSpent = 0;
+      this.humanTimeEstimate = '';
+      this.humanTimeSpent = '';
+      this.assignees = [];
+
+      SidebarStore.singleton = this;
+    }
+
+    return SidebarStore.singleton;
+  }
+
+  processAssigneeData(data) {
+    if (data.assignees) {
+      this.assignees = data.assignees;
+    }
+  }
+
+  processTimeTrackingData(data) {
+    this.timeEstimate = data.time_estimate;
+    this.totalTimeSpent = data.total_time_spent;
+    this.humanTimeEstimate = data.human_time_estimate;
+    this.humanTotalTimeSpent = data.human_total_time_spent;
+  }
+
+  addAssignee(assignee) {
+    if (!this.findAssignee(assignee)) {
+      this.assignees.push(assignee);
+    }
+  }
+
+  findAssignee(findAssignee) {
+    return this.assignees.filter(assignee => assignee.id === findAssignee.id)[0];
+  }
+
+  removeAssignee(removeAssignee) {
+    if (removeAssignee) {
+      this.assignees = this.assignees.filter(assignee => assignee.id !== removeAssignee.id);
+    }
+  }
+
+  removeAllAssignees() {
+    this.assignees = [];
+  }
+}
diff --git a/app/assets/javascripts/subbable_resource.js b/app/assets/javascripts/subbable_resource.js
deleted file mode 100644
index d8191605128..00000000000
--- a/app/assets/javascripts/subbable_resource.js
+++ /dev/null
@@ -1,51 +0,0 @@
-(() => {
-/*
-*   SubbableResource can be extended to provide a pubsub-style service for one-off REST
-*   calls. Subscribe by passing a callback or render method you will use to handle responses.
- *
-* */
-
-  class SubbableResource {
-    constructor(resourcePath) {
-      this.endpoint = resourcePath;
-
-      // TODO: Switch to axios.create
-      this.resource = $.ajax;
-      this.subscribers = [];
-    }
-
-    subscribe(callback) {
-      this.subscribers.push(callback);
-    }
-
-    publish(newResponse) {
-      const responseCopy = _.extend({}, newResponse);
-      this.subscribers.forEach((fn) => {
-        fn(responseCopy);
-      });
-      return newResponse;
-    }
-
-    get(payload) {
-      return this.resource(payload)
-        .then(data => this.publish(data));
-    }
-
-    post(payload) {
-      return this.resource(payload)
-        .then(data => this.publish(data));
-    }
-
-    put(payload) {
-      return this.resource(payload)
-        .then(data => this.publish(data));
-    }
-
-    delete(payload) {
-      return this.resource(payload)
-        .then(data => this.publish(data));
-    }
-  }
-
-  gl.SubbableResource = SubbableResource;
-})(window.gl || (window.gl = {}));
diff --git a/app/assets/javascripts/subscription_select.js b/app/assets/javascripts/subscription_select.js
index 8b25f43ffc7..0cd591c7320 100644
--- a/app/assets/javascripts/subscription_select.js
+++ b/app/assets/javascripts/subscription_select.js
@@ -19,8 +19,8 @@
               return label;
             };
           })(this),
-          clicked: function(item, $el, e) {
-            return e.preventDefault();
+          clicked: function(options) {
+            return options.e.preventDefault();
           },
           id: function(obj, el) {
             return $(el).data("id");
diff --git a/app/assets/javascripts/users_select.js b/app/assets/javascripts/users_select.js
index 68cf9ced3ef..7828bf86f60 100644
--- a/app/assets/javascripts/users_select.js
+++ b/app/assets/javascripts/users_select.js
@@ -1,6 +1,7 @@
 /* eslint-disable func-names, space-before-function-paren, one-var, no-var, prefer-rest-params, wrap-iife, quotes, max-len, one-var-declaration-per-line, vars-on-top, prefer-arrow-callback, consistent-return, comma-dangle, object-shorthand, no-shadow, no-unused-vars, no-else-return, no-self-compare, prefer-template, no-unused-expressions, no-lonely-if, yoda, prefer-spread, no-void, camelcase, no-param-reassign */
 /* global Issuable */
-/* global ListUser */
+
+import eventHub from './sidebar/event_hub';
 
 (function() {
   var bind = function(fn, me) { return function() { return fn.apply(me, arguments); }; },
@@ -54,42 +55,92 @@
           selectedIdDefault = (defaultNullUser && showNullUser) ? 0 : null;
           selectedId = $dropdown.data('selected') || selectedIdDefault;
 
-          var updateIssueBoardsIssue = function () {
-            $loading.removeClass('hidden').fadeIn();
-            gl.issueBoards.BoardsStore.detail.issue.update($dropdown.attr('data-issue-update'))
-              .then(function () {
-                $loading.fadeOut();
-              })
-              .catch(function () {
-                $loading.fadeOut();
-              });
+          const assignYourself = function () {
+            const unassignedSelected = $dropdown.closest('.selectbox')
+              .find(`input[name='${$dropdown.data('field-name')}'][value=0]`);
+
+            if (unassignedSelected) {
+              unassignedSelected.remove();
+            }
+
+            // Save current selected user to the DOM
+            const input = document.createElement('input');
+            input.type = 'hidden';
+            input.name = $dropdown.data('field-name');
+
+            const currentUserInfo = $dropdown.data('currentUserInfo');
+
+            if (currentUserInfo) {
+              input.value = currentUserInfo.id;
+              input.dataset.meta = currentUserInfo.name;
+            } else if (_this.currentUser) {
+              input.value = _this.currentUser.id;
+            }
+
+            $dropdown.before(input);
+          };
+
+          if ($block[0]) {
+            $block[0].addEventListener('assignYourself', assignYourself);
+          }
+
+          const getSelectedUserInputs = function() {
+            return $selectbox
+              .find(`input[name="${$dropdown.data('field-name')}"]`);
+          };
+
+          const getSelected = function() {
+            return getSelectedUserInputs()
+              .map((index, input) => parseInt(input.value, 10))
+              .get();
+          };
+
+          const getMultiSelectDropdownTitle = function(selectedUser, isSelected) {
+            const selectedUsers = getSelected()
+              .filter(u => u !== 0);
+
+            const firstUser = getSelectedUserInputs()
+              .map((index, input) => ({
+                name: input.dataset.meta,
+                value: parseInt(input.value, 10),
+              }))
+              .filter(u => u.id !== 0)
+              .get(0);
+
+            if (selectedUsers.length === 0) {
+              return 'Unassigned';
+            } else if (selectedUsers.length === 1) {
+              return firstUser.name;
+            } else if (isSelected) {
+              const otherSelected = selectedUsers.filter(s => s !== selectedUser.id);
+              return `${selectedUser.name} + ${otherSelected.length} more`;
+            } else {
+              return `${firstUser.name} + ${selectedUsers.length - 1} more`;
+            }
           };
 
           $('.assign-to-me-link').on('click', (e) => {
             e.preventDefault();
             $(e.currentTarget).hide();
-            const $input = $(`input[name="${$dropdown.data('field-name')}"]`);
-            $input.val(gon.current_user_id);
-            selectedId = $input.val();
-            $dropdown.find('.dropdown-toggle-text').text(gon.current_user_fullname).removeClass('is-default');
-          });
-
-          $block.on('click', '.js-assign-yourself', function(e) {
-            e.preventDefault();
 
-            if ($dropdown.hasClass('js-issue-board-sidebar')) {
-              gl.issueBoards.boardStoreIssueSet('assignee', new ListUser({
-                id: _this.currentUser.id,
-                username: _this.currentUser.username,
-                name: _this.currentUser.name,
-                avatar_url: _this.currentUser.avatar_url
-              }));
+            if ($dropdown.data('multiSelect')) {
+              assignYourself();
 
-              updateIssueBoardsIssue();
+              const currentUserInfo = $dropdown.data('currentUserInfo');
+              $dropdown.find('.dropdown-toggle-text').text(getMultiSelectDropdownTitle(currentUserInfo)).removeClass('is-default');
             } else {
-              return assignTo(_this.currentUser.id);
+              const $input = $(`input[name="${$dropdown.data('field-name')}"]`);
+              $input.val(gon.current_user_id);
+              selectedId = $input.val();
+              $dropdown.find('.dropdown-toggle-text').text(gon.current_user_fullname).removeClass('is-default');
             }
           });
+
+          $block.on('click', '.js-assign-yourself', (e) => {
+            e.preventDefault();
+            return assignTo(_this.currentUser.id);
+          });
+
           assignTo = function(selected) {
             var data;
             data = {};
@@ -97,6 +148,7 @@
             data[abilityName].assignee_id = selected != null ? selected : null;
             $loading.removeClass('hidden').fadeIn();
             $dropdown.trigger('loading.gl.dropdown');
+
             return $.ajax({
               type: 'PUT',
               dataType: 'json',
@@ -106,7 +158,6 @@
               var user;
               $dropdown.trigger('loaded.gl.dropdown');
               $loading.fadeOut();
-              $selectbox.hide();
               if (data.assignee) {
                 user = {
                   name: data.assignee.name,
@@ -133,51 +184,90 @@
               var isAuthorFilter;
               isAuthorFilter = $('.js-author-search');
               return _this.users(term, options, function(users) {
-                var anyUser, index, j, len, name, obj, showDivider;
-                if (term.length === 0) {
-                  showDivider = 0;
-                  if (firstUser) {
-                    // Move current user to the front of the list
-                    for (index = j = 0, len = users.length; j < len; index = (j += 1)) {
-                      obj = users[index];
-                      if (obj.username === firstUser) {
-                        users.splice(index, 1);
-                        users.unshift(obj);
-                        break;
-                      }
+                // GitLabDropdownFilter returns this.instance
+                // GitLabDropdownRemote returns this.options.instance
+                const glDropdown = this.instance || this.options.instance;
+                glDropdown.options.processData(term, users, callback);
+              }.bind(this));
+            },
+            processData: function(term, users, callback) {
+              let anyUser;
+              let index;
+              let j;
+              let len;
+              let name;
+              let obj;
+              let showDivider;
+              if (term.length === 0) {
+                showDivider = 0;
+                if (firstUser) {
+                  // Move current user to the front of the list
+                  for (index = j = 0, len = users.length; j < len; index = (j += 1)) {
+                    obj = users[index];
+                    if (obj.username === firstUser) {
+                      users.splice(index, 1);
+                      users.unshift(obj);
+                      break;
                     }
                   }
-                  if (showNullUser) {
-                    showDivider += 1;
-                    users.unshift({
-                      beforeDivider: true,
-                      name: 'Unassigned',
-                      id: 0
-                    });
-                  }
-                  if (showAnyUser) {
-                    showDivider += 1;
-                    name = showAnyUser;
-                    if (name === true) {
-                      name = 'Any User';
-                    }
-                    anyUser = {
-                      beforeDivider: true,
-                      name: name,
-                      id: null
-                    };
-                    users.unshift(anyUser);
+                }
+                if (showNullUser) {
+                  showDivider += 1;
+                  users.unshift({
+                    beforeDivider: true,
+                    name: 'Unassigned',
+                    id: 0
+                  });
+                }
+                if (showAnyUser) {
+                  showDivider += 1;
+                  name = showAnyUser;
+                  if (name === true) {
+                    name = 'Any User';
                   }
+                  anyUser = {
+                    beforeDivider: true,
+                    name: name,
+                    id: null
+                  };
+                  users.unshift(anyUser);
                 }
+
                 if (showDivider) {
-                  users.splice(showDivider, 0, "divider");
+                  users.splice(showDivider, 0, 'divider');
                 }
 
-                callback(users);
-                if (showMenuAbove) {
-                  $dropdown.data('glDropdown').positionMenuAbove();
+                if ($dropdown.hasClass('js-multiselect')) {
+                  const selected = getSelected().filter(i => i !== 0);
+
+                  if (selected.length > 0) {
+                    if ($dropdown.data('dropdown-header')) {
+                      showDivider += 1;
+                      users.splice(showDivider, 0, {
+                        header: $dropdown.data('dropdown-header'),
+                      });
+                    }
+
+                    const selectedUsers = users
+                      .filter(u => selected.indexOf(u.id) !== -1)
+                      .sort((a, b) => a.name > b.name);
+
+                    users = users.filter(u => selected.indexOf(u.id) === -1);
+
+                    selectedUsers.forEach((selectedUser) => {
+                      showDivider += 1;
+                      users.splice(showDivider, 0, selectedUser);
+                    });
+
+                    users.splice(showDivider + 1, 0, 'divider');
+                  }
                 }
-              });
+              }
+
+              callback(users);
+              if (showMenuAbove) {
+                $dropdown.data('glDropdown').positionMenuAbove();
+              }
             },
             filterable: true,
             filterRemote: true,
@@ -186,7 +276,22 @@
             },
             selectable: true,
             fieldName: $dropdown.data('field-name'),
-            toggleLabel: function(selected, el) {
+            toggleLabel: function(selected, el, glDropdown) {
+              const inputValue = glDropdown.filterInput.val();
+
+              if (this.multiSelect && inputValue === '') {
+                // Remove non-users from the fullData array
+                const users = glDropdown.filteredFullData();
+                const callback = glDropdown.parseData.bind(glDropdown);
+
+                // Update the data model
+                this.processData(inputValue, users, callback);
+              }
+
+              if (this.multiSelect) {
+                return getMultiSelectDropdownTitle(selected, $(el).hasClass('is-active'));
+              }
+
               if (selected && 'id' in selected && $(el).hasClass('is-active')) {
                 $dropdown.find('.dropdown-toggle-text').removeClass('is-default');
                 if (selected.text) {
@@ -200,15 +305,92 @@
               }
             },
             defaultLabel: defaultLabel,
-            inputId: 'issue_assignee_id',
             hidden: function(e) {
-              $selectbox.hide();
-              // display:block overrides the hide-collapse rule
-              return $value.css('display', '');
+              if ($dropdown.hasClass('js-multiselect')) {
+                eventHub.$emit('sidebar.saveAssignees');
+              }
+
+              if (!$dropdown.data('always-show-selectbox')) {
+                $selectbox.hide();
+
+                // Recalculate where .value is because vue might have changed it
+                $block = $selectbox.closest('.block');
+                $value = $block.find('.value');
+                // display:block overrides the hide-collapse rule
+                $value.css('display', '');
+              }
             },
+<<<<<<< HEAD
             vue: $dropdown.hasClass('js-issue-board-sidebar'),
             clicked: function(user, $el, e) {
               var isIssueIndex, isMRIndex, page, selected, isSelecting;
+=======
+            multiSelect: $dropdown.hasClass('js-multiselect'),
+            inputMeta: $dropdown.data('input-meta'),
+            clicked: function(options) {
+              const { $el, e, isMarking } = options;
+              const user = options.selectedObj;
+
+              if ($dropdown.hasClass('js-multiselect')) {
+                const isActive = $el.hasClass('is-active');
+                const previouslySelected = $dropdown.closest('.selectbox')
+                    .find("input[name='" + ($dropdown.data('field-name')) + "'][value!=0]");
+
+                // Enables support for limiting the number of users selected
+                // Automatically removes the first on the list if more users are selected
+                const maxSelect = $dropdown.data('max-select');
+                if (maxSelect) {
+                  const selected = getSelected();
+
+                  if (selected.length > maxSelect) {
+                    const firstSelectedId = selected[0];
+                    const firstSelected = $dropdown.closest('.selectbox')
+                      .find(`input[name='${$dropdown.data('field-name')}'][value=${firstSelectedId}]`);
+
+                    firstSelected.remove();
+                    eventHub.$emit('sidebar.removeAssignee', {
+                      id: firstSelectedId,
+                    });
+                  }
+                }
+
+                if (user.beforeDivider && user.name.toLowerCase() === 'unassigned') {
+                  // Unassigned selected
+                  previouslySelected.each((index, element) => {
+                    const id = parseInt(element.value, 10);
+                    element.remove();
+                  });
+                  eventHub.$emit('sidebar.removeAllAssignees');
+                } else if (isActive) {
+                  // user selected
+                  eventHub.$emit('sidebar.addAssignee', user);
+
+                  // Remove unassigned selection (if it was previously selected)
+                  const unassignedSelected = $dropdown.closest('.selectbox')
+                    .find("input[name='" + ($dropdown.data('field-name')) + "'][value=0]");
+
+                  if (unassignedSelected) {
+                    unassignedSelected.remove();
+                  }
+                } else {
+                  if (previouslySelected.length === 0) {
+                  // Select unassigned because there is no more selected users
+                    this.addInput($dropdown.data('field-name'), 0, {});
+                  }
+
+                  // User unselected
+                  eventHub.$emit('sidebar.removeAssignee', user);
+                }
+
+                if (getSelected().find(u => u === gon.current_user_id)) {
+                  $('.assign-to-me-link').hide();
+                } else {
+                  $('.assign-to-me-link').show();
+                }
+              }
+
+              var isIssueIndex, isMRIndex, page, selected;
+>>>>>>> b0a2435... Merge branch 'multiple_assignees_review_upstream' into ee_master
               page = $('body').data('page');
               isIssueIndex = page === 'projects:issues:index';
               isMRIndex = (page === page && page === 'projects:merge_requests:index');
@@ -229,6 +411,7 @@
                 return Issuable.filterResults($dropdown.closest('form'));
               } else if ($dropdown.hasClass('js-filter-submit')) {
                 return $dropdown.closest('form').submit();
+<<<<<<< HEAD
               } else if ($dropdown.hasClass('js-issue-board-sidebar')) {
                 if (user.id && isSelecting) {
                   gl.issueBoards.boardStoreIssueSet('assignee', new ListUser({
@@ -243,6 +426,9 @@
 
                 updateIssueBoardsIssue();
               } else {
+=======
+              } else if (!$dropdown.hasClass('js-multiselect')) {
+>>>>>>> b0a2435... Merge branch 'multiple_assignees_review_upstream' into ee_master
                 selected = $dropdown.closest('.selectbox').find("input[name='" + ($dropdown.data('field-name')) + "']").val();
                 return assignTo(selected);
               }
@@ -256,29 +442,54 @@
                 selectedId = parseInt($dropdown[0].dataset.selected, 10) || selectedIdDefault;
               }
               $el.find('.is-active').removeClass('is-active');
-              $el.find(`li[data-user-id="${selectedId}"] .dropdown-menu-user-link`).addClass('is-active');
+
+              function highlightSelected(id) {
+                $el.find(`li[data-user-id="${id}"] .dropdown-menu-user-link`).addClass('is-active');
+              }
+
+              if ($selectbox[0]) {
+                getSelected().forEach(selectedId => highlightSelected(selectedId));
+              } else {
+                highlightSelected(selectedId);
+              }
             },
+            updateLabel: $dropdown.data('dropdown-title'),
             renderRow: function(user) {
-              var avatar, img, listClosingTags, listWithName, listWithUserName, selected, username;
+              var avatar, img, listClosingTags, listWithName, listWithUserName, username;
               username = user.username ? "@" + user.username : "";
               avatar = user.avatar_url ? user.avatar_url : false;
-              selected = user.id === parseInt(selectedId, 10) ? "is-active" : "";
+
+              let selected = user.id === parseInt(selectedId, 10);
+
+              if (this.multiSelect) {
+                const fieldName = this.fieldName;
+                const field = $dropdown.closest('.selectbox').find("input[name='" + fieldName + "'][value='" + user.id + "']");
+
+                if (field.length) {
+                  selected = true;
+                }
+              }
+
               img = "";
               if (user.beforeDivider != null) {
-                "<li> <a href='#' class='" + selected + "'> " + user.name + " </a> </li>";
+                `<li><a href='#' class='${selected === true ? 'is-active' : ''}'>${user.name}</a></li>`;
               } else {
                 if (avatar) {
-                  img = "<img src='" + avatar + "' class='avatar avatar-inline' width='30' />";
+                  img = "<img src='" + avatar + "' class='avatar avatar-inline' width='32' />";
                 }
               }
-              // split into three parts so we can remove the username section if nessesary
-              listWithName = "<li data-user-id=" + user.id + "> <a href='#' class='dropdown-menu-user-link " + selected + "'> " + img + " <strong class='dropdown-menu-user-full-name'> " + user.name + " </strong>";
-              listWithUserName = "<span class='dropdown-menu-user-username'> " + username + " </span>";
-              listClosingTags = "</a> </li>";
-              if (username === '') {
-                listWithUserName = '';
-              }
-              return listWithName + listWithUserName + listClosingTags;
+
+              return `
+                <li data-user-id=${user.id}>
+                  <a href='#' class='dropdown-menu-user-link ${selected === true ? 'is-active' : ''}'>
+                    ${img}
+                    <strong class='dropdown-menu-user-full-name'>
+                      ${user.name}
+                    </strong>
+                    ${username ? `<span class='dropdown-menu-user-username'>${username}</span>` : ''}
+                  </a>
+                </li>
+              `;
             }
           });
         };
diff --git a/app/assets/stylesheets/framework/avatar.scss b/app/assets/stylesheets/framework/avatar.scss
index 3f5b78ed445..91c1ebd5a7d 100644
--- a/app/assets/stylesheets/framework/avatar.scss
+++ b/app/assets/stylesheets/framework/avatar.scss
@@ -93,3 +93,14 @@
     align-self: center;
   }
 }
+
+.avatar-counter {
+  background-color: $gray-darkest;
+  color: $white-light;
+  border: 1px solid $border-color;
+  border-radius: 1em;
+  font-family: $regular_font;
+  font-size: 9px;
+  line-height: 16px;
+  text-align: center;
+}
diff --git a/app/assets/stylesheets/framework/dropdowns.scss b/app/assets/stylesheets/framework/dropdowns.scss
index 73ded9f30d4..856989bccf1 100644
--- a/app/assets/stylesheets/framework/dropdowns.scss
+++ b/app/assets/stylesheets/framework/dropdowns.scss
@@ -251,11 +251,9 @@
   }
 
   .dropdown-header {
-    color: $gl-text-color;
+    color: $gl-text-color-secondary;
     font-size: 13px;
-    font-weight: 600;
     line-height: 22px;
-    text-transform: capitalize;
     padding: 0 16px;
   }
 
@@ -337,8 +335,8 @@
 .dropdown-menu-user {
   .avatar {
     float: left;
-    width: 30px;
-    height: 30px;
+    width: 2 * $gl-padding;
+    height: 2 * $gl-padding;
     margin: 0 10px 0 0;
   }
 }
@@ -381,6 +379,7 @@
 .dropdown-menu-selectable {
   a {
     padding-left: 26px;
+    position: relative;
 
     &.is-indeterminate,
     &.is-active {
@@ -406,6 +405,9 @@
 
     &.is-active::before {
       content: "\f00c";
+      position: absolute;
+      top: 50%;
+      transform: translateY(-50%);
     }
   }
 }
diff --git a/app/assets/stylesheets/framework/lists.scss b/app/assets/stylesheets/framework/lists.scss
index 15dc0aa6a52..c9a25946ffd 100644
--- a/app/assets/stylesheets/framework/lists.scss
+++ b/app/assets/stylesheets/framework/lists.scss
@@ -255,6 +255,7 @@ ul.controls {
       .avatar-inline {
         margin-left: 0;
         margin-right: 0;
+        margin-bottom: 0;
       }
     }
   }
diff --git a/app/assets/stylesheets/pages/boards.scss b/app/assets/stylesheets/pages/boards.scss
index 0be1c215959..68d7ab4bf84 100644
--- a/app/assets/stylesheets/pages/boards.scss
+++ b/app/assets/stylesheets/pages/boards.scss
@@ -207,8 +207,13 @@
     margin-bottom: 5px;
   }
 
-  &.is-active {
+  &.is-active,
+  &.is-active .card-assignee:hover a {
     background-color: $row-hover;
+
+    &:first-child:not(:only-child) {
+      box-shadow: -10px 0 10px 1px $row-hover;
+    }
   }
 
   .label {
@@ -224,7 +229,7 @@
 }
 
 .card-title {
-  margin: 0;
+  margin: 0 30px 0 0;
   font-size: 1em;
   line-height: inherit;
 
@@ -240,10 +245,69 @@
   min-height: 20px;
 
   .card-assignee {
-    margin-left: auto;
-    margin-right: 5px;
-    padding-left: 10px;
+    display: flex;
+    justify-content: flex-end;
+    position: absolute;
+    right: 15px;
     height: 20px;
+    width: 20px;
+
+    .avatar-counter {
+      display: none;
+      vertical-align: middle;
+      min-width: 20px;
+      line-height: 19px;
+      height: 20px;
+      padding-left: 2px;
+      padding-right: 2px;
+      border-radius: 2em;
+    }
+
+    img {
+      vertical-align: top;
+    }
+
+    a {
+      position: relative;
+      margin-left: -15px;
+    }
+
+    a:nth-child(1) {
+      z-index: 3;
+    }
+
+    a:nth-child(2) {
+      z-index: 2;
+    }
+
+    a:nth-child(3) {
+      z-index: 1;
+    }
+
+    a:nth-child(4) {
+      display: none;
+    }
+
+    &:hover {
+      .avatar-counter {
+        display: inline-block;
+      }
+
+      a {
+        position: static;
+        background-color: $white-light;
+        transition: background-color 0s;
+        margin-left: auto;
+
+        &:nth-child(4) {
+          display: block;
+        }
+
+        &:first-child:not(:only-child) {
+          box-shadow: -10px 0 10px 1px $white-light;
+        }
+      }
+    }
   }
 
   .avatar {
diff --git a/app/assets/stylesheets/pages/diff.scss b/app/assets/stylesheets/pages/diff.scss
index feefaad8a15..77f2638683a 100644
--- a/app/assets/stylesheets/pages/diff.scss
+++ b/app/assets/stylesheets/pages/diff.scss
@@ -570,14 +570,7 @@
 
 .diff-comments-more-count,
 .diff-notes-collapse {
-  background-color: $gray-darkest;
-  color: $white-light;
-  border: 1px solid $white-light;
-  border-radius: 1em;
-  font-family: $regular_font;
-  font-size: 9px;
-  line-height: 17px;
-  text-align: center;
+  @extend .avatar-counter;
 }
 
 .diff-notes-collapse {
diff --git a/app/assets/stylesheets/pages/issuable.scss b/app/assets/stylesheets/pages/issuable.scss
index ad6eb9f6fe0..485ea369f3d 100644
--- a/app/assets/stylesheets/pages/issuable.scss
+++ b/app/assets/stylesheets/pages/issuable.scss
@@ -95,10 +95,15 @@
 }
 
 .right-sidebar {
-  a {
+  a,
+  .btn-link {
     color: inherit;
   }
 
+  .btn-link {
+    outline: none;
+  }
+
   .issuable-header-text {
     margin-top: 7px;
   }
@@ -215,6 +220,10 @@
       }
     }
 
+    .assign-yourself .btn-link {
+      padding-left: 0;
+    }
+
     .light {
       font-weight: normal;
     }
@@ -239,6 +248,10 @@
       margin-left: 0;
     }
 
+    .assignee .user-list .avatar {
+      margin: 0;
+    }
+
     .username {
       display: block;
       margin-top: 4px;
@@ -301,6 +314,10 @@
         margin-top: 0;
       }
 
+      .sidebar-avatar-counter {
+        padding-top: 2px;
+      }
+
       .todo-undone {
         color: $gl-link-color;
       }
@@ -309,10 +326,15 @@
         display: none;
       }
 
-      .avatar:hover {
+      .avatar:hover,
+      .avatar-counter:hover {
         border-color: $issuable-sidebar-color;
       }
 
+      .avatar-counter:hover {
+        color: $issuable-sidebar-color;
+      }
+
       .btn-clipboard {
         border: none;
         color: $issuable-sidebar-color;
@@ -322,6 +344,17 @@
           color: $gl-text-color;
         }
       }
+
+      &.multiple-users {
+        display: flex;
+        justify-content: center;
+      }
+    }
+
+    .sidebar-avatar-counter {
+      width: 24px;
+      height: 24px;
+      border-radius: 12px;
     }
 
     .sidebar-collapsed-user {
@@ -332,6 +365,37 @@
     .issuable-header-btn {
       display: none;
     }
+
+    .multiple-users {
+      height: 24px;
+      margin-bottom: 17px;
+      margin-top: 4px;
+      padding-bottom: 4px;
+
+      .btn-link {
+        padding: 0;
+        border: 0;
+
+        .avatar {
+          margin: 0;
+        }
+      }
+
+      .btn-link:first-child {
+        position: absolute;
+        left: 10px;
+        z-index: 1;
+      }
+
+      .btn-link:last-child {
+        position: absolute;
+        right: 10px;
+
+        &:hover {
+          text-decoration: none;
+        }
+      }
+    }
   }
 
   a {
@@ -380,17 +444,21 @@
 }
 
 .participants-list {
+  display: flex;
+  flex-wrap: wrap;
+  justify-content: space-between;
   margin: -5px;
 }
 
+.user-list {
+  display: flex;
+  flex-wrap: wrap;
+}
+
 .participants-author {
-  display: inline-block;
+  flex-basis: 14%;
   padding: 5px;
 
-  &:nth-of-type(7n) {
-    padding-right: 0;
-  }
-
   .author_link {
     display: block;
   }
@@ -400,13 +468,39 @@
   }
 }
 
-.participants-more {
+.user-item {
+  display: inline-block;
+  padding: 5px;
+  flex-basis: 20%;
+
+  .user-link {
+    display: inline-block;
+  }
+}
+
+.participants-more,
+.user-list-more {
   margin-top: 5px;
   margin-left: 5px;
 
-  a {
+  a,
+  .btn-link {
     color: $gl-text-color-secondary;
   }
+
+  .btn-link {
+    outline: none;
+    padding: 0;
+  }
+
+  .btn-link:hover {
+    @extend a:hover;
+    text-decoration: none;
+  }
+
+  .btn-link:focus {
+    text-decoration: none;
+  }
 }
 
 .issuable-form-padding-top {
@@ -499,6 +593,19 @@
   }
 }
 
+.issuable-list li,
+.issue-info-container .controls {
+  .avatar-counter {
+    display: inline-block;
+    vertical-align: middle;
+    min-width: 16px;
+    line-height: 14px;
+    height: 16px;
+    padding-left: 2px;
+    padding-right: 2px;
+  }
+}
+
 .time_tracker {
   padding-bottom: 0;
   border-bottom: 0;
diff --git a/app/controllers/concerns/issuable_actions.rb b/app/controllers/concerns/issuable_actions.rb
index 3ccf2a9ce33..b199f18da1e 100644
--- a/app/controllers/concerns/issuable_actions.rb
+++ b/app/controllers/concerns/issuable_actions.rb
@@ -66,6 +66,7 @@ module IssuableActions
       :milestone_id,
       :state_event,
       :subscription_event,
+      assignee_ids: [],
       label_ids: [],
       add_label_ids: [],
       remove_label_ids: []
diff --git a/app/controllers/concerns/issuable_collections.rb b/app/controllers/concerns/issuable_collections.rb
index c8a501d7319..6df2c068745 100644
--- a/app/controllers/concerns/issuable_collections.rb
+++ b/app/controllers/concerns/issuable_collections.rb
@@ -43,7 +43,7 @@ module IssuableCollections
   end
 
   def issues_collection
-    issues_finder.execute.preload(:project, :author, :assignee, :labels, :milestone, project: :namespace)
+    issues_finder.execute.preload(:project, :author, :assignees, :labels, :milestone, project: :namespace)
   end
 
   def merge_requests_collection
diff --git a/app/controllers/projects/boards/issues_controller.rb b/app/controllers/projects/boards/issues_controller.rb
index 28c9646910d..da9b789d617 100644
--- a/app/controllers/projects/boards/issues_controller.rb
+++ b/app/controllers/projects/boards/issues_controller.rb
@@ -82,7 +82,7 @@ module Projects
           labels: true,
           only: [:id, :iid, :title, :confidential, :due_date, :relative_position],
           include: {
-            assignee: { only: [:id, :name, :username], methods: [:avatar_url] },
+            assignees: { only: [:id, :name, :username], methods: [:avatar_url] },
             milestone: { only: [:id, :title] }
           },
           user: current_user
diff --git a/app/controllers/projects/issues_controller.rb b/app/controllers/projects/issues_controller.rb
index af9157bfbb5..2cb38fd953d 100644
--- a/app/controllers/projects/issues_controller.rb
+++ b/app/controllers/projects/issues_controller.rb
@@ -67,7 +67,7 @@ class Projects::IssuesController < Projects::ApplicationController
 
   def new
     params[:issue] ||= ActionController::Parameters.new(
-      assignee_id: ""
+      assignee_ids: ""
     )
     build_params = issue_params.merge(
       merge_request_to_resolve_discussions_of: params[:merge_request_to_resolve_discussions_of],
@@ -150,7 +150,7 @@ class Projects::IssuesController < Projects::ApplicationController
         if @issue.valid?
           render json: @issue.to_json(methods: [:task_status, :task_status_short],
                                       include: { milestone: {},
-                                                 assignee: { only: [:name, :username], methods: [:avatar_url] },
+                                                 assignees: { only: [:id, :name, :username], methods: [:avatar_url] },
                                                  labels: { methods: :text_color } })
         else
           render json: { errors: @issue.errors.full_messages }, status: :unprocessable_entity
@@ -275,7 +275,7 @@ class Projects::IssuesController < Projects::ApplicationController
   def issue_params
     params.require(:issue).permit(
       :title, :assignee_id, :position, :description, :confidential,
-      :milestone_id, :due_date, :state_event, :task_num, :lock_version, label_ids: []
+      :milestone_id, :due_date, :state_event, :task_num, :lock_version, label_ids: [], assignee_ids: [],
     )
   end
 
diff --git a/app/finders/issuable_finder.rb b/app/finders/issuable_finder.rb
index 4cc42b88a2a..957ad875858 100644
--- a/app/finders/issuable_finder.rb
+++ b/app/finders/issuable_finder.rb
@@ -231,7 +231,7 @@ class IssuableFinder
     when 'created-by-me', 'authored'
       items.where(author_id: current_user.id)
     when 'assigned-to-me'
-      items.where(assignee_id: current_user.id)
+      items.assigned_to(current_user)
     else
       items
     end
diff --git a/app/finders/issues_finder.rb b/app/finders/issues_finder.rb
index 76715e5970d..b4c074bc69c 100644
--- a/app/finders/issues_finder.rb
+++ b/app/finders/issues_finder.rb
@@ -26,17 +26,28 @@ class IssuesFinder < IssuableFinder
     IssuesFinder.not_restricted_by_confidentiality(current_user)
   end
 
+  def by_assignee(items)
+    if assignee
+      items.assigned_to(assignee)
+    elsif no_assignee?
+      items.unassigned
+    elsif assignee_id? || assignee_username? # assignee not found
+      items.none
+    else
+      items
+    end
+  end
+
   def self.not_restricted_by_confidentiality(user)
-    return Issue.where('issues.confidential IS NULL OR issues.confidential IS FALSE') if user.blank?
+    return Issue.where('issues.confidential IS NOT TRUE') if user.blank?
 
     return Issue.all if user.admin?
 
     Issue.where('
-      issues.confidential IS NULL
-      OR issues.confidential IS FALSE
+      issues.confidential IS NOT TRUE
       OR (issues.confidential = TRUE
         AND (issues.author_id = :user_id
-          OR issues.assignee_id = :user_id
+          OR EXISTS (SELECT TRUE FROM issue_assignees WHERE user_id = :user_id AND issue_id = issues.id)
           OR issues.project_id IN(:project_ids)))',
       user_id: user.id,
       project_ids: user.authorized_projects(Gitlab::Access::REPORTER).select(:id))
diff --git a/app/helpers/form_helper.rb b/app/helpers/form_helper.rb
index 1182939f656..639a720b024 100644
--- a/app/helpers/form_helper.rb
+++ b/app/helpers/form_helper.rb
@@ -15,4 +15,37 @@ module FormHelper
         end
     end
   end
+
+  def issue_dropdown_options(issuable, has_multiple_assignees = true)
+    options = {
+      toggle_class: 'js-user-search js-assignee-search',
+      title: 'Select assignee',
+      filter: true,
+      dropdown_class: 'dropdown-menu-user dropdown-menu-selectable dropdown-menu-assignee',
+      placeholder: 'Search users',
+      data: {
+        first_user: current_user&.username,
+        null_user: true,
+        current_user: true,
+        project_id: issuable.project.try(:id),
+        field_name: "#{issuable.class.model_name.param_key}[assignee_ids][]",
+        default_label: 'Assignee',
+        'max-select': 1,
+        'dropdown-header': 'Assignee',
+      }
+    }
+
+    if has_multiple_assignees
+      options[:toggle_class] += ' js-multiselect js-save-user-data'
+      options[:title] = 'Select assignee(s)'
+      options[:data][:multi_select] = true
+      options[:data][:'input-meta'] = 'name'
+      options[:data][:'always-show-selectbox'] = true
+      options[:data][:current_user_info] = current_user.to_json(only: [:id, :name])
+      options[:data][:'dropdown-header'] = 'Assignee(s)'
+      options[:data].delete(:'max-select')
+    end
+
+    options
+  end
 end
diff --git a/app/helpers/issuables_helper.rb b/app/helpers/issuables_helper.rb
index 0b13dbf5f8d..7656929efe7 100644
--- a/app/helpers/issuables_helper.rb
+++ b/app/helpers/issuables_helper.rb
@@ -63,6 +63,16 @@ module IssuablesHelper
     end
   end
 
+  def users_dropdown_label(selected_users)
+    if selected_users.length == 0
+      "Unassigned"
+    elsif selected_users.length == 1
+      selected_users[0].name
+    else
+      "#{selected_users[0].name} + #{selected_users.length - 1} more"
+    end
+  end
+
   def user_dropdown_label(user_id, default_label)
     return default_label if user_id.nil?
     return "Unassigned" if user_id == "0"
diff --git a/app/mailers/emails/issues.rb b/app/mailers/emails/issues.rb
index d64e48f774b..0f847841295 100644
--- a/app/mailers/emails/issues.rb
+++ b/app/mailers/emails/issues.rb
@@ -11,10 +11,12 @@ module Emails
       mail_answer_thread(@issue, issue_thread_options(updated_by_user_id, recipient_id))
     end
 
-    def reassigned_issue_email(recipient_id, issue_id, previous_assignee_id, updated_by_user_id)
+    def reassigned_issue_email(recipient_id, issue_id, previous_assignee_ids, updated_by_user_id)
       setup_issue_mail(issue_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(@issue, issue_thread_options(updated_by_user_id, recipient_id))
     end
 
diff --git a/app/models/concerns/issuable.rb b/app/models/concerns/issuable.rb
index 26dbf4d9570..edf4e9e5d78 100644
--- a/app/models/concerns/issuable.rb
+++ b/app/models/concerns/issuable.rb
@@ -26,7 +26,6 @@ module Issuable
     cache_markdown_field :description, issuable_state_filter_enabled: true
 
     belongs_to :author, class_name: "User"
-    belongs_to :assignee, class_name: "User"
     belongs_to :updated_by, class_name: "User"
     belongs_to :milestone
     has_many :notes, as: :noteable, inverse_of: :noteable, dependent: :destroy do
@@ -65,11 +64,8 @@ module Issuable
     validates :title, presence: true, length: { maximum: 255 }
 
     scope :authored, ->(user) { where(author_id: user) }
-    scope :assigned_to, ->(u) { where(assignee_id: u.id)}
     scope :recent, -> { reorder(id: :desc) }
     scope :order_position_asc, -> { reorder(position: :asc) }
-    scope :assigned, -> { where("assignee_id IS NOT NULL") }
-    scope :unassigned, -> { where("assignee_id IS NULL") }
     scope :of_projects, ->(ids) { where(project_id: ids) }
     scope :of_milestones, ->(ids) { where(milestone_id: ids) }
     scope :with_milestone, ->(title) { left_joins_milestones.where(milestones: { title: title }) }
@@ -92,7 +88,6 @@ module Issuable
     attr_mentionable :description
 
     participant :author
-    participant :assignee
     participant :notes_with_associations
 
     strip_attributes :title
@@ -102,13 +97,6 @@ module Issuable
     after_save :update_assignee_cache_counts, if: :assignee_id_changed?
     after_save :record_metrics, unless: :imported?
 
-    def update_assignee_cache_counts
-      # make sure we flush the cache for both the old *and* new assignees(if they exist)
-      previous_assignee = User.find_by_id(assignee_id_was) if assignee_id_was
-      previous_assignee&.update_cache_counts
-      assignee&.update_cache_counts
-    end
-
     # We want to use optimistic lock for cases when only title or description are involved
     # http://api.rubyonrails.org/classes/ActiveRecord/Locking/Optimistic.html
     def locking_enabled?
@@ -237,10 +225,6 @@ module Issuable
     today? && created_at == updated_at
   end
 
-  def is_being_reassigned?
-    assignee_id_changed?
-  end
-
   def open?
     opened? || reopened?
   end
@@ -269,7 +253,11 @@ module Issuable
       # DEPRECATED
       repository: project.hook_attrs.slice(:name, :url, :description, :homepage)
     }
-    hook_data[:assignee] = assignee.hook_attrs if assignee
+    if self.is_a?(Issue)
+      hook_data[:assignees] = assignees.map(&:hook_attrs) if assignees.any?
+    else
+      hook_data[:assignee] = assignee.hook_attrs if assignee
+    end
 
     hook_data
   end
@@ -331,11 +319,6 @@ module Issuable
     false
   end
 
-  def assignee_or_author?(user)
-    # We're comparing IDs here so we don't need to load any associations.
-    author_id == user.id || assignee_id == user.id
-  end
-
   def record_metrics
     metrics = self.metrics || create_metrics
     metrics.record!
diff --git a/app/models/concerns/milestoneish.rb b/app/models/concerns/milestoneish.rb
index f449229864d..a3472af5c55 100644
--- a/app/models/concerns/milestoneish.rb
+++ b/app/models/concerns/milestoneish.rb
@@ -40,7 +40,7 @@ module Milestoneish
   def issues_visible_to_user(user)
     memoize_per_user(user, :issues_visible_to_user) do
       IssuesFinder.new(user, issues_finder_params)
-        .execute.where(milestone_id: milestoneish_ids)
+        .execute.includes(:assignees).where(milestone_id: milestoneish_ids)
     end
   end
 
diff --git a/app/models/global_milestone.rb b/app/models/global_milestone.rb
index 0afbca2cb32..538615130a7 100644
--- a/app/models/global_milestone.rb
+++ b/app/models/global_milestone.rb
@@ -36,7 +36,7 @@ class GlobalMilestone
     closed = count_by_state(milestones_by_state_and_title, 'closed')
     all = milestones_by_state_and_title.map { |(_, title), _| title }.uniq.count
 
-    { 
+    {
       opened: opened,
       closed: closed,
       all: all
@@ -86,7 +86,7 @@ class GlobalMilestone
   end
 
   def issues
-    @issues ||= Issue.of_milestones(milestoneish_ids).includes(:project, :assignee, :labels)
+    @issues ||= Issue.of_milestones(milestoneish_ids).includes(:project, :assignees, :labels)
   end
 
   def merge_requests
@@ -94,7 +94,7 @@ class GlobalMilestone
   end
 
   def participants
-    @participants ||= milestones.includes(:participants).map(&:participants).flatten.compact.uniq
+    @participants ||= milestones.map(&:participants).flatten.uniq
   end
 
   def labels
diff --git a/app/models/issue.rb b/app/models/issue.rb
index 78bde6820da..27e3ed9bc7f 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -24,10 +24,17 @@ class Issue < ActiveRecord::Base
 
   has_many :merge_requests_closing_issues, class_name: 'MergeRequestsClosingIssues', dependent: :delete_all
 
+  has_many :issue_assignees
+  has_many :assignees, class_name: "User", through: :issue_assignees
+
   validates :project, presence: true
 
   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 :without_due_date, -> { where(due_date: nil) }
   scope :due_before, ->(date) { where('issues.due_date < ?', date) }
   scope :due_between, ->(from_date, to_date) { where('issues.due_date >= ?', from_date).where('issues.due_date <= ?', to_date) }
@@ -37,13 +44,15 @@ class Issue < ActiveRecord::Base
 
   scope :created_after, -> (datetime) { where("created_at >= ?", datetime) }
 
-  scope :include_associations, -> { includes(:assignee, :labels, project: :namespace) }
+  scope :include_associations, -> { includes(:labels, project: :namespace) }
 
   after_save :expire_etag_cache
 
   attr_spammable :title, spam_title: true
   attr_spammable :description, spam_description: true
 
+  participant :assignees
+
   state_machine :state, initial: :opened do
     event :close do
       transition [:reopened, :opened] => :closed
@@ -63,10 +72,14 @@ class Issue < ActiveRecord::Base
   end
 
   def hook_attrs
+    assignee_ids = self.assignee_ids
+
     attrs = {
       total_time_spent: total_time_spent,
       human_total_time_spent: human_total_time_spent,
-      human_time_estimate: human_time_estimate
+      human_time_estimate: human_time_estimate,
+      assignee_ids: assignee_ids,
+      assignee_id: assignee_ids.first # This key is deprecated
     }
 
     attributes.merge!(attrs)
@@ -114,6 +127,22 @@ class Issue < ActiveRecord::Base
               "id DESC")
   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}"
@@ -248,7 +277,7 @@ class Issue < ActiveRecord::Base
       true
     elsif confidential?
       author == user ||
-        assignee == user ||
+        assignees.include?(user) ||
         project.team.member?(user, Gitlab::Access::REPORTER)
     else
       project.public? ||
diff --git a/app/models/issue_assignee.rb b/app/models/issue_assignee.rb
new file mode 100644
index 00000000000..b94b55bb1d1
--- /dev/null
+++ b/app/models/issue_assignee.rb
@@ -0,0 +1,29 @@
+class IssueAssignee < ActiveRecord::Base
+  extend Gitlab::CurrentSettings
+
+  belongs_to :issue
+  belongs_to :assignee, class_name: "User", foreign_key: :user_id
+
+  after_create :update_assignee_cache_counts
+  after_destroy :update_assignee_cache_counts
+
+  # EE-specific
+  after_create :update_elasticsearch_index
+  after_destroy :update_elasticsearch_index
+  # EE-specific
+
+  def update_assignee_cache_counts
+    assignee&.update_cache_counts
+  end
+
+  def update_elasticsearch_index
+    if current_application_settings.elasticsearch_indexing?
+      ElasticIndexerWorker.perform_async(
+        :update,
+        'Issue',
+        issue.id,
+        changed_fields: ['assignee_ids']
+      )
+    end
+  end
+end
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 12c5481cd6d..35231bab12e 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -17,6 +17,8 @@ class MergeRequest < ActiveRecord::Base
 
   has_many :merge_requests_closing_issues, class_name: 'MergeRequestsClosingIssues', dependent: :delete_all
 
+  belongs_to :assignee, class_name: "User"
+
   serialize :merge_params, Hash
 
   after_create :ensure_merge_request_diff, unless: :importing?
@@ -114,8 +116,14 @@ class MergeRequest < ActiveRecord::Base
 
   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)}
+
+  participant :assignee
 
   after_save :keep_around_commit
+  after_save :update_assignee_cache_counts, if: :assignee_id_changed?
 
   def self.reference_prefix
     '!'
@@ -177,6 +185,30 @@ class MergeRequest < ActiveRecord::Base
     work_in_progress?(title) ? title : "WIP: #{title}"
   end
 
+  def update_assignee_cache_counts
+    # make sure we flush the cache for both the old *and* new assignees(if they exist)
+    previous_assignee = User.find_by_id(assignee_id_was) if assignee_id_was
+    previous_assignee&.update_cache_counts
+    assignee&.update_cache_counts
+  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
+
+  # This method is needed for compatibility with issues to not mess view and other code
+  def assignees
+    Array(assignee)
+  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}"
diff --git a/app/models/milestone.rb b/app/models/milestone.rb
index 652b1551928..c06bfe0ccdd 100644
--- a/app/models/milestone.rb
+++ b/app/models/milestone.rb
@@ -21,7 +21,6 @@ class Milestone < ActiveRecord::Base
   has_many :issues
   has_many :labels, -> { distinct.reorder('labels.title') },  through: :issues
   has_many :merge_requests
-  has_many :participants, -> { distinct.reorder('users.name') }, through: :issues, source: :assignee
   has_many :events, as: :target, dependent: :destroy
 
   scope :active, -> { with_state(:active) }
@@ -107,6 +106,10 @@ class Milestone < ActiveRecord::Base
     end
   end
 
+  def participants
+    User.joins(assigned_issues: :milestone).where("milestones.id = ?", id)
+  end
+
   def self.sort(method)
     case method.to_s
     when 'due_date_asc'
diff --git a/app/models/user.rb b/app/models/user.rb
index 2b7ebe6c1a7..a3126cbb644 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -89,6 +89,7 @@ class User < ActiveRecord::Base
   has_many :subscriptions,            dependent: :destroy
   has_many :recent_events, -> { order "id DESC" }, foreign_key: :author_id,   class_name: "Event"
   has_many :oauth_applications, class_name: 'Doorkeeper::Application', as: :owner, dependent: :destroy
+
   has_one  :abuse_report,             dependent: :destroy, foreign_key: :user_id
   has_many :reported_abuse_reports,   dependent: :destroy, foreign_key: :reporter_id, class_name: "AbuseReport"
   has_many :spam_logs,                dependent: :destroy
@@ -99,6 +100,10 @@ class User < ActiveRecord::Base
   has_many :award_emoji,              dependent: :destroy
   has_many :triggers,                 dependent: :destroy, class_name: 'Ci::Trigger', foreign_key: :owner_id
 
+  has_many :issue_assignees
+  has_many :assigned_issues, class_name: "Issue", through: :issue_assignees, source: :issue
+  has_many :assigned_merge_requests,  dependent: :nullify, foreign_key: :assignee_id, class_name: "MergeRequest"
+
   # Issues that a user owns are expected to be moved to the "ghost" user before
   # the user is destroyed. If the user owns any issues during deletion, this
   # should be treated as an exceptional condition.
diff --git a/app/serializers/issuable_entity.rb b/app/serializers/issuable_entity.rb
index 29aecb50849..65b204d4dd2 100644
--- a/app/serializers/issuable_entity.rb
+++ b/app/serializers/issuable_entity.rb
@@ -1,7 +1,6 @@
 class IssuableEntity < Grape::Entity
   expose :id
   expose :iid
-  expose :assignee_id
   expose :author_id
   expose :description
   expose :lock_version
diff --git a/app/serializers/issue_entity.rb b/app/serializers/issue_entity.rb
index 6429159ebe1..bc4f68710b2 100644
--- a/app/serializers/issue_entity.rb
+++ b/app/serializers/issue_entity.rb
@@ -1,6 +1,7 @@
 class IssueEntity < IssuableEntity
   expose :branch_name
   expose :confidential
+  expose :assignees, using: API::Entities::UserBasic
   expose :due_date
   expose :moved_to_id
   expose :project_id
diff --git a/app/serializers/merge_request_entity.rb b/app/serializers/merge_request_entity.rb
index 5f80ab397a9..453ba52b892 100644
--- a/app/serializers/merge_request_entity.rb
+++ b/app/serializers/merge_request_entity.rb
@@ -1,4 +1,5 @@
 class MergeRequestEntity < IssuableEntity
+  expose :assignee_id
   expose :in_progress_merge_commit_sha
   expose :locked_at
   expose :merge_commit_sha
diff --git a/app/services/issuable/bulk_update_service.rb b/app/services/issuable/bulk_update_service.rb
index 60891cbb255..40ff9b8b867 100644
--- a/app/services/issuable/bulk_update_service.rb
+++ b/app/services/issuable/bulk_update_service.rb
@@ -7,10 +7,14 @@ module Issuable
       ids = params.delete(:issuable_ids).split(",")
       items = model_class.where(id: ids)
 
-      %i(state_event milestone_id assignee_id add_label_ids remove_label_ids subscription_event).each do |key|
+      %i(state_event milestone_id assignee_id assignee_ids add_label_ids remove_label_ids subscription_event).each do |key|
         params.delete(key) unless params[key].present?
       end
 
+      if params[:assignee_ids] == [IssuableFinder::NONE.to_s]
+        params[:assignee_ids] = []
+      end
+
       items.each do |issuable|
         next unless can?(current_user, :"update_#{type}", issuable)
 
diff --git a/app/services/issuable_base_service.rb b/app/services/issuable_base_service.rb
index b071a398481..6c2777a8d2d 100644
--- a/app/services/issuable_base_service.rb
+++ b/app/services/issuable_base_service.rb
@@ -1,11 +1,6 @@
 class IssuableBaseService < BaseService
   private
 
-  def create_assignee_note(issuable)
-    SystemNoteService.change_assignee(
-      issuable, issuable.project, current_user, issuable.assignee)
-  end
-
   def create_milestone_note(issuable)
     SystemNoteService.change_milestone(
       issuable, issuable.project, current_user, issuable.milestone)
@@ -53,6 +48,7 @@ class IssuableBaseService < BaseService
       params.delete(:add_label_ids)
       params.delete(:remove_label_ids)
       params.delete(:label_ids)
+      params.delete(:assignee_ids)
       params.delete(:assignee_id)
       params.delete(:due_date)
     end
@@ -77,7 +73,7 @@ class IssuableBaseService < BaseService
   def assignee_can_read?(issuable, assignee_id)
     new_assignee = User.find_by_id(assignee_id)
 
-    return false unless new_assignee.present?
+    return false unless new_assignee
 
     ability_name = :"read_#{issuable.to_ability_name}"
     resource     = issuable.persisted? ? issuable : project
@@ -207,6 +203,7 @@ class IssuableBaseService < BaseService
     filter_params(issuable)
     old_labels = issuable.labels.to_a
     old_mentioned_users = issuable.mentioned_users.to_a
+    old_assignees = issuable.assignees.to_a
 
     label_ids = process_label_ids(params, existing_label_ids: issuable.label_ids)
     params[:label_ids] = label_ids if labels_changing?(issuable.label_ids, label_ids)
@@ -222,7 +219,13 @@ class IssuableBaseService < BaseService
           handle_common_system_notes(issuable, old_labels: old_labels)
         end
 
-        handle_changes(issuable, old_labels: old_labels, old_mentioned_users: old_mentioned_users)
+        handle_changes(
+          issuable,
+          old_labels: old_labels,
+          old_mentioned_users: old_mentioned_users,
+          old_assignees: old_assignees
+        )
+
         after_update(issuable)
         issuable.create_new_cross_references!(current_user)
         execute_hooks(issuable, 'update')
@@ -272,7 +275,7 @@ class IssuableBaseService < BaseService
     end
   end
 
-  def has_changes?(issuable, old_labels: [])
+  def has_changes?(issuable, old_labels: [], old_assignees: [])
     valid_attrs = [:title, :description, :assignee_id, :milestone_id, :target_branch]
 
     attrs_changed = valid_attrs.any? do |attr|
@@ -281,7 +284,9 @@ class IssuableBaseService < BaseService
 
     labels_changed = issuable.labels != old_labels
 
-    attrs_changed || labels_changed
+    assignees_changed = issuable.assignees != old_assignees
+
+    attrs_changed || labels_changed || assignees_changed
   end
 
   def handle_common_system_notes(issuable, old_labels: [])
diff --git a/app/services/issues/base_service.rb b/app/services/issues/base_service.rb
index ee1b40db718..eedbfa724ff 100644
--- a/app/services/issues/base_service.rb
+++ b/app/services/issues/base_service.rb
@@ -9,11 +9,30 @@ module Issues
 
     private
 
+    def create_assignee_note(issue, old_assignees)
+      SystemNoteService.change_issue_assignees(
+        issue, issue.project, current_user, old_assignees)
+    end
+
     def execute_hooks(issue, action = 'open')
       issue_data  = hook_data(issue, action)
       hooks_scope = issue.confidential? ? :confidential_issue_hooks : :issue_hooks
       issue.project.execute_hooks(issue_data, hooks_scope)
       issue.project.execute_services(issue_data, hooks_scope)
     end
+
+    def filter_assignee(issuable)
+      return if params[:assignee_ids].blank?
+
+      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
   end
 end
diff --git a/app/services/issues/update_service.rb b/app/services/issues/update_service.rb
index b7fe5cb168b..cd9f9a4a16e 100644
--- a/app/services/issues/update_service.rb
+++ b/app/services/issues/update_service.rb
@@ -12,8 +12,12 @@ module Issues
       spam_check(issue, current_user)
     end
 
-    def handle_changes(issue, old_labels: [], old_mentioned_users: [])
-      if has_changes?(issue, old_labels: old_labels)
+    def handle_changes(issue, options)
+      old_labels = options[:old_labels] || []
+      old_mentioned_users = options[:old_mentioned_users] || []
+      old_assignees = options[:old_assignees] || []
+
+      if has_changes?(issue, old_labels: old_labels, old_assignees: old_assignees)
         todo_service.mark_pending_todos_as_done(issue, current_user)
       end
 
@@ -26,9 +30,9 @@ module Issues
         create_milestone_note(issue)
       end
 
-      if issue.previous_changes.include?('assignee_id')
-        create_assignee_note(issue)
-        notification_service.reassigned_issue(issue, current_user)
+      if issue.assignees != old_assignees
+        create_assignee_note(issue, old_assignees)
+        notification_service.reassigned_issue(issue, current_user, old_assignees)
         todo_service.reassigned_issue(issue, current_user)
       end
 
diff --git a/app/services/merge_requests/assign_issues_service.rb b/app/services/merge_requests/assign_issues_service.rb
index 066efa1acc3..8c6c4841020 100644
--- a/app/services/merge_requests/assign_issues_service.rb
+++ b/app/services/merge_requests/assign_issues_service.rb
@@ -4,7 +4,7 @@ module MergeRequests
       @assignable_issues ||= begin
         if current_user == merge_request.author
           closes_issues.select do |issue|
-            !issue.is_a?(ExternalIssue) && !issue.assignee_id? && can?(current_user, :admin_issue, issue)
+            !issue.is_a?(ExternalIssue) && !issue.assignees.present? && can?(current_user, :admin_issue, issue)
           end
         else
           []
@@ -14,7 +14,7 @@ module MergeRequests
 
     def execute
       assignable_issues.each do |issue|
-        Issues::UpdateService.new(issue.project, current_user, assignee_id: current_user.id).execute(issue)
+        Issues::UpdateService.new(issue.project, current_user, assignee_ids: [current_user.id]).execute(issue)
       end
 
       {
diff --git a/app/services/merge_requests/base_service.rb b/app/services/merge_requests/base_service.rb
index 582d5c47b66..3542a41ac83 100644
--- a/app/services/merge_requests/base_service.rb
+++ b/app/services/merge_requests/base_service.rb
@@ -38,6 +38,11 @@ module MergeRequests
 
     private
 
+    def create_assignee_note(merge_request)
+      SystemNoteService.change_assignee(
+        merge_request, merge_request.project, current_user, merge_request.assignee)
+    end
+
     # Returns all origin and fork merge requests from `@project` satisfying passed arguments.
     def merge_requests_for(source_branch, mr_states: [:opened, :reopened])
       MergeRequest
diff --git a/app/services/merge_requests/update_service.rb b/app/services/merge_requests/update_service.rb
index ab7fcf3b6e2..5c843a258fb 100644
--- a/app/services/merge_requests/update_service.rb
+++ b/app/services/merge_requests/update_service.rb
@@ -21,7 +21,10 @@ module MergeRequests
       update(merge_request)
     end
 
-    def handle_changes(merge_request, old_labels: [], old_mentioned_users: [])
+    def handle_changes(merge_request, options)
+      old_labels = options[:old_labels] || []
+      old_mentioned_users = options[:old_mentioned_users] || []
+
       if has_changes?(merge_request, old_labels: old_labels)
         todo_service.mark_pending_todos_as_done(merge_request, current_user)
       end
diff --git a/app/services/notification_recipient_service.rb b/app/services/notification_recipient_service.rb
index 8bb995158de..988bd0a7cdb 100644
--- a/app/services/notification_recipient_service.rb
+++ b/app/services/notification_recipient_service.rb
@@ -19,9 +19,14 @@ class NotificationRecipientService
     # Re-assign is considered as a mention of the new assignee so we add the
     # new assignee to the list of recipients after we rejected users with
     # the "on mention" notification level
-    if [:reassign_merge_request, :reassign_issue].include?(custom_action)
+    case custom_action
+    when :reassign_merge_request
       recipients << previous_assignee if previous_assignee
       recipients << target.assignee
+    when :reassign_issue
+      previous_assignees = Array(previous_assignee)
+      recipients.concat(previous_assignees)
+      recipients.concat(target.assignees)
     end
 
     recipients = reject_muted_users(recipients)
diff --git a/app/services/notification_service.rb b/app/services/notification_service.rb
index 6b186263bd1..fe9f5ae2b33 100644
--- a/app/services/notification_service.rb
+++ b/app/services/notification_service.rb
@@ -66,8 +66,23 @@ class NotificationService
   #  * issue new assignee if their notification level is not Disabled
   #  * users with custom level checked with "reassign issue"
   #
-  def reassigned_issue(issue, current_user)
-    reassign_resource_email(issue, issue.project, current_user, :reassigned_issue_email)
+  def reassigned_issue(issue, current_user, previous_assignees = [])
+    recipients = NotificationRecipientService.new(issue.project).build_recipients(
+      issue,
+      current_user,
+      action: "reassign",
+      previous_assignee: previous_assignees
+    )
+
+    recipients.each do |recipient|
+      mailer.send(
+        :reassigned_issue_email,
+        recipient.id,
+        issue.id,
+        previous_assignees.map(&:id),
+        current_user.id
+      ).deliver_later
+    end
   end
 
   # When we add labels to an issue we should send an email to:
@@ -367,10 +382,10 @@ class NotificationService
   end
 
   def previous_record(object, attribute)
-    if object && attribute
-      if object.previous_changes.include?(attribute)
-        object.previous_changes[attribute].first
-      end
+    return unless object && attribute
+
+    if object.previous_changes.include?(attribute)
+      object.previous_changes[attribute].first
     end
   end
 end
diff --git a/app/services/slash_commands/interpret_service.rb b/app/services/slash_commands/interpret_service.rb
index 6aeebc26685..dc2ee695384 100644
--- a/app/services/slash_commands/interpret_service.rb
+++ b/app/services/slash_commands/interpret_service.rb
@@ -88,20 +88,33 @@ module SlashCommands
       current_user.can?(:"admin_#{issuable.to_ability_name}", project)
     end
     command :assign do |assignee_param|
-      user = extract_references(assignee_param, :user).first
-      user ||= User.find_by(username: assignee_param)
+      user_ids = extract_references(assignee_param, :user).map(&:id)
 
-      @updates[:assignee_id] = user.id if user
+      if user_ids.empty?
+        user_ids = User.where(username: assignee_param.split(' ').map(&:strip)).pluck(:id)
+      end
+
+      next if user_ids.empty?
+
+      if issuable.is_a?(Issue)
+        @updates[:assignee_ids] = user_ids
+      else
+        @updates[:assignee_id] = user_ids.last
+      end
     end
 
     desc 'Remove assignee'
     condition do
       issuable.persisted? &&
-        issuable.assignee_id? &&
+        issuable.assignees.any? &&
         current_user.can?(:"admin_#{issuable.to_ability_name}", project)
     end
     command :unassign do
-      @updates[:assignee_id] = nil
+      if issuable.is_a?(Issue)
+        @updates[:assignee_ids] = []
+      else
+        @updates[:assignee_id] = nil
+      end
     end
 
     desc 'Set milestone'
diff --git a/app/services/system_note_service.rb b/app/services/system_note_service.rb
index c9e25c7aaa2..898c69b3f8c 100644
--- a/app/services/system_note_service.rb
+++ b/app/services/system_note_service.rb
@@ -49,6 +49,44 @@ module SystemNoteService
     create_note(NoteSummary.new(noteable, project, author, body, action: 'assignee'))
   end
 
+  # Called when the assignees of an Issue is changed or removed
+  #
+  # issue - Issue object
+  # project  - Project owning noteable
+  # author   - User performing the change
+  # assignees - Users being assigned, or nil
+  #
+  # Example Note text:
+  #
+  #   "removed all assignees"
+  #
+  #   "assigned to @user1 additionally to @user2"
+  #
+  #   "assigned to @user1, @user2 and @user3 and unassigned from @user4 and @user5"
+  #
+  #   "assigned to @user1 and @user2"
+  #
+  # Returns the created Note object
+  def change_issue_assignees(issue, project, author, old_assignees)
+    body =
+      if issue.assignees.any? && old_assignees.any?
+        unassigned_users = old_assignees - issue.assignees
+        added_users = issue.assignees.to_a - old_assignees
+
+        text_parts = []
+        text_parts << "assigned to #{added_users.map(&:to_reference).to_sentence}" if added_users.any?
+        text_parts << "unassigned #{unassigned_users.map(&:to_reference).to_sentence}" if unassigned_users.any?
+
+        text_parts.join(' and ')
+      elsif old_assignees.any?
+        "removed all assignees"
+      elsif issue.assignees.any?
+        "assigned to #{issue.assignees.map(&:to_reference).to_sentence}"
+      end
+
+    create_note(noteable: issue, project: project, author: author, note: body)
+  end
+
   # Called when one or more labels on a Noteable are added and/or removed
   #
   # noteable       - Noteable object
diff --git a/app/services/todo_service.rb b/app/services/todo_service.rb
index 8ae61694b50..322c6286365 100644
--- a/app/services/todo_service.rb
+++ b/app/services/todo_service.rb
@@ -251,9 +251,9 @@ class TodoService
   end
 
   def create_assignment_todo(issuable, author)
-    if issuable.assignee
+    if issuable.assignees.any?
       attributes = attributes_for_todo(issuable.project, issuable, author, Todo::ASSIGNED)
-      create_todos(issuable.assignee, attributes)
+      create_todos(issuable.assignees, attributes)
     end
   end
 
diff --git a/app/views/issues/_issue.atom.builder b/app/views/issues/_issue.atom.builder
index 23a88448055..9ec765c45f9 100644
--- a/app/views/issues/_issue.atom.builder
+++ b/app/views/issues/_issue.atom.builder
@@ -23,10 +23,12 @@ xml.entry do
     end
   end
 
-  if issue.assignee
-    xml.assignee do
-      xml.name issue.assignee.name
-      xml.email issue.assignee_public_email
+  if issue.assignees.any?
+    xml.assignees do
+      issue.assignees.each do |assignee|
+        xml.name assignee.name
+        xml.email assignee.public_email
+      end
     end
   end
 end
diff --git a/app/views/notify/_reassigned_issuable_email.text.erb b/app/views/notify/_reassigned_issuable_email.text.erb
deleted file mode 100644
index daf20a226dd..00000000000
--- a/app/views/notify/_reassigned_issuable_email.text.erb
+++ /dev/null
@@ -1,6 +0,0 @@
-Reassigned <%= issuable.class.model_name.human.titleize %> <%= issuable.iid %>
-
-<%= url_for([issuable.project.namespace.becomes(Namespace), issuable.project, issuable, { only_path: false }]) %>
-
-Assignee changed <%= "from #{@previous_assignee.name}" if @previous_assignee -%>
- to <%= "#{issuable.assignee_id ? issuable.assignee_name : 'Unassigned'}" %>
diff --git a/app/views/notify/new_issue_email.html.haml b/app/views/notify/new_issue_email.html.haml
index c762578971a..eb5157ccac9 100644
--- a/app/views/notify/new_issue_email.html.haml
+++ b/app/views/notify/new_issue_email.html.haml
@@ -2,9 +2,9 @@
   %p.details
     #{link_to @issue.author_name, user_url(@issue.author)} created an issue:
 
-- if @issue.assignee_id.present?
+- if @issue.assignees.any?
   %p
-    Assignee: #{@issue.assignee_name}
+    Assignee: #{@issue.assignee_list}
 
 - 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 ca5c2f2688c..13f1ac08e94 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(namespace_project_issue_url(@issue.project.namespace, @issue.project, @issue)) %>
 Author:    <%= @issue.author_name %>
-Assignee:  <%= @issue.assignee_name %>
+Assignee:  <%= @issue.assignee_list %>
 
 <%= @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 457e94b4800..f19ac3adfc7 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(namespace_project_issue_url(@issue.project.namespace, @issue.project, @issue)) %>
 Author:    <%= @issue.author_name %>
-Assignee:  <%= @issue.assignee_name %>
+Assignee:  <%= @issue.assignee_list %>
 
 <%= @issue.description %>
diff --git a/app/views/notify/reassigned_issue_email.html.haml b/app/views/notify/reassigned_issue_email.html.haml
index 498ba8b8365..ee2f40e1683 100644
--- a/app/views/notify/reassigned_issue_email.html.haml
+++ b/app/views/notify/reassigned_issue_email.html.haml
@@ -1 +1,10 @@
-= render 'reassigned_issuable_email', issuable: @issue
+%p
+  Assignee changed
+  - if @previous_assignees.any?
+    from
+    %strong= @previous_assignees.map(&:name).to_sentence
+  to
+  - if @issue.assignees.any?
+    %strong= @issue.assignee_list
+  - else
+    %strong Unassigned
diff --git a/app/views/notify/reassigned_issue_email.text.erb b/app/views/notify/reassigned_issue_email.text.erb
index 710253be984..6c357f1074a 100644
--- a/app/views/notify/reassigned_issue_email.text.erb
+++ b/app/views/notify/reassigned_issue_email.text.erb
@@ -1 +1,6 @@
-<%= render 'reassigned_issuable_email', issuable: @issue %>
+Reassigned Issue <%= @issue.iid %>
+
+<%= url_for([@issue.project.namespace.becomes(Namespace), @issue.project, @issue, { only_path: false }]) %>
+
+Assignee changed <%= "from #{@previous_assignees.map(&:name).to_sentence}" if @previous_assignees.any? -%>
+ to <%= "#{@issue.assignees.any? ? @issue.assignee_list : 'Unassigned'}" %>
diff --git a/app/views/notify/reassigned_merge_request_email.html.haml b/app/views/notify/reassigned_merge_request_email.html.haml
index 2a650130f59..841df872857 100644
--- a/app/views/notify/reassigned_merge_request_email.html.haml
+++ b/app/views/notify/reassigned_merge_request_email.html.haml
@@ -1 +1,9 @@
-= render 'reassigned_issuable_email', issuable: @merge_request
+Reassigned Merge Request #{ @merge_request.iid }
+
+= url_for([@merge_request.project.namespace.becomes(Namespace), @merge_request.project, @merge_request, { only_path: false }])
+
+Assignee changed
+- if @previous_assignee
+  from #{@previous_assignee.name}
+to
+= @merge_request.assignee_id ? @merge_request.assignee_name : 'Unassigned'
diff --git a/app/views/notify/reassigned_merge_request_email.text.erb b/app/views/notify/reassigned_merge_request_email.text.erb
index b5b4f1ff99a..998a40fefde 100644
--- a/app/views/notify/reassigned_merge_request_email.text.erb
+++ b/app/views/notify/reassigned_merge_request_email.text.erb
@@ -1 +1,6 @@
-<%= render 'reassigned_issuable_email', issuable: @merge_request %>
+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 #{@previous_assignee.name}" if @previous_assignee -%>
+ to <%= "#{@merge_request.assignee_id ? @merge_request.assignee_name : 'Unassigned'}" %>
diff --git a/app/views/projects/boards/components/sidebar/_assignee.html.haml b/app/views/projects/boards/components/sidebar/_assignee.html.haml
index 0f424334521..96f7f12b1d7 100644
--- a/app/views/projects/boards/components/sidebar/_assignee.html.haml
+++ b/app/views/projects/boards/components/sidebar/_assignee.html.haml
@@ -1,40 +1,32 @@
-.block.assignee
-  .title.hide-collapsed
-    Assignee
-    - if can?(current_user, :admin_issue, @project)
-      = icon("spinner spin", class: "block-loading")
-      = link_to "Edit", "#", class: "edit-link pull-right"
-  .value.hide-collapsed
-    %span.assign-yourself.no-value{ "v-if" => "!issue.assignee" }
-      No assignee
-      - if can?(current_user, :admin_issue, @project)
-        \-
-        %a.js-assign-yourself{ href: "#" }
-          assign yourself
-    %a.author_link.bold{ ":href" => "'#{root_url}' + issue.assignee.username",
-      "v-if" => "issue.assignee" }
-      %img.avatar.avatar-inline.s32{ ":src" => "issue.assignee.avatar",
-        width: "32", alt: "Avatar" }
-      %span.author
-        {{ issue.assignee.name }}
-      %span.username
-        = precede "@" do
-          {{ issue.assignee.username }}
+.block.assignee{ ref: "assigneeBlock" }
+  %template{ "v-if" => "issue.assignees" }
+    %assignee-title{ ":number-of-assignees" => "issue.assignees.length",
+      ":loading" => "loadingAssignees",
+      ":editable" => can?(current_user, :admin_issue, @project) }
+    %assignees.value{ "root-path" => "#{root_url}",
+      ":users" => "issue.assignees",
+      ":editable" => can?(current_user, :admin_issue, @project),
+      "@assign-self" => "assignSelf" }
+
   - if can?(current_user, :admin_issue, @project)
     .selectbox.hide-collapsed
       %input{ type: "hidden",
-        name: "issue[assignee_id]",
-        id: "issue_assignee_id",
-        ":value" => "issue.assignee.id",
-        "v-if" => "issue.assignee" }
+        name: "issue[assignee_ids][]",
+        ":value" => "assignee.id",
+        "v-if" => "issue.assignees",
+        "v-for" => "assignee in issue.assignees" }
       .dropdown
+<<<<<<< HEAD
         %button.dropdown-menu-toggle.js-user-search.js-author-search.js-issue-board-sidebar{ type: "button", data: { toggle: "dropdown", field_name: "issue[assignee_id]", first_user: (current_user.username if current_user), current_user: "true", project_id: @project.id, null_user: "true", null_user_default: "true" },
+=======
+        %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: { toggle: "dropdown", field_name: "issue[assignee_ids][]", first_user: (current_user.username if current_user), current_user: "true", project_id: @project.id, null_user: "true", multi_select: "true", dropdown: { header: 'Assignee(s)'} },
+>>>>>>> b0a2435... Merge branch 'multiple_assignees_review_upstream' into ee_master
           ":data-issuable-id" => "issue.id",
           ":data-selected" => "assigneeId",
           ":data-issue-update" => "'#{namespace_project_issues_path(@project.namespace, @project)}/' + issue.id + '.json'" }
-          Select assignee
+          Select assignee(s)
           = icon("chevron-down")
-        .dropdown-menu.dropdown-menu-user.dropdown-menu-selectable.dropdown-menu-author
+        .dropdown-menu.dropdown-select.dropdown-menu-user.dropdown-menu-selectable.dropdown-menu-author
           = dropdown_title("Assign to")
           = dropdown_filter("Search users")
           = dropdown_content
diff --git a/app/views/projects/issues/_issue.html.haml b/app/views/projects/issues/_issue.html.haml
index 0e3902c066a..c184e0e0022 100644
--- a/app/views/projects/issues/_issue.html.haml
+++ b/app/views/projects/issues/_issue.html.haml
@@ -13,9 +13,9 @@
             %li
               CLOSED
 
-          - if issue.assignee
+          - if issue.assignees.any?
             %li
-              = link_to_member(@project, issue.assignee, name: false, title: "Assigned to :name")
+              = render 'shared/issuable/assignees', project: @project, issue: issue
 
           = render 'shared/issuable_meta_data', issuable: issue
 
diff --git a/app/views/shared/issuable/_assignees.html.haml b/app/views/shared/issuable/_assignees.html.haml
new file mode 100644
index 00000000000..36bbb1148d4
--- /dev/null
+++ b/app/views/shared/issuable/_assignees.html.haml
@@ -0,0 +1,15 @@
+- max_render = 3
+- max = [max_render, issue.assignees.length].min
+
+- issue.assignees.each_with_index do |assignee, index|
+  - if index < max
+    = link_to_member(@project, assignee, name: false, title: "Assigned to :name")
+
+- if issue.assignees.length > max_render
+  - counter = issue.assignees.length - max_render
+
+  %span{ class: 'avatar-counter has-tooltip', data: { container: 'body', placement: 'bottom', 'line-type' => 'old', 'original-title' => "+#{counter} more assignees" } }
+    - if counter < 99
+      = "+#{counter}"
+    - else
+      99+
diff --git a/app/views/shared/issuable/_participants.html.haml b/app/views/shared/issuable/_participants.html.haml
index 171da899937..db407363a09 100644
--- a/app/views/shared/issuable/_participants.html.haml
+++ b/app/views/shared/issuable/_participants.html.haml
@@ -12,9 +12,9 @@
     - participants.each do |participant|
       .participants-author.js-participants-author
         = link_to_member(@project, participant, name: false, size: 24)
-    - if participants_extra > 0
-      .participants-more
-        %a.js-participants-more{ href: "#", data: { original_text: "+ #{participants_size - 7} more", less_text: "- show less" } }
-          + #{participants_extra} more
+  - if participants_extra > 0
+    .hide-collapsed.participants-more
+      %a.js-participants-more{ href: "#", data: { original_text: "+ #{participants_size - 7} more", less_text: "- show less" } }
+        + #{participants_extra} more
 :javascript
   IssuableContext.prototype.PARTICIPANTS_ROW_COUNT = #{participants_row};
diff --git a/app/views/shared/issuable/_search_bar.html.haml b/app/views/shared/issuable/_search_bar.html.haml
index b6fce5e3cd4..dd727f1abfb 100644
--- a/app/views/shared/issuable/_search_bar.html.haml
+++ b/app/views/shared/issuable/_search_bar.html.haml
@@ -124,8 +124,17 @@
                 %li
                   %a{ href: "#", data: { id: "close" } } Closed
           .filter-item.inline
+            - if type == :issues
+              - field_name = "update[assignee_ids][]"
+            - else
+              - field_name = "update[assignee_id]"
+
             = dropdown_tag("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",
+<<<<<<< HEAD
               placeholder: "Search authors", data: { first_user: (current_user.username if current_user), null_user: true, current_user: true, project_id: @project.id, field_name: "update[assignee_id]", default_label: "Assignee" } })
+=======
+              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 } })
+>>>>>>> b0a2435... Merge branch 'multiple_assignees_review_upstream' into ee_master
           .filter-item.inline
             = dropdown_tag("Milestone", options: { title: "Assign milestone", toggle_class: 'js-milestone-select js-extra-options js-filter-submit js-filter-bulk-update', filter: true, dropdown_class: "dropdown-menu-selectable dropdown-menu-milestone", placeholder: "Search milestones", data: { show_no: true, field_name: "update[milestone_id]", project_id: @project.id, milestones: namespace_project_milestones_path(@project.namespace, @project, :json), use_id: true, default_label: "Milestone" } })
           .filter-item.inline.labels-filter
diff --git a/app/views/shared/issuable/_sidebar.html.haml b/app/views/shared/issuable/_sidebar.html.haml
index bc638e994f3..9bf73c87c88 100644
--- a/app/views/shared/issuable/_sidebar.html.haml
+++ b/app/views/shared/issuable/_sidebar.html.haml
@@ -1,10 +1,10 @@
 - todo = issuable_todo(issuable)
 - content_for :page_specific_javascripts do
   = page_specific_javascript_bundle_tag('common_vue')
-  = page_specific_javascript_bundle_tag('issuable')
+  = page_specific_javascript_bundle_tag('sidebar')
 
 %aside.right-sidebar.js-right-sidebar{ data: { "offset-top" => "102", "spy" => "affix" }, class: sidebar_gutter_collapsed_class, 'aria-live' => 'polite' }
-  .issuable-sidebar
+  .issuable-sidebar{ data: { endpoint: "#{issuable_json_path(issuable)}" } }
     - can_edit_issuable = can?(current_user, :"admin_#{issuable.to_ability_name}", @project)
     .block.issuable-sidebar-header
       - if current_user
@@ -20,36 +20,59 @@
         .block.todo.hide-expanded
           = render "shared/issuable/sidebar_todo", todo: todo, issuable: issuable, is_collapsed: true
       .block.assignee
-        .sidebar-collapsed-icon.sidebar-collapsed-user{ data: { toggle: "tooltip", placement: "left", container: "body" }, title: (issuable.assignee.name if issuable.assignee) }
-          - if issuable.assignee
-            = link_to_member(@project, issuable.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: 'edit-link pull-right'
-        .value.hide-collapsed
-          - if issuable.assignee
-            = link_to_member(@project, issuable.assignee, size: 32, extra_class: 'bold') do
-              - if issuable.instance_of?(MergeRequest) && !issuable.can_be_merged_by?(issuable.assignee)
-                %span.pull-right.cannot-be-merged{ data: { toggle: 'tooltip', placement: 'left' }, title: 'Not allowed to merge' }
-                  = icon('exclamation-triangle', 'aria-hidden': 'true')
-              %span.username
-                = issuable.assignee.to_reference
-          - else
-            %span.assign-yourself.no-value
-              No assignee
-              - if can_edit_issuable
-                \-
-                %a.js-assign-yourself{ href: '#' }
-                  assign yourself
+        - if issuable.instance_of?(Issue)
+          #js-vue-sidebar-assignees{ data: { field: "#{issuable.to_ability_name}[assignee_ids]" } }
+        - else
+          .sidebar-collapsed-icon.sidebar-collapsed-user{ data: { toggle: "tooltip", placement: "left", container: "body" }, title: (issuable.assignee.name if issuable.assignee) }
+            - if issuable.assignee
+              = link_to_member(@project, issuable.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: 'edit-link pull-right'
+          .value.hide-collapsed
+            - if issuable.assignee
+              = link_to_member(@project, issuable.assignee, size: 32, extra_class: 'bold') do
+                - if issuable.instance_of?(MergeRequest) && !issuable.can_be_merged_by?(issuable.assignee)
+                  %span.pull-right.cannot-be-merged{ data: { toggle: 'tooltip', placement: 'left' }, title: 'Not allowed to merge' }
+                    = icon('exclamation-triangle', 'aria-hidden': 'true')
+                %span.username
+                  = issuable.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
+<<<<<<< HEAD
           = f.hidden_field 'assignee_id', value: issuable.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 if @project), author_id: issuable.author_id, field_name: "#{issuable.to_ability_name}[assignee_id]", issue_update: issuable_json_path(issuable), ability_name: issuable.to_ability_name, null_user: true, null_user_default: true, selected: issuable.assignee_id } })
+=======
+          - issuable.assignees.each do |assignee|
+            = hidden_field_tag "#{issuable.to_ability_name}[assignee_ids][]", assignee.id, id: nil
+>>>>>>> b0a2435... Merge branch 'multiple_assignees_review_upstream' into ee_master
 
+          - 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 if @project), author_id: issuable.author_id, field_name: "#{issuable.to_ability_name}[assignee_id]", issue_update: issuable_json_path(issuable), ability_name: issuable.to_ability_name, null_user: true } }
+
+          - if issuable.instance_of?(Issue)
+            - if issuable.assignees.length == 0
+              = hidden_field_tag "#{issuable.to_ability_name}[assignee_ids][]", 0, id: nil
+            - title = 'Select assignee(s)'
+            - options[:toggle_class] += ' js-multiselect js-save-user-data'
+            - options[:data][:field_name] = "#{issuable.to_ability_name}[assignee_ids][]"
+            - options[:data][:multi_select] = true
+            - options[:data]['dropdown-title'] = title
+            - options[:data]['dropdown-header'] = 'Assignee(s)'
+          - else
+            - title = 'Select assignee'
+
+          = dropdown_tag(title, options: options)
       .block.milestone
         .sidebar-collapsed-icon
           = icon('clock-o', 'aria-hidden': 'true')
@@ -75,11 +98,10 @@
           = dropdown_tag('Milestone', options: { title: 'Assign milestone', toggle_class: 'js-milestone-select js-extra-options', filter: true, dropdown_class: 'dropdown-menu-selectable', placeholder: 'Search milestones', data: { show_no: true, field_name: "#{issuable.to_ability_name}[milestone_id]", project_id: @project.id, issuable_id: issuable.id, milestones: namespace_project_milestones_path(@project.namespace, @project, :json), ability_name: issuable.to_ability_name, issue_update: issuable_json_path(issuable), use_id: true }})
       - if issuable.has_attribute?(:time_estimate)
         #issuable-time-tracker.block
-          %issuable-time-tracker{ ':time_estimate' => 'issuable.time_estimate', ':time_spent' => 'issuable.total_time_spent', ':human_time_estimate' => 'issuable.human_time_estimate', ':human_time_spent' => 'issuable.human_total_time_spent', 'docs-url' => help_page_path('workflow/time_tracking.md') }
-            // Fallback while content is loading
-            .title.hide-collapsed
-              Time tracking
-              = icon('spinner spin', 'aria-hidden': 'true')
+          // Fallback while content is loading
+          .title.hide-collapsed
+            Time tracking
+            = icon('spinner spin', 'aria-hidden': 'true')
       - if issuable.has_attribute?(:due_date)
         .block.due_date
           .sidebar-collapsed-icon
@@ -169,8 +191,13 @@
           = clipboard_button(text: project_ref, title: "Copy reference to clipboard", placement: "left")
 
     :javascript
-      gl.IssuableResource = new gl.SubbableResource('#{issuable_json_path(issuable)}');
-      new gl.IssuableTimeTracking("#{escape_javascript(serialize_issuable(issuable))}");
+      gl.sidebarOptions = {
+        endpoint: "#{issuable_json_path(issuable)}",
+        editable: #{can_edit_issuable ? true : false},
+        currentUser: #{current_user.to_json(only: [:username, :id, :name], methods: :avatar_url)},
+        rootPath: "#{root_path}"
+      };
+
       new MilestoneSelect('{"full_path":"#{@project.full_path}"}');
       new LabelsSelect();
       new IssuableContext('#{escape_javascript(current_user.to_json(only: [:username, :id, :name]))}');
diff --git a/app/views/shared/issuable/form/_issue_assignee.html.haml b/app/views/shared/issuable/form/_issue_assignee.html.haml
new file mode 100644
index 00000000000..c33474ac3b4
--- /dev/null
+++ b/app/views/shared/issuable/form/_issue_assignee.html.haml
@@ -0,0 +1,30 @@
+- issue = issuable
+.block.assignee
+  .sidebar-collapsed-icon.sidebar-collapsed-user{ data: { toggle: "tooltip", placement: "left", container: "body" }, title: (issuable.assignee_list) }
+    - if issue.assignees.any?
+      - issue.assignees.each do |assignee|
+        = link_to_member(@project, 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: 'edit-link pull-right'
+  .value.hide-collapsed
+    - if issue.assignees.any?
+      - issue.assignees.each do |assignee|
+        = link_to_member(@project, assignee, size: 32, extra_class: 'bold') do
+          %span.username
+            = 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_ids', value: issuable.assignee_ids, id: 'issue_assignee_ids'
+    = 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 if @project), author_id: issuable.author_id, field_name: "#{issuable.to_ability_name}[assignee_id]", issue_update: issuable_json_path(issuable), ability_name: issuable.to_ability_name, null_user: true } })
diff --git a/app/views/shared/issuable/form/_merge_request_assignee.html.haml b/app/views/shared/issuable/form/_merge_request_assignee.html.haml
new file mode 100644
index 00000000000..18011d528a0
--- /dev/null
+++ b/app/views/shared/issuable/form/_merge_request_assignee.html.haml
@@ -0,0 +1,31 @@
+- merge_request = issuable
+.block.assignee
+  .sidebar-collapsed-icon.sidebar-collapsed-user{ data: { toggle: "tooltip", placement: "left", container: "body" }, title: (merge_request.assignee.name if merge_request.assignee) }
+    - 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: 'edit-link pull-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.pull-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 9dbfedb84f1..411cb717fc7 100644
--- a/app/views/shared/issuable/form/_metadata.html.haml
+++ b/app/views/shared/issuable/form/_metadata.html.haml
@@ -10,13 +10,24 @@
 .row
   %div{ class: (has_due_date ? "col-lg-6" : "col-sm-12") }
     .form-group.issue-assignee
-      = form.label :assignee_id, "Assignee", class: "control-label #{"col-lg-4" if has_due_date}"
-      .col-sm-10{ class: ("col-lg-8" if has_due_date) }
-        .issuable-form-select-holder
-          = form.hidden_field :assignee_id
-          = dropdown_tag(user_dropdown_label(issuable.assignee_id, "Assignee"), options: { toggle_class: "js-dropdown-keep-input js-user-search js-issuable-form-dropdown js-assignee-search", title: "Select assignee", filter: true, dropdown_class: "dropdown-menu-user dropdown-menu-selectable dropdown-menu-assignee js-filter-submit",
-            placeholder: "Search assignee", data: { first_user: current_user.try(:username), null_user: true, current_user: true, project_id: issuable.project.try(:id), selected: issuable.assignee_id, field_name: "#{issuable.class.model_name.param_key}[assignee_id]", default_label: "Assignee"} })
-        = link_to 'Assign to me', '#', class: "assign-to-me-link #{'hide' if issuable.assignee_id == current_user.id}"
+      - if issuable.is_a?(Issue)
+        = form.label :assignee_ids, "Assignee", class: "control-label #{"col-lg-4" if has_due_date}"
+        .col-sm-10{ class: ("col-lg-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 }
+
+            = dropdown_tag(users_dropdown_label(issuable.assignees), options: issue_dropdown_options(issuable, true))
+          = link_to 'Assign to me', '#', class: "assign-to-me-link #{'hide' if issuable.assignees.include?(current_user)}"
+      - else
+        = form.label :assignee_id, "Assignee", class: "control-label #{"col-lg-4" if has_due_date}"
+        .col-sm-10{ class: ("col-lg-8" if has_due_date) }
+          .issuable-form-select-holder
+            = form.hidden_field :assignee_id
+
+            = dropdown_tag(user_dropdown_label(issuable.assignee_id, "Assignee"), options: { toggle_class: "js-dropdown-keep-input js-user-search js-issuable-form-dropdown js-assignee-search", title: "Select assignee", filter: true, dropdown_class: "dropdown-menu-user dropdown-menu-selectable dropdown-menu-assignee js-filter-submit",
+              placeholder: "Search assignee", data: { first_user: current_user.try(:username), null_user: true, current_user: true, project_id: issuable.project.try(:id), selected: issuable.assignee_id, field_name: "#{issuable.class.model_name.param_key}[assignee_id]", default_label: "Assignee"} })
+          = link_to 'Assign to me', '#', class: "assign-to-me-link #{'hide' if issuable.assignee_id == current_user.id}"
     .form-group.issue-milestone
       = form.label :milestone_id, "Milestone", class: "control-label #{"col-lg-4" if has_due_date}"
       .col-sm-10{ class: ("col-lg-8" if has_due_date) }
diff --git a/app/views/shared/milestones/_issuable.html.haml b/app/views/shared/milestones/_issuable.html.haml
index 5247d6a51e6..22547a30cdf 100644
--- a/app/views/shared/milestones/_issuable.html.haml
+++ b/app/views/shared/milestones/_issuable.html.haml
@@ -1,7 +1,7 @@
 -# @project is present when viewing Project's milestone
 - project = @project || issuable.project
 - namespace = @project_namespace || project.namespace.becomes(Namespace)
-- assignee = issuable.assignee
+- assignees = issuable.assignees
 - issuable_type = issuable.class.table_name
 - base_url_args = [namespace, project]
 - issuable_type_args = base_url_args + [issuable_type]
@@ -26,7 +26,7 @@
         - render_colored_label(label)
 
     %span.assignee-icon
-      - if assignee
-        = link_to polymorphic_path(issuable_type_args, { milestone_title: @milestone.title, assignee_id: issuable.assignee_id, state: 'all' }),
+      - assignees.each do |assignee|
+        = link_to polymorphic_path(base_url_args, { milestone_title: @milestone.title, assignee_id: assignee.id, state: 'all' }),
                   class: 'has-tooltip', title: "Assigned to #{assignee.name}", data: { container: 'body' } do
-          - image_tag(avatar_icon(issuable.assignee, 16), class: "avatar s16", alt: '')
+          - image_tag(avatar_icon(assignee, 16), class: "avatar s16", alt: '')