From a04d9ba90c0d1df02f613b7aa01ef598e1ac5b28 Mon Sep 17 00:00:00 2001
From: Heinrich Lee Yu <heinrich@gitlab.com>
Date: Wed, 6 Feb 2019 10:31:46 +0000
Subject: Add reply to notes to turn into discussions

---
 .../javascripts/notes/components/note_actions.vue  |  20 +++
 .../notes/components/note_actions/reply_button.vue |  40 ++++++
 .../notes/components/noteable_discussion.vue       |   1 +
 .../javascripts/notes/components/noteable_note.vue |  22 ++-
 .../javascripts/notes/components/notes_app.vue     |   7 +-
 app/assets/javascripts/notes/stores/actions.js     |   3 +
 .../javascripts/notes/stores/mutation_types.js     |   1 +
 app/assets/javascripts/notes/stores/mutations.js   |   5 +
 app/controllers/concerns/issuable_actions.rb       |   3 +
 app/models/concerns/noteable.rb                    |  15 ++-
 app/models/discussion.rb                           |   6 +
 app/models/individual_note_discussion.rb           |   8 ++
 app/models/sent_notification.rb                    |  25 +---
 app/services/notes/build_service.rb                |   2 +
 app/services/notes/create_service.rb               |   4 +
 locale/gitlab.pot                                  |   3 +
 .../merge_request/user_posts_notes_spec.rb         |  32 +++++
 .../components/note_actions/reply_button_spec.js   |  46 +++++++
 .../notes/components/note_actions_spec.js          | 149 +++++++++++++++------
 spec/javascripts/notes/stores/actions_spec.js      |  14 ++
 spec/javascripts/notes/stores/mutation_spec.js     |  23 ++++
 .../email/handler/create_note_handler_spec.rb      |  26 +++-
 spec/models/sent_notification_spec.rb              |  34 ++++-
 spec/services/notes/build_service_spec.rb          |  40 ++++++
 spec/services/notes/create_service_spec.rb         |  37 +++++
 25 files changed, 493 insertions(+), 73 deletions(-)
 create mode 100644 app/assets/javascripts/notes/components/note_actions/reply_button.vue
 create mode 100644 spec/javascripts/notes/components/note_actions/reply_button_spec.js

diff --git a/app/assets/javascripts/notes/components/note_actions.vue b/app/assets/javascripts/notes/components/note_actions.vue
index d99694b06e9..394f2a80a67 100644
--- a/app/assets/javascripts/notes/components/note_actions.vue
+++ b/app/assets/javascripts/notes/components/note_actions.vue
@@ -2,11 +2,13 @@
 import { mapGetters } from 'vuex';
 import Icon from '~/vue_shared/components/icon.vue';
 import { GlLoadingIcon, GlTooltipDirective } from '@gitlab/ui';
+import ReplyButton from './note_actions/reply_button.vue';
 
 export default {
   name: 'NoteActions',
   components: {
     Icon,
+    ReplyButton,
     GlLoadingIcon,
   },
   directives: {
@@ -21,6 +23,11 @@ export default {
       type: [String, Number],
       required: true,
     },
+    discussionId: {
+      type: String,
+      required: false,
+      default: '',
+    },
     noteUrl: {
       type: String,
       required: false,
@@ -36,6 +43,10 @@ export default {
       required: false,
       default: null,
     },
+    showReply: {
+      type: Boolean,
+      required: true,
+    },
     canEdit: {
       type: Boolean,
       required: true,
@@ -80,6 +91,9 @@ export default {
   },
   computed: {
     ...mapGetters(['getUserDataByProp']),
+    showReplyButton() {
+      return gon.features && gon.features.replyToIndividualNotes && this.showReply;
+    },
     shouldShowActionsDropdown() {
       return this.currentUserId && (this.canEdit || this.canReportAsAbuse);
     },
@@ -153,6 +167,12 @@ export default {
         <icon css-classes="link-highlight award-control-icon-super-positive" name="emoji_smiley" />
       </a>
     </div>
+    <reply-button
+      v-if="showReplyButton"
+      ref="replyButton"
+      class="js-reply-button"
+      :note-id="discussionId"
+    />
     <div v-if="canEdit" class="note-actions-item">
       <button
         v-gl-tooltip.bottom
diff --git a/app/assets/javascripts/notes/components/note_actions/reply_button.vue b/app/assets/javascripts/notes/components/note_actions/reply_button.vue
new file mode 100644
index 00000000000..b2f9d7f128a
--- /dev/null
+++ b/app/assets/javascripts/notes/components/note_actions/reply_button.vue
@@ -0,0 +1,40 @@
+<script>
+import { mapActions } from 'vuex';
+import { GlTooltipDirective, GlButton } from '@gitlab/ui';
+import Icon from '~/vue_shared/components/icon.vue';
+
+export default {
+  name: 'ReplyButton',
+  components: {
+    Icon,
+    GlButton,
+  },
+  directives: {
+    GlTooltip: GlTooltipDirective,
+  },
+  props: {
+    noteId: {
+      type: String,
+      required: true,
+    },
+  },
+  methods: {
+    ...mapActions(['convertToDiscussion']),
+  },
+};
+</script>
+
+<template>
+  <div class="note-actions-item">
+    <gl-button
+      ref="button"
+      v-gl-tooltip.bottom
+      class="note-action-button"
+      variant="transparent"
+      :title="__('Reply to comment')"
+      @click="convertToDiscussion(noteId)"
+    >
+      <icon name="comment" css-classes="link-highlight" />
+    </gl-button>
+  </div>
+</template>
diff --git a/app/assets/javascripts/notes/components/noteable_discussion.vue b/app/assets/javascripts/notes/components/noteable_discussion.vue
index e26cce1c47f..b7e9f7c2028 100644
--- a/app/assets/javascripts/notes/components/noteable_discussion.vue
+++ b/app/assets/javascripts/notes/components/noteable_discussion.vue
@@ -398,6 +398,7 @@ Please check your network connection and try again.`;
                     :line="line"
                     :commit="commit"
                     :help-page-path="helpPagePath"
+                    :show-reply-button="canReply"
                     @handleDeleteNote="deleteNoteHandler"
                   >
                     <note-edited-text
diff --git a/app/assets/javascripts/notes/components/noteable_note.vue b/app/assets/javascripts/notes/components/noteable_note.vue
index 3c48d81ed05..56108a58010 100644
--- a/app/assets/javascripts/notes/components/noteable_note.vue
+++ b/app/assets/javascripts/notes/components/noteable_note.vue
@@ -29,6 +29,11 @@ export default {
       type: Object,
       required: true,
     },
+    discussion: {
+      type: Object,
+      required: false,
+      default: null,
+    },
     line: {
       type: Object,
       required: false,
@@ -54,7 +59,7 @@ export default {
     };
   },
   computed: {
-    ...mapGetters(['targetNoteHash', 'getNoteableData', 'getUserData']),
+    ...mapGetters(['targetNoteHash', 'getNoteableData', 'getUserData', 'commentsDisabled']),
     author() {
       return this.note.author;
     },
@@ -80,6 +85,19 @@ export default {
     isTarget() {
       return this.targetNoteHash === this.noteAnchorId;
     },
+    discussionId() {
+      if (this.discussion) {
+        return this.discussion.id;
+      }
+      return '';
+    },
+    showReplyButton() {
+      if (!this.discussion || !this.getNoteableData.current_user.can_create_note) {
+        return false;
+      }
+
+      return this.discussion.individual_note && !this.commentsDisabled;
+    },
     actionText() {
       if (!this.commit) {
         return '';
@@ -231,6 +249,7 @@ export default {
           :note-id="note.id"
           :note-url="note.noteable_note_url"
           :access-level="note.human_access"
+          :show-reply="showReplyButton"
           :can-edit="note.current_user.can_edit"
           :can-award-emoji="note.current_user.can_award_emoji"
           :can-delete="note.current_user.can_edit"
@@ -241,6 +260,7 @@ export default {
           :is-resolved="note.resolved"
           :is-resolving="isResolving"
           :resolved-by="note.resolved_by"
+          :discussion-id="discussionId"
           @handleEdit="editHandler"
           @handleDelete="deleteHandler"
           @handleResolve="resolveHandler"
diff --git a/app/assets/javascripts/notes/components/notes_app.vue b/app/assets/javascripts/notes/components/notes_app.vue
index 5edceea043c..6d72b72e628 100644
--- a/app/assets/javascripts/notes/components/notes_app.vue
+++ b/app/assets/javascripts/notes/components/notes_app.vue
@@ -199,7 +199,12 @@ export default {
             :key="discussion.id"
             :note="discussion.notes[0]"
           />
-          <noteable-note v-else :key="discussion.id" :note="discussion.notes[0]" />
+          <noteable-note
+            v-else
+            :key="discussion.id"
+            :note="discussion.notes[0]"
+            :discussion="discussion"
+          />
         </template>
         <noteable-discussion
           v-else
diff --git a/app/assets/javascripts/notes/stores/actions.js b/app/assets/javascripts/notes/stores/actions.js
index 2105a62cecb..ff65f14d529 100644
--- a/app/assets/javascripts/notes/stores/actions.js
+++ b/app/assets/javascripts/notes/stores/actions.js
@@ -426,5 +426,8 @@ export const submitSuggestion = (
     });
 };
 
+export const convertToDiscussion = ({ commit }, noteId) =>
+  commit(types.CONVERT_TO_DISCUSSION, noteId);
+
 // prevent babel-plugin-rewire from generating an invalid default during karma tests
 export default () => {};
diff --git a/app/assets/javascripts/notes/stores/mutation_types.js b/app/assets/javascripts/notes/stores/mutation_types.js
index df943c155f4..2bffedad336 100644
--- a/app/assets/javascripts/notes/stores/mutation_types.js
+++ b/app/assets/javascripts/notes/stores/mutation_types.js
@@ -17,6 +17,7 @@ export const SET_NOTES_FETCHED_STATE = 'SET_NOTES_FETCHED_STATE';
 export const SET_NOTES_LOADING_STATE = 'SET_NOTES_LOADING_STATE';
 export const DISABLE_COMMENTS = 'DISABLE_COMMENTS';
 export const APPLY_SUGGESTION = 'APPLY_SUGGESTION';
+export const CONVERT_TO_DISCUSSION = 'CONVERT_TO_DISCUSSION';
 
 // DISCUSSION
 export const COLLAPSE_DISCUSSION = 'COLLAPSE_DISCUSSION';
diff --git a/app/assets/javascripts/notes/stores/mutations.js b/app/assets/javascripts/notes/stores/mutations.js
index 33d39ad2ec9..d167f8ef421 100644
--- a/app/assets/javascripts/notes/stores/mutations.js
+++ b/app/assets/javascripts/notes/stores/mutations.js
@@ -264,4 +264,9 @@ export default {
     ).length;
     state.hasUnresolvedDiscussions = state.unresolvedDiscussionsCount > 1;
   },
+
+  [types.CONVERT_TO_DISCUSSION](state, discussionId) {
+    const discussion = utils.findNoteObjectById(state.discussions, discussionId);
+    Object.assign(discussion, { individual_note: false });
+  },
 };
diff --git a/app/controllers/concerns/issuable_actions.rb b/app/controllers/concerns/issuable_actions.rb
index 8ef3b6502df..cd3fa641e89 100644
--- a/app/controllers/concerns/issuable_actions.rb
+++ b/app/controllers/concerns/issuable_actions.rb
@@ -7,6 +7,9 @@ module IssuableActions
   included do
     before_action :authorize_destroy_issuable!, only: :destroy
     before_action :authorize_admin_issuable!, only: :bulk_update
+    before_action only: :show do
+      push_frontend_feature_flag(:reply_to_individual_notes)
+    end
   end
 
   def permitted_keys
diff --git a/app/models/concerns/noteable.rb b/app/models/concerns/noteable.rb
index 29476654bf7..3c74034b527 100644
--- a/app/models/concerns/noteable.rb
+++ b/app/models/concerns/noteable.rb
@@ -1,9 +1,18 @@
 # frozen_string_literal: true
 
 module Noteable
-  # Names of all implementers of `Noteable` that support resolvable notes.
+  extend ActiveSupport::Concern
+
+  # `Noteable` class names that support resolvable notes.
   RESOLVABLE_TYPES = %w(MergeRequest).freeze
 
+  class_methods do
+    # `Noteable` class names that support replying to individual notes.
+    def replyable_types
+      %w(Issue MergeRequest)
+    end
+  end
+
   def base_class_name
     self.class.base_class.name
   end
@@ -26,6 +35,10 @@ module Noteable
     DiscussionNote.noteable_types.include?(base_class_name)
   end
 
+  def supports_replying_to_individual_notes?
+    supports_discussions? && self.class.replyable_types.include?(base_class_name)
+  end
+
   def supports_suggestion?
     false
   end
diff --git a/app/models/discussion.rb b/app/models/discussion.rb
index dbc7b6e67be..f2678e0597d 100644
--- a/app/models/discussion.rb
+++ b/app/models/discussion.rb
@@ -17,6 +17,8 @@ class Discussion
             :for_commit?,
             :for_merge_request?,
 
+            :save,
+
             to: :first_note
 
   def project_id
@@ -116,6 +118,10 @@ class Discussion
     false
   end
 
+  def can_convert_to_discussion?
+    false
+  end
+
   def new_discussion?
     notes.length == 1
   end
diff --git a/app/models/individual_note_discussion.rb b/app/models/individual_note_discussion.rb
index 07ee7470ea2..aab0ff93468 100644
--- a/app/models/individual_note_discussion.rb
+++ b/app/models/individual_note_discussion.rb
@@ -13,6 +13,14 @@ class IndividualNoteDiscussion < Discussion
     true
   end
 
+  def can_convert_to_discussion?
+    noteable.supports_replying_to_individual_notes? && Feature.enabled?(:reply_to_individual_notes)
+  end
+
+  def convert_to_discussion!
+    first_note.becomes!(Discussion.note_class).to_discussion
+  end
+
   def reply_attributes
     super.tap { |attrs| attrs.delete(:discussion_id) }
   end
diff --git a/app/models/sent_notification.rb b/app/models/sent_notification.rb
index e65b3df0fb6..6caab24143b 100644
--- a/app/models/sent_notification.rb
+++ b/app/models/sent_notification.rb
@@ -48,7 +48,7 @@ class SentNotification < ActiveRecord::Base
     end
 
     def record_note(note, recipient_id, reply_key = self.reply_key, attrs = {})
-      attrs[:in_reply_to_discussion_id] = note.discussion_id
+      attrs[:in_reply_to_discussion_id] = note.discussion_id if note.part_of_discussion?
 
       record(note.noteable, recipient_id, reply_key, attrs)
     end
@@ -99,29 +99,12 @@ class SentNotification < ActiveRecord::Base
   private
 
   def reply_params
-    attrs = {
+    {
       noteable_type: self.noteable_type,
       noteable_id: self.noteable_id,
-      commit_id: self.commit_id
+      commit_id: self.commit_id,
+      in_reply_to_discussion_id: self.in_reply_to_discussion_id
     }
-
-    if self.in_reply_to_discussion_id.present?
-      attrs[:in_reply_to_discussion_id] = self.in_reply_to_discussion_id
-    else
-      # Remove in GitLab 10.0, when we will not support replying to SentNotifications
-      # that don't have `in_reply_to_discussion_id` anymore.
-      attrs.merge!(
-        type: self.note_type,
-
-        # LegacyDiffNote
-        line_code: self.line_code,
-
-        # DiffNote
-        position: self.position.to_json
-      )
-    end
-
-    attrs
   end
 
   def note_valid
diff --git a/app/services/notes/build_service.rb b/app/services/notes/build_service.rb
index bae98ede561..541f3e0d23c 100644
--- a/app/services/notes/build_service.rb
+++ b/app/services/notes/build_service.rb
@@ -15,6 +15,8 @@ module Notes
           return note
         end
 
+        discussion = discussion.convert_to_discussion! if discussion.can_convert_to_discussion?
+
         params.merge!(discussion.reply_attributes)
         should_resolve = discussion.resolved?
       end
diff --git a/app/services/notes/create_service.rb b/app/services/notes/create_service.rb
index c4546f30235..b975c3a8cb6 100644
--- a/app/services/notes/create_service.rb
+++ b/app/services/notes/create_service.rb
@@ -34,6 +34,10 @@ module Notes
       end
 
       if !only_commands && note.save
+        if note.part_of_discussion? && note.discussion.can_convert_to_discussion?
+          note.discussion.convert_to_discussion!.save(touch: false)
+        end
+
         todo_service.new_note(note, current_user)
         clear_noteable_diffs_cache(note)
         Suggestions::CreateService.new(note).execute
diff --git a/locale/gitlab.pot b/locale/gitlab.pot
index bdb2ed1e26e..1caf263b9f3 100644
--- a/locale/gitlab.pot
+++ b/locale/gitlab.pot
@@ -5959,6 +5959,9 @@ msgstr ""
 msgid "Reopen milestone"
 msgstr ""
 
+msgid "Reply to comment"
+msgstr ""
+
 msgid "Reply to this email directly or %{view_it_on_gitlab}."
 msgstr ""
 
diff --git a/spec/features/merge_request/user_posts_notes_spec.rb b/spec/features/merge_request/user_posts_notes_spec.rb
index ee5f5377ca6..1bbcf455ac7 100644
--- a/spec/features/merge_request/user_posts_notes_spec.rb
+++ b/spec/features/merge_request/user_posts_notes_spec.rb
@@ -66,6 +66,38 @@ describe 'Merge request > User posts notes', :js do
         is_expected.to have_css('.js-note-text', visible: true)
       end
     end
+
+    describe 'when reply_to_individual_notes feature flag is not set' do
+      before do
+        stub_feature_flags(reply_to_individual_notes: false)
+        visit project_merge_request_path(project, merge_request)
+      end
+
+      it 'does not show a reply button' do
+        expect(page).to have_no_selector('.js-reply-button')
+      end
+    end
+
+    describe 'when reply_to_individual_notes feature flag is set' do
+      before do
+        stub_feature_flags(reply_to_individual_notes: true)
+        visit project_merge_request_path(project, merge_request)
+      end
+
+      it 'shows a reply button' do
+        reply_button = find('.js-reply-button', match: :first)
+
+        expect(reply_button).to have_selector('.ic-comment')
+      end
+
+      it 'shows reply placeholder when clicking reply button' do
+        reply_button = find('.js-reply-button', match: :first)
+
+        reply_button.click
+
+        expect(page).to have_selector('.discussion-reply-holder')
+      end
+    end
   end
 
   describe 'when previewing a note' do
diff --git a/spec/javascripts/notes/components/note_actions/reply_button_spec.js b/spec/javascripts/notes/components/note_actions/reply_button_spec.js
new file mode 100644
index 00000000000..11e1664a3f4
--- /dev/null
+++ b/spec/javascripts/notes/components/note_actions/reply_button_spec.js
@@ -0,0 +1,46 @@
+import Vuex from 'vuex';
+import { createLocalVue, mount } from '@vue/test-utils';
+import ReplyButton from '~/notes/components/note_actions/reply_button.vue';
+
+describe('ReplyButton', () => {
+  const noteId = 'dummy-note-id';
+
+  let wrapper;
+  let convertToDiscussion;
+
+  beforeEach(() => {
+    const localVue = createLocalVue();
+    convertToDiscussion = jasmine.createSpy('convertToDiscussion');
+
+    localVue.use(Vuex);
+    const store = new Vuex.Store({
+      actions: {
+        convertToDiscussion,
+      },
+    });
+
+    wrapper = mount(ReplyButton, {
+      propsData: {
+        noteId,
+      },
+      store,
+      sync: false,
+      localVue,
+    });
+  });
+
+  afterEach(() => {
+    wrapper.destroy();
+  });
+
+  it('dispatches convertToDiscussion with note ID on click', () => {
+    const button = wrapper.find({ ref: 'button' });
+
+    button.trigger('click');
+
+    expect(convertToDiscussion).toHaveBeenCalledTimes(1);
+    const [, payload] = convertToDiscussion.calls.argsFor(0);
+
+    expect(payload).toBe(noteId);
+  });
+});
diff --git a/spec/javascripts/notes/components/note_actions_spec.js b/spec/javascripts/notes/components/note_actions_spec.js
index b102b7aecf7..0c1962912b4 100644
--- a/spec/javascripts/notes/components/note_actions_spec.js
+++ b/spec/javascripts/notes/components/note_actions_spec.js
@@ -2,14 +2,38 @@ import Vue from 'vue';
 import { shallowMount, createLocalVue } from '@vue/test-utils';
 import createStore from '~/notes/stores';
 import noteActions from '~/notes/components/note_actions.vue';
+import { TEST_HOST } from 'spec/test_constants';
 import { userDataMock } from '../mock_data';
 
 describe('noteActions', () => {
   let wrapper;
   let store;
+  let props;
+
+  const createWrapper = propsData => {
+    const localVue = createLocalVue();
+    return shallowMount(noteActions, {
+      store,
+      propsData,
+      localVue,
+      sync: false,
+    });
+  };
 
   beforeEach(() => {
     store = createStore();
+    props = {
+      accessLevel: 'Maintainer',
+      authorId: 26,
+      canDelete: true,
+      canEdit: true,
+      canAwardEmoji: true,
+      canReportAsAbuse: true,
+      noteId: '539',
+      noteUrl: `${TEST_HOST}/group/project/merge_requests/1#note_1`,
+      reportAbusePath: `${TEST_HOST}/abuse_reports/new?ref_url=http%3A%2F%2Flocalhost%3A3000%2Fgitlab-org%2Fgitlab-ce%2Fissues%2F7%23note_539&user_id=26`,
+      showReply: false,
+    };
   });
 
   afterEach(() => {
@@ -17,31 +41,10 @@ describe('noteActions', () => {
   });
 
   describe('user is logged in', () => {
-    let props;
-
     beforeEach(() => {
-      props = {
-        accessLevel: 'Maintainer',
-        authorId: 26,
-        canDelete: true,
-        canEdit: true,
-        canAwardEmoji: true,
-        canReportAsAbuse: true,
-        noteId: '539',
-        noteUrl: 'https://localhost:3000/group/project/merge_requests/1#note_1',
-        reportAbusePath:
-          '/abuse_reports/new?ref_url=http%3A%2F%2Flocalhost%3A3000%2Fgitlab-org%2Fgitlab-ce%2Fissues%2F7%23note_539&user_id=26',
-      };
-
       store.dispatch('setUserData', userDataMock);
 
-      const localVue = createLocalVue();
-      wrapper = shallowMount(noteActions, {
-        store,
-        propsData: props,
-        localVue,
-        sync: false,
-      });
+      wrapper = createWrapper(props);
     });
 
     it('should render access level badge', () => {
@@ -91,28 +94,14 @@ describe('noteActions', () => {
   });
 
   describe('user is not logged in', () => {
-    let props;
-
     beforeEach(() => {
       store.dispatch('setUserData', {});
-      props = {
-        accessLevel: 'Maintainer',
-        authorId: 26,
+      wrapper = createWrapper({
+        ...props,
         canDelete: false,
         canEdit: false,
         canAwardEmoji: false,
         canReportAsAbuse: false,
-        noteId: '539',
-        noteUrl: 'https://localhost:3000/group/project/merge_requests/1#note_1',
-        reportAbusePath:
-          '/abuse_reports/new?ref_url=http%3A%2F%2Flocalhost%3A3000%2Fgitlab-org%2Fgitlab-ce%2Fissues%2F7%23note_539&user_id=26',
-      };
-      const localVue = createLocalVue();
-      wrapper = shallowMount(noteActions, {
-        store,
-        propsData: props,
-        localVue,
-        sync: false,
       });
     });
 
@@ -124,4 +113,88 @@ describe('noteActions', () => {
       expect(wrapper.find('.more-actions').exists()).toBe(false);
     });
   });
+
+  describe('with feature flag replyToIndividualNotes enabled', () => {
+    beforeEach(() => {
+      gon.features = {
+        replyToIndividualNotes: true,
+      };
+    });
+
+    afterEach(() => {
+      gon.features = {};
+    });
+
+    describe('for showReply = true', () => {
+      beforeEach(() => {
+        wrapper = createWrapper({
+          ...props,
+          showReply: true,
+        });
+      });
+
+      it('shows a reply button', () => {
+        const replyButton = wrapper.find({ ref: 'replyButton' });
+
+        expect(replyButton.exists()).toBe(true);
+      });
+    });
+
+    describe('for showReply = false', () => {
+      beforeEach(() => {
+        wrapper = createWrapper({
+          ...props,
+          showReply: false,
+        });
+      });
+
+      it('does not show a reply button', () => {
+        const replyButton = wrapper.find({ ref: 'replyButton' });
+
+        expect(replyButton.exists()).toBe(false);
+      });
+    });
+  });
+
+  describe('with feature flag replyToIndividualNotes disabled', () => {
+    beforeEach(() => {
+      gon.features = {
+        replyToIndividualNotes: false,
+      };
+    });
+
+    afterEach(() => {
+      gon.features = {};
+    });
+
+    describe('for showReply = true', () => {
+      beforeEach(() => {
+        wrapper = createWrapper({
+          ...props,
+          showReply: true,
+        });
+      });
+
+      it('does not show a reply button', () => {
+        const replyButton = wrapper.find({ ref: 'replyButton' });
+
+        expect(replyButton.exists()).toBe(false);
+      });
+    });
+
+    describe('for showReply = false', () => {
+      beforeEach(() => {
+        wrapper = createWrapper({
+          ...props,
+          showReply: false,
+        });
+      });
+
+      it('does not show a reply button', () => {
+        const replyButton = wrapper.find({ ref: 'replyButton' });
+
+        expect(replyButton.exists()).toBe(false);
+      });
+    });
+  });
 });
diff --git a/spec/javascripts/notes/stores/actions_spec.js b/spec/javascripts/notes/stores/actions_spec.js
index 2e3cd5e8f36..73f960dd21e 100644
--- a/spec/javascripts/notes/stores/actions_spec.js
+++ b/spec/javascripts/notes/stores/actions_spec.js
@@ -585,4 +585,18 @@ describe('Actions Notes Store', () => {
       );
     });
   });
+
+  describe('convertToDiscussion', () => {
+    it('commits CONVERT_TO_DISCUSSION with noteId', done => {
+      const noteId = 'dummy-note-id';
+      testAction(
+        actions.convertToDiscussion,
+        noteId,
+        {},
+        [{ type: 'CONVERT_TO_DISCUSSION', payload: noteId }],
+        [],
+        done,
+      );
+    });
+  });
 });
diff --git a/spec/javascripts/notes/stores/mutation_spec.js b/spec/javascripts/notes/stores/mutation_spec.js
index b6b2c7d60a5..4f8d3069bb5 100644
--- a/spec/javascripts/notes/stores/mutation_spec.js
+++ b/spec/javascripts/notes/stores/mutation_spec.js
@@ -517,4 +517,27 @@ describe('Notes Store mutations', () => {
       );
     });
   });
+
+  describe('CONVERT_TO_DISCUSSION', () => {
+    let discussion;
+    let state;
+
+    beforeEach(() => {
+      discussion = {
+        id: 42,
+        individual_note: true,
+      };
+      state = { discussions: [discussion] };
+    });
+
+    it('toggles individual_note', () => {
+      mutations.CONVERT_TO_DISCUSSION(state, discussion.id);
+
+      expect(discussion.individual_note).toBe(false);
+    });
+
+    it('throws if discussion was not found', () => {
+      expect(() => mutations.CONVERT_TO_DISCUSSION(state, 99)).toThrow();
+    });
+  });
 });
diff --git a/spec/lib/gitlab/email/handler/create_note_handler_spec.rb b/spec/lib/gitlab/email/handler/create_note_handler_spec.rb
index e5420ea6bea..50e473c459e 100644
--- a/spec/lib/gitlab/email/handler/create_note_handler_spec.rb
+++ b/spec/lib/gitlab/email/handler/create_note_handler_spec.rb
@@ -155,11 +155,7 @@ describe Gitlab::Email::Handler::CreateNoteHandler do
     it_behaves_like "checks permissions on noteable"
   end
 
-  context "when everything is fine" do
-    before do
-      setup_attachment
-    end
-
+  shared_examples 'a reply to existing comment' do
     it "creates a comment" do
       expect { receiver.execute }.to change { noteable.notes.count }.by(1)
       new_note = noteable.notes.last
@@ -168,7 +164,21 @@ describe Gitlab::Email::Handler::CreateNoteHandler do
       expect(new_note.position).to eq(note.position)
       expect(new_note.note).to include("I could not disagree more.")
       expect(new_note.in_reply_to?(note)).to be_truthy
+
+      if note.part_of_discussion?
+        expect(new_note.discussion_id).to eq(note.discussion_id)
+      else
+        expect(new_note.discussion_id).not_to eq(note.discussion_id)
+      end
     end
+  end
+
+  context "when everything is fine" do
+    before do
+      setup_attachment
+    end
+
+    it_behaves_like 'a reply to existing comment'
 
     it "adds all attachments" do
       receiver.execute
@@ -207,4 +217,10 @@ describe Gitlab::Email::Handler::CreateNoteHandler do
       end
     end
   end
+
+  context "when note is not a discussion" do
+    let(:note) { create(:note_on_merge_request, project: project) }
+
+    it_behaves_like 'a reply to existing comment'
+  end
 end
diff --git a/spec/models/sent_notification_spec.rb b/spec/models/sent_notification_spec.rb
index 677613b7980..6c35ed8f649 100644
--- a/spec/models/sent_notification_spec.rb
+++ b/spec/models/sent_notification_spec.rb
@@ -36,19 +36,41 @@ describe SentNotification do
     end
   end
 
+  shared_examples 'a successful sent notification' do
+    it 'creates a new SentNotification' do
+      expect { subject }.to change { described_class.count }.by(1)
+    end
+  end
+
   describe '.record' do
     let(:issue) { create(:issue) }
 
-    it 'creates a new SentNotification' do
-      expect { described_class.record(issue, user.id) }.to change { described_class.count }.by(1)
-    end
+    subject { described_class.record(issue, user.id) }
+
+    it_behaves_like 'a successful sent notification'
   end
 
   describe '.record_note' do
-    let(:note) { create(:diff_note_on_merge_request) }
+    subject { described_class.record_note(note, note.author.id) }
 
-    it 'creates a new SentNotification' do
-      expect { described_class.record_note(note, note.author.id) }.to change { described_class.count }.by(1)
+    context 'for a discussion note' do
+      let(:note) { create(:diff_note_on_merge_request) }
+
+      it_behaves_like 'a successful sent notification'
+
+      it 'sets in_reply_to_discussion_id' do
+        expect(subject.in_reply_to_discussion_id).to eq(note.discussion_id)
+      end
+    end
+
+    context 'for an individual note' do
+      let(:note) { create(:note_on_merge_request) }
+
+      it_behaves_like 'a successful sent notification'
+
+      it 'does not set in_reply_to_discussion_id' do
+        expect(subject.in_reply_to_discussion_id).to be_nil
+      end
     end
   end
 
diff --git a/spec/services/notes/build_service_spec.rb b/spec/services/notes/build_service_spec.rb
index 9aaccb4bffe..af4daff336b 100644
--- a/spec/services/notes/build_service_spec.rb
+++ b/spec/services/notes/build_service_spec.rb
@@ -123,6 +123,46 @@ describe Notes::BuildService do
       end
     end
 
+    context 'when replying to individual note' do
+      let(:note) { create(:note_on_issue) }
+
+      subject { described_class.new(project, author, note: 'Test', in_reply_to_discussion_id: note.discussion_id).execute }
+
+      shared_examples 'an individual note reply' do
+        it 'builds another individual note' do
+          expect(subject).to be_valid
+          expect(subject).to be_a(Note)
+          expect(subject.discussion_id).not_to eq(note.discussion_id)
+        end
+      end
+
+      context 'when reply_to_individual_notes is disabled' do
+        before do
+          stub_feature_flags(reply_to_individual_notes: false)
+        end
+
+        it_behaves_like 'an individual note reply'
+      end
+
+      context 'when reply_to_individual_notes is enabled' do
+        before do
+          stub_feature_flags(reply_to_individual_notes: true)
+        end
+
+        it 'sets the note up to be in reply to that note' do
+          expect(subject).to be_valid
+          expect(subject).to be_a(DiscussionNote)
+          expect(subject.discussion_id).to eq(note.discussion_id)
+        end
+
+        context 'when noteable does not support replies' do
+          let(:note) { create(:note_on_commit) }
+
+          it_behaves_like 'an individual note reply'
+        end
+      end
+    end
+
     it 'builds a note without saving it' do
       new_note = described_class.new(project,
                                     author,
diff --git a/spec/services/notes/create_service_spec.rb b/spec/services/notes/create_service_spec.rb
index 1b9ba42cfd6..48f1d696ff6 100644
--- a/spec/services/notes/create_service_spec.rb
+++ b/spec/services/notes/create_service_spec.rb
@@ -278,5 +278,42 @@ describe Notes::CreateService do
         expect(note.note).to eq(':smile:')
       end
     end
+
+    context 'reply to individual note' do
+      let(:existing_note) { create(:note_on_issue, noteable: issue, project: project) }
+      let(:reply_opts) { opts.merge(in_reply_to_discussion_id: existing_note.discussion_id) }
+
+      subject { described_class.new(project, user, reply_opts).execute }
+
+      context 'when reply_to_individual_notes is disabled' do
+        before do
+          stub_feature_flags(reply_to_individual_notes: false)
+        end
+
+        it 'creates an individual note' do
+          expect(subject.type).to eq(nil)
+          expect(subject.discussion_id).not_to eq(existing_note.discussion_id)
+        end
+
+        it 'does not convert existing note' do
+          expect { subject }.not_to change { existing_note.reload.type }
+        end
+      end
+
+      context 'when reply_to_individual_notes is enabled' do
+        before do
+          stub_feature_flags(reply_to_individual_notes: true)
+        end
+
+        it 'creates a DiscussionNote in reply to existing note' do
+          expect(subject).to be_a(DiscussionNote)
+          expect(subject.discussion_id).to eq(existing_note.discussion_id)
+        end
+
+        it 'converts existing note to DiscussionNote' do
+          expect { subject }.to change { existing_note.reload.type }.from(nil).to('DiscussionNote')
+        end
+      end
+    end
   end
 end
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