Add option to start a new discussion on an MR

39 files changed, 695 insertions, 479 deletions

diff --git a/app/assets/javascripts/files_comment_button.js b/app/assets/javascripts/files_comment_button.js
index 3f041172ff3..d1d24b411a4 100644
--- a/app/assets/javascripts/files_comment_button.js
+++ b/app/assets/javascripts/files_comment_button.js
@@ -5,7 +5,7 @@
 let $commentButtonTemplate;
 var bind = function(fn, me) { return function() { return fn.apply(me, arguments); }; };
 
-window.FilesCommentButton = (function() {
+this.FilesCommentButton = (function() {
   var COMMENT_BUTTON_CLASS, EMPTY_CELL_CLASS, LINE_COLUMN_CLASSES, LINE_CONTENT_CLASS, LINE_HOLDER_CLASS, LINE_NUMBER_CLASS, OLD_LINE_CLASS, TEXT_FILE_SELECTOR, UNFOLDABLE_LINE_CLASS;
 
   COMMENT_BUTTON_CLASS = '.add-diff-note';
@@ -55,14 +55,19 @@ window.FilesCommentButton = (function() {
 
     textFileElement = this.getTextFileElement($currentTarget);
     buttonParentElement.append(this.buildButton({
+      discussionID: lineContentElement.attr('data-discussion-id'),
+      lineType: lineContentElement.attr('data-line-type'),
+
       noteableType: textFileElement.attr('data-noteable-type'),
       noteableID: textFileElement.attr('data-noteable-id'),
       commitID: textFileElement.attr('data-commit-id'),
       noteType: lineContentElement.attr('data-note-type'),
-      position: lineContentElement.attr('data-position'),
-      lineType: lineContentElement.attr('data-line-type'),
-      discussionID: lineContentElement.attr('data-discussion-id'),
-      lineCode: lineContentElement.attr('data-line-code')
+
+      // LegacyDiffNote
+      lineCode: lineContentElement.attr('data-line-code'),
+
+      // DiffNote
+      position: lineContentElement.attr('data-position')
     }));
   };
 
@@ -76,14 +81,19 @@ window.FilesCommentButton = (function() {
 
   FilesCommentButton.prototype.buildButton = function(buttonAttributes) {
     return $commentButtonTemplate.clone().attr({
+      'data-discussion-id': buttonAttributes.discussionID,
+      'data-line-type': buttonAttributes.lineType,
+
       'data-noteable-type': buttonAttributes.noteableType,
       'data-noteable-id': buttonAttributes.noteableID,
       'data-commit-id': buttonAttributes.commitID,
       'data-note-type': buttonAttributes.noteType,
+
+      // LegacyDiffNote
       'data-line-code': buttonAttributes.lineCode,
-      'data-position': buttonAttributes.position,
-      'data-discussion-id': buttonAttributes.discussionID,
-      'data-line-type': buttonAttributes.lineType
+
+      // DiffNote
+      'data-position': buttonAttributes.position
     });
   };
 
diff --git a/app/assets/javascripts/notes.js b/app/assets/javascripts/notes.js
index 1d563c63f39..71c03c89314 100644
--- a/app/assets/javascripts/notes.js
+++ b/app/assets/javascripts/notes.js
@@ -213,11 +213,7 @@ require('./task_list');
             _this.last_fetched_at = data.last_fetched_at;
             _this.setPollingInterval(data.notes.length);
             return $.each(notes, function(i, note) {
-              if (note.discussion_html != null) {
-                return _this.renderDiscussionNote(note);
-              } else {
-                return _this.renderNote(note);
-              }
+              _this.renderNote(note);
             });
           };
         })(this)
@@ -278,6 +274,10 @@ require('./task_list');
 
     Notes.prototype.renderNote = function(note) {
       var $notesList;
+      if (note.discussion_html != null) {
+        return this.renderDiscussionNote(note);
+      }
+
       if (!note.valid) {
         if (note.errors.commands_only) {
           new Flash(note.errors.commands_only, 'notice', this.parentTimeline);
@@ -323,9 +323,9 @@ require('./task_list');
         return;
       }
       this.note_ids.push(note.id);
-      form = $("#new-discussion-note-form-" + note.discussion_id);
-      if ((note.original_discussion_id != null) && form.length === 0) {
-        form = $("#new-discussion-note-form-" + note.original_discussion_id);
+      form = $(".js-discussion-note-form[data-discussion-id='" + note.discussion_id + "']");
+      if (form.length === 0) {
+        form = $(".js-discussion-note-form[data-original-discussion-id='" + note.original_discussion_id + "']");
       }
       row = form.closest("tr");
       lineType = this.isParallelView() ? form.find('#line_type').val() : 'old';
@@ -334,8 +334,8 @@ require('./task_list');
       note_html.renderGFM();
       // is this the first note of discussion?
       discussionContainer = $(".notes[data-discussion-id='" + note.discussion_id + "']");
-      if ((note.original_discussion_id != null) && discussionContainer.length === 0) {
-        discussionContainer = $(".notes[data-discussion-id='" + note.original_discussion_id + "']");
+      if (discussionContainer.length === 0) {
+        discussionContainer = $(".notes[data-original-discussion-id='" + note.original_discussion_id + "']");
       }
       if (discussionContainer.length === 0) {
         if (!this.isParallelView() || row.hasClass('js-temp-notes-holder')) {
@@ -363,7 +363,7 @@ require('./task_list');
         // Add note to 'Changes' page discussions
         discussionContainer.append(note_html);
         // Init discussion on 'Discussion' page if it is merge request page
-        if ($('body').attr('data-page').indexOf('projects:merge_request') === 0) {
+        if ($('body').attr('data-page').indexOf('projects:merge_request') === 0 || !note.diff_discussion_html) {
           $('ul.main-notes-list').append(note.discussion_html).renderGFM();
         }
       } else {
@@ -456,6 +456,7 @@ require('./task_list');
       form.find("#note_line_code").remove();
       form.find("#note_position").remove();
       form.find("#note_type").remove();
+      form.find("#note_in_reply_to_discussion_id").remove();
       form.find('.js-comment-resolve-button').closest('comment-and-resolve-btn').remove();
       return this.parentTimeline = form.parents('.timeline');
     };
@@ -470,10 +471,24 @@ require('./task_list');
      */
 
     Notes.prototype.setupNoteForm = function(form) {
-      var textarea;
+      var textarea, key;
       new gl.GLForm(form);
       textarea = form.find(".js-note-text");
-      return new Autosave(textarea, ["Note", form.find("#note_noteable_type").val(), form.find("#note_noteable_id").val(), form.find("#note_commit_id").val(), form.find("#note_type").val(), form.find("#note_line_code").val(), form.find("#note_position").val()]);
+      key = [
+        "Note",
+        form.find("#note_noteable_type").val(),
+        form.find("#note_noteable_id").val(),
+        form.find("#note_commit_id").val(),
+        form.find("#note_type").val(),
+        form.find("#in_reply_to_discussion_id").val(),
+
+        // LegacyDiffNote
+        form.find("#note_line_code").val(),
+
+        // DiffNote
+        form.find("#note_position").val()
+      ];
+      return new Autosave(textarea, key);
     };
 
     /*
@@ -510,7 +525,7 @@ require('./task_list');
         }
       }
 
-      this.renderDiscussionNote(note);
+      this.renderNote(note);
       // cleanup after successfully creating a diff/discussion note
       this.removeDiscussionNoteForm($form);
     };
@@ -727,23 +742,35 @@ require('./task_list');
 
     Sets some hidden fields in the form.
 
-    Note: dataHolder must have the "discussionId", "lineCode", "noteableType"
-    and "noteableId" data attributes set.
+    Note: dataHolder must have the "discussionId" and "lineCode" data attributes set.
      */
 
     Notes.prototype.setupDiscussionNoteForm = function(dataHolder, form) {
       // setup note target
-      form.attr('id', "new-discussion-note-form-" + (dataHolder.data("discussionId")));
+      var discussionID = dataHolder.data("discussionId");
+
+      form.attr('id', "new-discussion-note-form-" + discussionID);
+      form.attr("data-discussion-id", discussionID);
+      form.attr("data-original-discussion-id", dataHolder.data("originalDiscussionId") || discussionID);
       form.attr("data-line-code", dataHolder.data("lineCode"));
-      form.find("#note_type").val(dataHolder.data("noteType"));
+
       form.find("#line_type").val(dataHolder.data("lineType"));
+      form.find("#in_reply_to_discussion_id").val(dataHolder.data("originalDiscussionId"));
+
+      form.find("#note_noteable_type").val(dataHolder.data("noteableType"));
+      form.find("#note_noteable_id").val(dataHolder.data("noteableId"));
       form.find("#note_commit_id").val(dataHolder.data("commitId"));
+      form.find("#note_type").val(dataHolder.data("noteType"));
+
+      // LegacyDiffNote
       form.find("#note_line_code").val(dataHolder.data("lineCode"));
+
+      // DiffNote
       form.find("#note_position").val(dataHolder.attr("data-position"));
-      form.find("#note_noteable_type").val(dataHolder.data("noteableType"));
-      form.find("#note_noteable_id").val(dataHolder.data("noteableId"));
+
       form.find('.js-note-discard').show().removeClass('js-note-discard').addClass('js-close-discussion-note-form').text(form.find('.js-close-discussion-note-form').data('cancel-text'));
       form.find('.js-note-target-close').remove();
+      form.find('.js-note-new-discussion').remove();
       this.setupNoteForm(form);
 
       if (typeof gl.diffNotesCompileComponents !== 'undefined') {
diff --git a/app/controllers/projects/commit_controller.rb b/app/controllers/projects/commit_controller.rb
index cc67f688d51..98cd4ce344a 100644
--- a/app/controllers/projects/commit_controller.rb
+++ b/app/controllers/projects/commit_controller.rb
@@ -2,6 +2,7 @@
 #
 # Not to be confused with CommitsController, plural.
 class Projects::CommitController < Projects::ApplicationController
+  include NotesHelper
   include CreatesCommit
   include DiffForPath
   include DiffHelper
@@ -111,22 +112,19 @@ class Projects::CommitController < Projects::ApplicationController
   end
 
   def define_note_vars
-    @grouped_diff_discussions = commit.notes.grouped_diff_discussions
-    @notes = commit.notes.non_diff_notes.fresh
-
-    Banzai::NoteRenderer.render(
-      @grouped_diff_discussions.values.flat_map(&:notes) + @notes,
-      @project,
-      current_user,
-    )
-
+    @noteable = @commit
     @note = @project.build_commit_note(commit)
 
-    @noteable = @commit
-    @comments_target = {
+    @new_diff_note_attrs = {
       noteable_type: 'Commit',
       commit_id: @commit.id
     }
+
+    @grouped_diff_discussions = commit.grouped_diff_discussions
+    @discussions = commit.discussions
+
+    @notes = (@grouped_diff_discussions.values + @discussions).flat_map(&:notes)
+    @notes = prepare_notes_for_rendering(@notes)
   end
 
   def assign_change_commit_vars
diff --git a/app/controllers/projects/discussions_controller.rb b/app/controllers/projects/discussions_controller.rb
index 1349b015a63..f4a18a5e8f7 100644
--- a/app/controllers/projects/discussions_controller.rb
+++ b/app/controllers/projects/discussions_controller.rb
@@ -28,7 +28,7 @@ class Projects::DiscussionsController < Projects::ApplicationController
   end
 
   def discussion
-    @discussion ||= @merge_request.find_diff_discussion(params[:id]) || render_404
+    @discussion ||= @merge_request.find_discussion(params[:id]) || render_404
   end
 
   def authorize_resolve_discussion!
diff --git a/app/controllers/projects/issues_controller.rb b/app/controllers/projects/issues_controller.rb
index d984e6d3918..0c6267f065b 100644
--- a/app/controllers/projects/issues_controller.rb
+++ b/app/controllers/projects/issues_controller.rb
@@ -84,15 +84,11 @@ class Projects::IssuesController < Projects::ApplicationController
   end
 
   def show
-    raw_notes = @issue.notes.inc_relations_for_view.fresh
-
-    @notes = Banzai::NoteRenderer.
-      render(raw_notes, @project, current_user, @path, @project_wiki, @ref)
-
-    @note     = @project.notes.new(noteable: @issue)
     @noteable = @issue
+    @note     = @project.notes.new(noteable: @issue)
 
-    preload_max_access_for_authors(@notes, @project)
+    @discussions = @issue.discussions
+    @notes = prepare_notes_for_rendering(@discussions.flat_map(&:notes))
 
     respond_to do |format|
       format.html
diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 37e3ac05916..7863fe18f32 100755
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/app/controllers/projects/merge_requests_controller.rb
@@ -570,20 +570,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     @note = @project.notes.new(noteable: @merge_request)
 
     @discussions = @merge_request.discussions
-
-    preload_noteable_for_regular_notes(@discussions.flat_map(&:notes))
-
-    # This is not executed lazily
-    @notes = Banzai::NoteRenderer.render(
-      @discussions.flat_map(&:notes),
-      @project,
-      current_user,
-      @path,
-      @project_wiki,
-      @ref
-    )
-
-    preload_max_access_for_authors(@notes, @project)
+    @notes = prepare_notes_for_rendering(@discussions.flat_map(&:notes))
   end
 
   def define_widget_vars
@@ -596,22 +583,15 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   end
 
   def define_diff_comment_vars
-    @comments_target = {
+    @new_diff_note_attrs = {
       noteable_type: 'MergeRequest',
       noteable_id: @merge_request.id
     }
 
     @use_legacy_diff_notes = !@merge_request.has_complete_diff_refs?
-    @grouped_diff_discussions = @merge_request.notes.inc_relations_for_view.grouped_diff_discussions
-
-    Banzai::NoteRenderer.render(
-      @grouped_diff_discussions.values.flat_map(&:notes),
-      @project,
-      current_user,
-      @path,
-      @project_wiki,
-      @ref
-    )
+
+    @grouped_diff_discussions = @merge_request.grouped_diff_discussions
+    @notes = prepare_notes_for_rendering(@grouped_diff_discussions.values.flat_map(&:notes))
   end
 
   def define_pipelines_vars
diff --git a/app/controllers/projects/notes_controller.rb b/app/controllers/projects/notes_controller.rb
index d00177e7612..f80313bbee0 100644
--- a/app/controllers/projects/notes_controller.rb
+++ b/app/controllers/projects/notes_controller.rb
@@ -6,13 +6,14 @@ class Projects::NotesController < Projects::ApplicationController
   before_action :authorize_create_note!, only: [:create]
   before_action :authorize_admin_note!, only: [:update, :destroy]
   before_action :authorize_resolve_note!, only: [:resolve, :unresolve]
-  before_action :find_current_user_notes, only: [:index]
 
   def index
     current_fetched_at = Time.now.to_i
 
     notes_json = { notes: [], last_fetched_at: current_fetched_at }
 
+    @notes = notes_finder.execute.inc_author
+
     @notes.each do |note|
       next if note.cross_reference_not_visible_for?(current_user)
 
@@ -23,7 +24,11 @@ class Projects::NotesController < Projects::ApplicationController
   end
 
   def create
-    create_params = note_params.merge(merge_request_diff_head_sha: params[:merge_request_diff_head_sha])
+    create_params = note_params.merge(
+      merge_request_diff_head_sha: params[:merge_request_diff_head_sha],
+      in_reply_to_discussion_id: params[:in_reply_to_discussion_id],
+      new_discussion: params[:new_discussion],
+    )
     @note = Notes::CreateService.new(project, current_user, create_params).execute
 
     if @note.is_a?(Note)
@@ -111,6 +116,17 @@ class Projects::NotesController < Projects::ApplicationController
     )
   end
 
+  def discussion_html(discussion)
+    return if discussion.render_as_individual_notes?
+
+    render_to_string(
+      "discussions/_discussion",
+      layout: false,
+      formats: [:html],
+      locals: { discussion: discussion }
+    )
+  end
+
   def diff_discussion_html(discussion)
     return unless discussion.diff_discussion?
 
@@ -135,17 +151,6 @@ class Projects::NotesController < Projects::ApplicationController
     )
   end
 
-  def discussion_html(discussion)
-    return unless discussion.diff_discussion?
-
-    render_to_string(
-      "discussions/_discussion",
-      layout: false,
-      formats: [:html],
-      locals: { discussion: discussion }
-    )
-  end
-
   def note_json(note)
     attrs = {
       id: note.id
@@ -156,33 +161,22 @@ class Projects::NotesController < Projects::ApplicationController
 
       attrs.merge!(
         valid: true,
-        discussion_id: note.discussion_id,
+        discussion_id: note.discussion_id(noteable),
         html: note_html(note),
         note: note.note
       )
 
-      if note.diff_note?
-        discussion = note.to_discussion
-
+      discussion = note.to_discussion(noteable)
+      unless discussion.render_as_individual_notes?
         attrs.merge!(
           diff_discussion_html: diff_discussion_html(discussion),
-          discussion_html: discussion_html(discussion)
-        )
+          discussion_html: discussion_html(discussion),
 
-        # The discussion_id is used to add the comment to the correct discussion
-        # element on the merge request page. Among other things, the discussion_id
-        # contains the sha of head commit of the merge request.
-        # When new commits are pushed into the merge request after the initial
-        # load of the merge request page, the discussion elements will still have
-        # the old discussion_ids, with the old head commit sha. The new comment,
-        # however, will have the new discussion_id with the new commit sha.
-        # To ensure that these new comments will still end up in the correct
-        # discussion element, we also send the original discussion_id, with the
-        # old commit sha, along, and fall back on this value when no discussion
-        # element with the new discussion_id could be found.
-        if note.new_diff_note? && note.position != note.original_position
-          attrs[:original_discussion_id] = note.original_discussion_id
-        end
+          # Since the `discussion_id` can change, for example when new commits are pushed into an MR,
+          # the never-changing `original_discussion_id` is used as a fallback to the find the relevant
+          # discussion container to add this note to.
+          original_discussion_id: note.original_discussion_id
+        )
       end
     else
       attrs.merge!(
@@ -205,14 +199,30 @@ class Projects::NotesController < Projects::ApplicationController
 
   def note_params
     params.require(:note).permit(
-      :note, :noteable, :noteable_id, :noteable_type, :project_id,
-      :attachment, :line_code, :commit_id, :type, :position
+      :project_id,
+      :noteable_type,
+      :noteable_id,
+      :commit_id,
+      :noteable,
+      :type,
+
+      :note,
+      :attachment,
+
+      # LegacyDiffNote
+      :line_code,
+
+      # DiffNote
+      :position
     )
   end
 
-  def find_current_user_notes
-    @notes = NotesFinder.new(project, current_user, params.merge(last_fetched_at: last_fetched_at))
-      .execute.inc_author
+  def notes_finder
+    @notes_finder ||= NotesFinder.new(project, current_user, params.merge(last_fetched_at: last_fetched_at))
+  end
+
+  def noteable
+    @noteable ||= notes_finder.target
   end
 
   def last_fetched_at
diff --git a/app/controllers/projects/snippets_controller.rb b/app/controllers/projects/snippets_controller.rb
index ea1a97b7cf0..2600a9d7256 100644
--- a/app/controllers/projects/snippets_controller.rb
+++ b/app/controllers/projects/snippets_controller.rb
@@ -1,4 +1,5 @@
 class Projects::SnippetsController < Projects::ApplicationController
+  include NotesHelper
   include ToggleAwardEmoji
   include SpammableActions
   include SnippetsActions
@@ -55,8 +56,10 @@ class Projects::SnippetsController < Projects::ApplicationController
 
   def show
     @note = @project.notes.new(noteable: @snippet)
-    @notes = Banzai::NoteRenderer.render(@snippet.notes.fresh, @project, current_user)
     @noteable = @snippet
+
+    @discussions = @snippet.discussions
+    @notes = prepare_notes_for_rendering(@discussions.flat_map(&:notes))
   end
 
   def destroy
diff --git a/app/finders/notes_finder.rb b/app/finders/notes_finder.rb
index 6630c6384f2..855940762f3 100644
--- a/app/finders/notes_finder.rb
+++ b/app/finders/notes_finder.rb
@@ -17,29 +17,46 @@ class NotesFinder
     @project = project
     @current_user = current_user
     @params = params
-    init_collection
   end
 
   def execute
-    @notes = since_fetch_at(@params[:last_fetched_at]) if @params[:last_fetched_at]
+    @notes = init_collection
+    @notes = since_fetch_at(@params[:last_fetched_at], @notes) if @params[:last_fetched_at]
     @notes
   end
 
-  private
+  def target
+    return @target if defined?(@target)
 
-  def init_collection
-    @notes =
-      if @params[:target_id]
-        on_target(@params[:target_type], @params[:target_id])
+    target_type = @params[:target_type]
+    target_id   = @params[:target_id]
+
+    return @target = nil unless target_type && target_id
+
+    @target =
+      if target_type == "commit"
+        if Ability.allowed?(@current_user, :download_code, @project)
+          @project.commit(target_id)
+        end
       else
-        notes_of_any_type
+        noteables_for_type(target_type).find(target_id)
       end
   end
 
+  private
+
+  def init_collection
+    if target
+      notes_on_target
+    else
+      notes_of_any_type
+    end
+  end
+
   def notes_of_any_type
     types = %w(commit issue merge_request snippet)
     note_relations = types.map { |t| notes_for_type(t) }
-    note_relations.map!{ |notes| search(@params[:search], notes) } if @params[:search]
+    note_relations.map! { |notes| search(@params[:search], notes) } if @params[:search]
     UnionFinder.new.find_union(note_relations, Note)
   end
 
@@ -69,17 +86,11 @@ class NotesFinder
     end
   end
 
-  def on_target(target_type, target_id)
-    if target_type == "commit"
-      notes_for_type('commit').for_commit_id(target_id)
+  def notes_on_target
+    if target.respond_to?(:related_notes)
+      target.related_notes
     else
-      target = noteables_for_type(target_type).find(target_id)
-
-      if target.respond_to?(:related_notes)
-        target.related_notes
-      else
-        target.notes
-      end
+      target.notes
     end
   end
 
@@ -94,10 +105,9 @@ class NotesFinder
 
   # Notes changed since last fetch
   # Uses overlapping intervals to avoid worrying about race conditions
-  def since_fetch_at(fetch_time)
+  def since_fetch_at(fetch_time, notes_relation = @notes)
     # Default to 0 to remain compatible with old clients
     last_fetched_at = Time.at(@params.fetch(:last_fetched_at, 0).to_i)
-
-    @notes.where('updated_at > ?', last_fetched_at - FETCH_OVERLAP).fresh
+    notes_relation.where('updated_at > ?', last_fetched_at - FETCH_OVERLAP).fresh
   end
 end
diff --git a/app/helpers/notes_helper.rb b/app/helpers/notes_helper.rb
index b0331f36a2f..01ecac983cf 100644
--- a/app/helpers/notes_helper.rb
+++ b/app/helpers/notes_helper.rb
@@ -24,9 +24,9 @@ module NotesHelper
   end
 
   def diff_view_data
-    return {} unless @comments_target
+    return {} unless @new_diff_note_attrs
 
-    @comments_target.slice(:noteable_id, :noteable_type, :commit_id)
+    @new_diff_note_attrs.slice(:noteable_id, :noteable_type, :commit_id)
   end
 
   def diff_view_line_data(line_code, position, line_type)
@@ -53,37 +53,26 @@ module NotesHelper
     }
 
     if use_legacy_diff_note
-      discussion_id = LegacyDiffNote.discussion_id(
-        @comments_target[:noteable_type],
-        @comments_target[:noteable_id] || @comments_target[:commit_id],
-        line_code
-      )
-
-      data.merge!(
-        note_type: LegacyDiffNote.name,
-        discussion_id: discussion_id
-      )
+      new_note = LegacyDiffNote.new(@new_diff_note_attrs.merge(line_code: line_code))
+      discussion_id = new_note.discussion_id
     else
-      discussion_id = DiffNote.discussion_id(
-        @comments_target[:noteable_type],
-        @comments_target[:noteable_id] || @comments_target[:commit_id],
-        position
-      )
-
-      data.merge!(
-        position: position.to_json,
-        note_type: DiffNote.name,
-        discussion_id: discussion_id
-      )
+      new_note = DiffNote.new(@new_diff_note_attrs.merge(position: position))
+      discussion_id = new_note.discussion_id
+
+      data[:position] = position.to_json
     end
 
-    data
+    data.merge(
+      note_type: new_note.type,
+      discussion_id: discussion_id
+    )
   end
 
   def link_to_reply_discussion(discussion, line_type = nil)
     return unless current_user
 
-    data = discussion.reply_attributes.merge(line_type: line_type)
+    data = { discussion_id: discussion.id, original_discussion_id: discussion.original_id, line_type: line_type }
+    data[:line_code] = discussion.line_code if discussion.respond_to?(:line_code)
 
     button_tag 'Reply...', class: 'btn btn-text-field js-discussion-reply-button',
                            data: data, title: 'Add a reply'
@@ -95,7 +84,15 @@ module NotesHelper
   end
 
   def preload_noteable_for_regular_notes(notes)
-    ActiveRecord::Associations::Preloader.new.preload(notes.select { |note| !note.for_commit? }, :noteable)
+    ActiveRecord::Associations::Preloader.new.preload(notes.reject(&:for_commit?), :noteable)
+  end
+
+  def prepare_notes_for_rendering(notes)
+    preload_noteable_for_regular_notes(notes)
+    preload_max_access_for_authors(notes, @project)
+    Banzai::NoteRenderer.render(notes, @project, current_user)
+
+    notes
   end
 
   def note_max_access_for_user(note)
diff --git a/app/models/commit.rb b/app/models/commit.rb
index ce92cc369ad..5c452f78546 100644
--- a/app/models/commit.rb
+++ b/app/models/commit.rb
@@ -2,6 +2,7 @@ class Commit
   extend ActiveModel::Naming
 
   include ActiveModel::Conversion
+  include Noteable
   include Participable
   include Mentionable
   include Referable
@@ -203,6 +204,10 @@ class Commit
     project.notes.for_commit_id(self.id)
   end
 
+  def discussion_notes
+    notes.non_diff_notes
+  end
+
   def notes_with_associations
     notes.includes(:author)
   end
diff --git a/app/models/commit_discussion.rb b/app/models/commit_discussion.rb
new file mode 100644
index 00000000000..bcca3155335
--- /dev/null
+++ b/app/models/commit_discussion.rb
@@ -0,0 +1,9 @@
+class CommitDiscussion < Discussion
+  def self.override_discussion_id(note)
+    discussion_id(note)
+  end
+
+  def potentially_resolvable?
+    false
+  end
+end
diff --git a/app/models/concerns/note_on_diff.rb b/app/models/concerns/note_on_diff.rb
index b8dd27a7afe..9ce40f329d8 100644
--- a/app/models/concerns/note_on_diff.rb
+++ b/app/models/concerns/note_on_diff.rb
@@ -24,12 +24,4 @@ module NoteOnDiff
   def diff_attributes
     raise NotImplementedError
   end
-
-  def can_be_award_emoji?
-    false
-  end
-
-  def to_discussion
-    Discussion.new([self])
-  end
 end
diff --git a/app/models/concerns/noteable.rb b/app/models/concerns/noteable.rb
new file mode 100644
index 00000000000..7900af6aaac
--- /dev/null
+++ b/app/models/concerns/noteable.rb
@@ -0,0 +1,17 @@
+module Noteable
+  def discussion_notes
+    notes
+  end
+
+  delegate :find_discussion, :find_original_discussion, to: :discussion_notes
+
+  def discussions
+    @discussions ||= discussion_notes
+      .inc_relations_for_view
+      .discussions(self)
+  end
+
+  def grouped_diff_discussions
+    notes.inc_relations_for_view.grouped_diff_discussions
+  end
+end
diff --git a/app/models/concerns/resolvable_note.rb b/app/models/concerns/resolvable_note.rb
new file mode 100644
index 00000000000..eecb77ebf80
--- /dev/null
+++ b/app/models/concerns/resolvable_note.rb
@@ -0,0 +1,61 @@
+module ResolvableNote
+  extend ActiveSupport::Concern
+
+  included do
+    belongs_to :resolved_by, class_name: "User"
+
+    validates :resolved_by, presence: true, if: :resolved?
+
+    # Keep this scope in sync with the logic in `#resolvable?` in `Note` subclasses that are resolvable
+    scope :resolvable, -> { where(type: %w(DiffNote DiscussionNote)).user.where(noteable_type: 'MergeRequest') }
+    scope :resolved, -> { resolvable.where.not(resolved_at: nil) }
+    scope :unresolved, -> { resolvable.where(resolved_at: nil) }
+  end
+
+  module ClassMethods
+    # This method must be kept in sync with `#resolve!`
+    def resolve!(current_user)
+      unresolved.update_all(resolved_at: Time.now, resolved_by_id: current_user.id)
+    end
+
+    # This method must be kept in sync with `#unresolve!`
+    def unresolve!
+      resolved.update_all(resolved_at: nil, resolved_by_id: nil)
+    end
+  end
+
+  # If you update this method remember to also update the scope `resolvable`
+  def resolvable?
+    to_discussion.potentially_resolvable? && !system?
+  end
+
+  def resolved?
+    return false unless resolvable?
+
+    self.resolved_at.present?
+  end
+
+  def to_be_resolved?
+    resolvable? && !resolved?
+  end
+
+  # If you update this method remember to also update `.resolve!`
+  def resolve!(current_user)
+    return unless resolvable?
+    return if resolved?
+
+    self.resolved_at = Time.now
+    self.resolved_by = current_user
+    save!
+  end
+
+  # If you update this method remember to also update `.unresolve!`
+  def unresolve!
+    return unless resolvable?
+    return unless resolved?
+
+    self.resolved_at = nil
+    self.resolved_by = nil
+    save!
+  end
+end
diff --git a/app/models/diff_discussion.rb b/app/models/diff_discussion.rb
new file mode 100644
index 00000000000..9a934c7520c
--- /dev/null
+++ b/app/models/diff_discussion.rb
@@ -0,0 +1,69 @@
+class DiffDiscussion < Discussion
+  NUMBER_OF_TRUNCATED_DIFF_LINES = 16
+
+  delegate  :line_code,
+            :original_line_code,
+            :diff_file,
+            :for_line?,
+            :active?,
+
+            to: :first_note
+
+  delegate  :blob,
+            :highlighted_diff_lines,
+            :diff_lines,
+
+            to: :diff_file,
+            allow_nil: true
+
+  def self.build_discussion_id(note)
+    [*super(note), *unique_position_identifier(note)]
+  end
+
+  def self.build_original_discussion_id(note)
+    [*Discussion.build_discussion_id(note), *note.original_position.key]
+  end
+
+  def self.unique_position_identifier(note)
+    note.position.key
+  end
+
+  def diff_discussion?
+    true
+  end
+
+  def legacy_diff_discussion?
+    false
+  end
+
+  def active?
+    return @active if @active.present?
+
+    @active = first_note.active?
+  end
+  MEMOIZED_VALUES << :active
+
+  def reply_attributes
+    super.merge(first_note.diff_attributes)
+  end
+
+  # Returns an array of at most 16 highlighted lines above a diff note
+  def truncated_diff_lines(highlight: true)
+    lines = highlight ? highlighted_diff_lines : diff_lines
+    prev_lines = []
+
+    lines.each do |line|
+      if line.meta?
+        prev_lines.clear
+      else
+        prev_lines << line
+
+        break if for_line?(line)
+
+        prev_lines.shift if prev_lines.length >= NUMBER_OF_TRUNCATED_DIFF_LINES
+      end
+    end
+
+    prev_lines
+  end
+end
diff --git a/app/models/diff_note.rb b/app/models/diff_note.rb
index 895a91139c9..e5f437f8647 100644
--- a/app/models/diff_note.rb
+++ b/app/models/diff_note.rb
@@ -9,58 +9,44 @@ class DiffNote < Note
   validates :diff_line, presence: true
   validates :line_code, presence: true, line_code: true
   validates :noteable_type, inclusion: { in: %w(Commit MergeRequest) }
-  validates :resolved_by, presence: true, if: :resolved?
   validate :positions_complete
   validate :verify_supported
 
-  # Keep this scope in sync with the logic in `#resolvable?`
-  scope :resolvable, -> { user.where(noteable_type: 'MergeRequest') }
-  scope :resolved, -> { resolvable.where.not(resolved_at: nil) }
-  scope :unresolved, -> { resolvable.where(resolved_at: nil) }
-
-  after_initialize :ensure_original_discussion_id
   before_validation :set_original_position, :update_position, on: :create
-  before_validation :set_line_code, :set_original_discussion_id
+  before_validation :set_line_code
   # We need to do this again, because it's already in `Note`, but is affected by
   # `update_position` and needs to run after that.
-  before_validation :set_discussion_id
+  before_validation :set_discussion_id, if: :position_changed?
   after_save :keep_around_commits
 
-  class << self
-    def build_discussion_id(noteable_type, noteable_id, position)
-      [super(noteable_type, noteable_id), *position.key].join("-")
-    end
-
-    # This method must be kept in sync with `#resolve!`
-    def resolve!(current_user)
-      unresolved.update_all(resolved_at: Time.now, resolved_by_id: current_user.id)
-    end
-
-    # This method must be kept in sync with `#unresolve!`
-    def unresolve!
-      resolved.update_all(resolved_at: nil, resolved_by_id: nil)
-    end
-  end
-
   def new_diff_note?
     true
   end
 
+  def discussion_class(*)
+    DiffDiscussion
+  end
+
   def diff_attributes
-    { position: position.to_json }
+    {
+      original_position: original_position.to_json,
+      position: position.to_json,
+    }
   end
 
-  def position=(new_position)
-    if new_position.is_a?(String)
-      new_position = JSON.parse(new_position) rescue nil
-    end
+  %i(original_position= position=).each do |meth|
+    define_method meth do |new_position|
+      if new_position.is_a?(String)
+        new_position = JSON.parse(new_position) rescue nil
+      end
 
-    if new_position.is_a?(Hash)
-      new_position = new_position.with_indifferent_access
-      new_position = Gitlab::Diff::Position.new(new_position)
-    end
+      if new_position.is_a?(Hash)
+        new_position = new_position.with_indifferent_access
+        new_position = Gitlab::Diff::Position.new(new_position)
+      end
 
-    super(new_position)
+      super(new_position)
+    end
   end
 
   def diff_file
@@ -88,43 +74,6 @@ class DiffNote < Note
     self.position.diff_refs == diff_refs
   end
 
-  # If you update this method remember to also update the scope `resolvable`
-  def resolvable?
-    !system? && for_merge_request?
-  end
-
-  def resolved?
-    return false unless resolvable?
-
-    self.resolved_at.present?
-  end
-
-  # If you update this method remember to also update `.resolve!`
-  def resolve!(current_user)
-    return unless resolvable?
-    return if resolved?
-
-    self.resolved_at = Time.now
-    self.resolved_by = current_user
-    save!
-  end
-
-  # If you update this method remember to also update `.unresolve!`
-  def unresolve!
-    return unless resolvable?
-    return unless resolved?
-
-    self.resolved_at = nil
-    self.resolved_by = nil
-    save!
-  end
-
-  def discussion
-    return unless resolvable?
-
-    self.noteable.find_diff_discussion(self.discussion_id)
-  end
-
   private
 
   def supported?
@@ -140,33 +89,13 @@ class DiffNote < Note
   end
 
   def set_original_position
-    self.original_position = self.position.dup
+    self.original_position = self.position.dup unless self.original_position&.complete?
   end
 
   def set_line_code
     self.line_code = self.position.line_code(self.project.repository)
   end
 
-  def ensure_original_discussion_id
-    return unless self.persisted?
-    return if self.original_discussion_id
-
-    set_original_discussion_id
-    update_column(:original_discussion_id, self.original_discussion_id)
-  end
-
-  def set_original_discussion_id
-    self.original_discussion_id = Digest::SHA1.hexdigest(build_original_discussion_id)
-  end
-
-  def build_discussion_id
-    self.class.build_discussion_id(noteable_type, noteable_id || commit_id, position)
-  end
-
-  def build_original_discussion_id
-    self.class.build_discussion_id(noteable_type, noteable_id || commit_id, original_position)
-  end
-
   def update_position
     return unless supported?
     return if for_commit?
diff --git a/app/models/discussion.rb b/app/models/discussion.rb
index bbe813db823..314aea2c63a 100644
--- a/app/models/discussion.rb
+++ b/app/models/discussion.rb
@@ -1,5 +1,5 @@
 class Discussion
-  NUMBER_OF_TRUNCATED_DIFF_LINES = 16
+  MEMOIZED_VALUES = [] # rubocop:disable Style/MutableConstant
 
   attr_reader :notes
 
@@ -11,12 +11,6 @@ class Discussion
             :for_commit?,
             :for_merge_request?,
 
-            :line_code,
-            :original_line_code,
-            :diff_file,
-            :for_line?,
-            :active?,
-
             to: :first_note
 
   delegate  :resolved_at,
@@ -25,29 +19,46 @@ class Discussion
             to: :last_resolved_note,
             allow_nil: true
 
-  delegate  :blob,
-            :highlighted_diff_lines,
-            :diff_lines,
+  def self.build(notes, noteable = nil)
+    notes.first.discussion_class(noteable).new(notes, noteable)
+  end
 
-            to: :diff_file,
-            allow_nil: true
+  def self.build_collection(notes, noteable = nil)
+    notes.group_by { |n| n.discussion_id(noteable) }.values.map { |notes| build(notes, noteable) }
+  end
 
-  def self.for_notes(notes)
-    notes.group_by(&:discussion_id).values.map { |notes| new(notes) }
+  def self.discussion_id(note)
+    Digest::SHA1.hexdigest(build_discussion_id(note).join("-"))
   end
 
-  def self.for_diff_notes(notes)
-    notes.group_by(&:line_code).values.map { |notes| new(notes) }
+  # Optionally override the discussion ID at runtime depending on circumstances
+  def self.override_discussion_id(note)
+    nil
   end
 
-  def initialize(notes)
-    @notes = notes
+  def self.build_discussion_id(note)
+    noteable_id = note.noteable_id || note.commit_id
+    [:discussion, note.noteable_type.try(:underscore), noteable_id]
   end
 
-  def last_resolved_note
-    return unless resolved?
+  def self.original_discussion_id(note)
+    original_discussion_id = build_original_discussion_id(note)
+    if original_discussion_id
+      Digest::SHA1.hexdigest(original_discussion_id.join("-"))
+    else
+      note.discussion_id
+    end
+  end
 
-    @last_resolved_note ||= resolved_notes.sort_by(&:resolved_at).last
+  # Optionally build a separate original discussion ID that will never change,
+  # if the main discussion ID _can_ change, like in the case of DiffDiscussion.
+  def self.build_original_discussion_id(note)
+    nil
+  end
+
+  def initialize(notes, noteable = nil)
+    @notes = notes
+    @noteable = noteable
   end
 
   def last_updated_at
@@ -59,42 +70,64 @@ class Discussion
   end
 
   def id
-    first_note.discussion_id
+    first_note.discussion_id(noteable)
   end
 
   alias_method :to_param, :id
 
+  def original_id
+    first_note.original_discussion_id
+  end
+
   def diff_discussion?
-    first_note.diff_note?
+    false
+  end
+
+  def render_as_individual_notes?
+    false
   end
 
-  def legacy_diff_discussion?
-    notes.any?(&:legacy_diff_note?)
+  def potentially_resolvable?
+    first_note.for_merge_request?
   end
 
   def resolvable?
     return @resolvable if @resolvable.present?
 
-    @resolvable = diff_discussion? && notes.any?(&:resolvable?)
+    @resolvable = potentially_resolvable? && notes.any?(&:resolvable?)
   end
+  MEMOIZED_VALUES << :resolvable
 
   def resolved?
     return @resolved if @resolved.present?
 
     @resolved = resolvable? && notes.none?(&:to_be_resolved?)
   end
+  MEMOIZED_VALUES << :resolved
 
   def first_note
-    @first_note ||= @notes.first
+    @first_note ||= notes.first
   end
+  MEMOIZED_VALUES << :first_note
 
   def first_note_to_resolve
-    @first_note_to_resolve ||= notes.detect(&:to_be_resolved?)
+    return unless resolvable?
+
+    @first_note_to_resolve ||= notes.find(&:to_be_resolved?)
   end
+  MEMOIZED_VALUES << :first_note_to_resolve
+
+  def last_resolved_note
+    return unless resolved?
+
+    @last_resolved_note ||= resolved_notes.sort_by(&:resolved_at).last
+  end
+  MEMOIZED_VALUES << :last_resolved_note
 
   def last_note
-    @last_note ||= @notes.last
+    @last_note ||= notes.last
   end
+  MEMOIZED_VALUES << :last_note
 
   def resolved_notes
     notes.select(&:resolved?)
@@ -124,25 +157,12 @@ class Discussion
     update { |notes| notes.unresolve! }
   end
 
-  def for_target?(target)
-    self.noteable == target && !diff_discussion?
-  end
-
-  def active?
-    return @active if @active.present?
-
-    @active = first_note.active?
-  end
-
   def collapsed?
-    return false unless diff_discussion?
-
     if resolvable?
       # New diff discussions only disappear once they are marked resolved
       resolved?
     else
-      # Old diff discussions disappear once they become outdated
-      !active?
+      false
     end
   end
 
@@ -151,52 +171,22 @@ class Discussion
   end
 
   def reply_attributes
-    data = {
-      noteable_type: first_note.noteable_type,
-      noteable_id:   first_note.noteable_id,
-      commit_id:     first_note.commit_id,
-      discussion_id: self.id,
-    }
-
-    if diff_discussion?
-      data[:note_type] = first_note.type
-
-      data.merge!(first_note.diff_attributes)
-    end
-
-    data
-  end
-
-  # Returns an array of at most 16 highlighted lines above a diff note
-  def truncated_diff_lines(highlight: true)
-    lines = highlight ? highlighted_diff_lines : diff_lines
-    prev_lines = []
-
-    lines.each do |line|
-      if line.meta?
-        prev_lines.clear
-      else
-        prev_lines << line
-
-        break if for_line?(line)
-
-        prev_lines.shift if prev_lines.length >= NUMBER_OF_TRUNCATED_DIFF_LINES
-      end
-    end
-
-    prev_lines
+    first_note.slice(:type, :noteable_type, :noteable_id, :commit_id)
   end
 
   private
 
   def update
-    notes_relation = DiffNote.where(id: notes.map(&:id)).fresh
+    # Do not select `Note.resolvable`, so that system notes remain in the collection
+    notes_relation = Note.where(id: notes.map(&:id))
+
     yield(notes_relation)
 
     # Set the notes array to the updated notes
-    @notes = notes_relation.to_a
+    @notes = notes_relation.fresh.to_a
 
-    # Reset the memoized values
-    @last_resolved_note = @resolvable = @resolved = @first_note = @last_note = nil
+    MEMOIZED_VALUES.each do |var|
+      instance_variable_set(:"@#{var}", nil)
+    end
   end
 end
diff --git a/app/models/discussion_note.rb b/app/models/discussion_note.rb
new file mode 100644
index 00000000000..fc168ae74a9
--- /dev/null
+++ b/app/models/discussion_note.rb
@@ -0,0 +1,9 @@
+class DiscussionNote < Note
+  NOTEABLE_TYPES = %w(MergeRequest).freeze
+
+  validates :noteable_type, inclusion: { in: NOTEABLE_TYPES }
+
+  def discussion_class(*)
+    SimpleDiscussion
+  end
+end
diff --git a/app/models/individual_note_discussion.rb b/app/models/individual_note_discussion.rb
new file mode 100644
index 00000000000..8c82d217126
--- /dev/null
+++ b/app/models/individual_note_discussion.rb
@@ -0,0 +1,13 @@
+class IndividualNoteDiscussion < Discussion
+  def self.build_discussion_id(note)
+    [*super(note), SecureRandom.hex]
+  end
+
+  def potentially_resolvable?
+    false
+  end
+
+  def render_as_individual_notes?
+    true
+  end
+end
diff --git a/app/models/issue.rb b/app/models/issue.rb
index 472796df9df..5010b6e1edf 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -3,6 +3,7 @@ require 'carrierwave/orm/activerecord'
 class Issue < ActiveRecord::Base
   include InternalId
   include Issuable
+  include Noteable
   include Referable
   include Sortable
   include Spammable
diff --git a/app/models/legacy_diff_discussion.rb b/app/models/legacy_diff_discussion.rb
new file mode 100644
index 00000000000..e13f17abdbe
--- /dev/null
+++ b/app/models/legacy_diff_discussion.rb
@@ -0,0 +1,21 @@
+class LegacyDiffDiscussion < DiffDiscussion
+  def self.unique_position_identifier(note)
+    note.line_code
+  end
+
+  def self.build_original_discussion_id(note)
+    Discussion.build_original_discussion_id(note)
+  end
+
+  def legacy_diff_discussion?
+    true
+  end
+
+  def potentially_resolvable?
+    false
+  end
+
+  def collapsed?
+    !active?
+  end
+end
diff --git a/app/models/legacy_diff_note.rb b/app/models/legacy_diff_note.rb
index 40277a9b139..4952a1ce2ca 100644
--- a/app/models/legacy_diff_note.rb
+++ b/app/models/legacy_diff_note.rb
@@ -7,10 +7,8 @@ class LegacyDiffNote < Note
 
   before_create :set_diff
 
-  class << self
-    def build_discussion_id(noteable_type, noteable_id, line_code)
-      [super(noteable_type, noteable_id), line_code].join("-")
-    end
+  def discussion_class(*)
+    LegacyDiffDiscussion
   end
 
   def legacy_diff_note?
@@ -119,8 +117,4 @@ class LegacyDiffNote < Note
     diffs = noteable.raw_diffs(Commit.max_diff_options)
     diffs.find { |d| d.new_path == self.diff.new_path }
   end
-
-  def build_discussion_id
-    self.class.build_discussion_id(noteable_type, noteable_id || commit_id, line_code)
-  end
 end
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 8d740adb771..546fd2b8e35 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -1,6 +1,7 @@
 class MergeRequest < ActiveRecord::Base
   include InternalId
   include Issuable
+  include Noteable
   include Referable
   include Sortable
 
@@ -475,44 +476,32 @@ class MergeRequest < ActiveRecord::Base
     )
   end
 
-  def discussions
-    @discussions ||= self.related_notes.
-      inc_relations_for_view.
-      fresh.
-      discussions
-  end
-
-  def diff_discussions
-    @diff_discussions ||= self.notes.diff_notes.discussions
-  end
+  alias_method :discussion_notes, :related_notes
 
   def resolvable_discussions
-    @resolvable_discussions ||= diff_discussions.select(&:to_be_resolved?)
-  end
-
-  def discussions_can_be_resolved_by?(user)
-    resolvable_discussions.all? { |discussion| discussion.can_resolve?(user) }
-  end
-
-  def find_diff_discussion(discussion_id)
-    notes = self.notes.diff_notes.where(discussion_id: discussion_id).fresh.to_a
-    return if notes.empty?
-
-    Discussion.new(notes)
+    @resolvable_discussions ||= notes.resolvable.discussions
   end
 
   def discussions_resolvable?
-    diff_discussions.any?(&:resolvable?)
+    resolvable_discussions.any?(&:resolvable?)
   end
 
   def discussions_resolved?
-    discussions_resolvable? && diff_discussions.none?(&:to_be_resolved?)
+    discussions_resolvable? && resolvable_discussions.none?(&:to_be_resolved?)
   end
 
   def discussions_to_be_resolved?
     discussions_resolvable? && !discussions_resolved?
   end
 
+  def discussions_to_be_resolved
+    @discussions_to_be_resolved ||= resolvable_discussions.select(&:to_be_resolved?)
+  end
+
+  def discussions_can_be_resolved_by?(user)
+    discussions_to_be_resolved.all? { |discussion| discussion.can_resolve?(user) }
+  end
+
   def mergeable_discussions_state?
     return true unless project.only_allow_merge_if_all_discussions_are_resolved?
 
diff --git a/app/models/note.rb b/app/models/note.rb
index 16d66cb1427..6385747b571 100644
--- a/app/models/note.rb
+++ b/app/models/note.rb
@@ -8,6 +8,7 @@ class Note < ActiveRecord::Base
   include FasterCacheKeys
   include CacheMarkdownField
   include AfterCommitQueue
+  include ResolvableNote
 
   cache_markdown_field :note, pipeline: :note
 
@@ -32,9 +33,6 @@ class Note < ActiveRecord::Base
   belongs_to :author, class_name: "User"
   belongs_to :updated_by, class_name: "User"
 
-  # Only used by DiffNote, but defined here so that it can be used in `Note.includes`
-  belongs_to :resolved_by, class_name: "User"
-
   has_many :todos, dependent: :destroy
   has_many :events, as: :target, dependent: :destroy
   has_one :system_note_metadata
@@ -54,6 +52,7 @@ class Note < ActiveRecord::Base
   validates :noteable_id, presence: true, unless: [:for_commit?, :importing?]
   validates :commit_id, presence: true, if: :for_commit?
   validates :author, presence: true
+  validates :discussion_id, :original_discussion_id, presence: true, format: { with: /\A\h{40}\z/ }
 
   validate unless: [:for_commit?, :importing?, :for_personal_snippet?] do |note|
     unless note.noteable.try(:project) == note.project
@@ -76,7 +75,7 @@ class Note < ActiveRecord::Base
   end
 
   scope :diff_notes, ->{ where(type: %w(LegacyDiffNote DiffNote)) }
-  scope :non_diff_notes, ->{ where(type: ['Note', nil]) }
+  scope :non_diff_notes, ->{ where(type: ['Note', 'DiscussionNote', nil]) }
 
   scope :with_associations, -> do
     # FYI noteable cannot be loaded for LegacyDiffNote for commits
@@ -84,9 +83,9 @@ class Note < ActiveRecord::Base
              project: [:project_members, { group: [:group_members] }])
   end
 
-  after_initialize :ensure_discussion_id
+  after_initialize :ensure_discussion_id, :ensure_original_discussion_id
   before_validation :nullify_blank_type, :nullify_blank_line_code
-  before_validation :set_discussion_id
+  before_validation :set_discussion_id, :set_original_discussion_id, on: :create
   after_save :keep_around_commit, unless: :for_personal_snippet?
   after_save :expire_etag_cache
 
@@ -95,22 +94,31 @@ class Note < ActiveRecord::Base
       ActiveModel::Name.new(self, nil, 'note')
     end
 
-    def build_discussion_id(noteable_type, noteable_id)
-      [:discussion, noteable_type.try(:underscore), noteable_id].join("-")
+    def discussions(noteable = nil)
+      Discussion.build_collection(fresh, noteable)
     end
 
-    def discussion_id(*args)
-      Digest::SHA1.hexdigest(build_discussion_id(*args))
+    def find_original_discussion(discussion_id)
+      note = find_by(original_discussion_id: discussion_id)
+      return unless note
+
+      note.to_discussion
     end
 
-    def discussions
-      Discussion.for_notes(fresh)
+    def find_discussion(discussion_id)
+      notes = where(discussion_id: discussion_id).fresh.to_a
+      return if notes.empty?
+
+      Discussion.build(notes)
     end
 
     def grouped_diff_discussions
-      active_notes = diff_notes.fresh.select(&:active?)
-      Discussion.for_diff_notes(active_notes).
-        map { |d| [d.line_code, d] }.to_h
+      diff_notes.
+        fresh.
+        select(&:active?).
+        group_by(&:line_code).
+        map { |line_code, notes| [line_code, DiffDiscussion.build(notes)] }.
+        to_h
     end
 
     def count_for_collection(ids, type)
@@ -121,7 +129,7 @@ class Note < ActiveRecord::Base
   end
 
   def cross_reference?
-    system && SystemNoteService.cross_reference?(note)
+    system? && SystemNoteService.cross_reference?(note)
   end
 
   def diff_note?
@@ -140,18 +148,6 @@ class Note < ActiveRecord::Base
     true
   end
 
-  def resolvable?
-    false
-  end
-
-  def resolved?
-    false
-  end
-
-  def to_be_resolved?
-    resolvable? && !resolved?
-  end
-
   def max_attachment_size
     current_application_settings.max_attachment_size.megabytes.to_i
   end
@@ -228,7 +224,7 @@ class Note < ActiveRecord::Base
   end
 
   def can_be_award_emoji?
-    noteable.is_a?(Awardable)
+    noteable.is_a?(Awardable) && !part_of_discussion?
   end
 
   def contains_emoji_only?
@@ -239,6 +235,42 @@ class Note < ActiveRecord::Base
     for_personal_snippet? ? 'personal_snippet' : noteable_type.underscore
   end
 
+  def can_be_discussion_note?
+    DiscussionNote::NOTEABLE_TYPES.include?(self.noteable_type)
+  end
+
+  def discussion_class(noteable = nil)
+    # When commit notes are rendered on an MR's Discussion page, they are
+    # displayed in one discussion instead of individually
+    if noteable && noteable != self.noteable && for_commit?
+      CommitDiscussion
+    else
+      IndividualNoteDiscussion
+    end
+  end
+
+  def discussion_id(noteable = nil)
+    discussion_class(noteable).override_discussion_id(self) || super()
+  end
+
+  # Returns a discussion containing just this note
+  def to_discussion(noteable = nil)
+    Discussion.build([self], noteable)
+  end
+
+  # Returns the entire discussion this note is part of
+  def discussion
+    if part_of_discussion?
+      self.noteable.notes.find_discussion(self.discussion_id)
+    else
+      to_discussion
+    end
+  end
+
+  def part_of_discussion?
+    !to_discussion.render_as_individual_notes?
+  end
+
   private
 
   def keep_around_commit
@@ -264,17 +296,21 @@ class Note < ActiveRecord::Base
   end
 
   def set_discussion_id
-    self.discussion_id = Digest::SHA1.hexdigest(build_discussion_id)
+    self.discussion_id ||= discussion_class.discussion_id(self)
   end
 
-  def build_discussion_id
-    if for_merge_request?
-      # Notes on merge requests are always in a discussion of their own,
-      # so we generate a unique discussion ID.
-      [:discussion, :note, SecureRandom.hex].join("-")
-    else
-      self.class.build_discussion_id(noteable_type, noteable_id || commit_id)
-    end
+  def ensure_original_discussion_id
+    return unless self.persisted?
+    # Needed in case the SELECT statement doesn't ask for `original_discussion_id`
+    return unless self.has_attribute?(:original_discussion_id)
+    return if self.original_discussion_id
+
+    set_original_discussion_id
+    update_column(:original_discussion_id, self.original_discussion_id)
+  end
+
+  def set_original_discussion_id
+    self.original_discussion_id = discussion_class.original_discussion_id(self)
   end
 
   def expire_etag_cache
diff --git a/app/models/sent_notification.rb b/app/models/sent_notification.rb
index f4bcb49b34d..6770af6fffd 100644
--- a/app/models/sent_notification.rb
+++ b/app/models/sent_notification.rb
@@ -5,10 +5,11 @@ class SentNotification < ActiveRecord::Base
   belongs_to :noteable, polymorphic: true
   belongs_to :recipient, class_name: "User"
 
-  validates :project, :recipient, :reply_key, presence: true
-  validates :reply_key, uniqueness: true
+  validates :project, :recipient, presence: true
+  validates :reply_key, presence: true, uniqueness: true
   validates :noteable_id, presence: true, unless: :for_commit?
   validates :commit_id, presence: true, if: :for_commit?
+  validates :in_reply_to_discussion_id, format: { with: /\A\h{40}\z/, allow_nil: true }
   validate :note_valid
 
   after_save :keep_around_commit
@@ -34,23 +35,20 @@ class SentNotification < ActiveRecord::Base
       end
 
       attrs.reverse_merge!(
-        project:        noteable.project,
-        noteable_type:  noteable.class.name,
-        noteable_id:    noteable_id,
-        commit_id:      commit_id,
-        recipient_id:   recipient_id,
-        reply_key:      reply_key
+        project: noteable.project,
+        recipient_id: recipient_id,
+        reply_key: reply_key,
+
+        noteable_type: noteable.class.name,
+        noteable_id: noteable_id,
+        commit_id: commit_id,
       )
 
       create(attrs)
     end
 
     def record_note(note, recipient_id, reply_key, attrs = {})
-      if note.diff_note?
-        attrs[:note_type] = note.type
-
-        attrs.merge!(note.diff_attributes)
-      end
+      attrs[:in_reply_to_discussion_id] = note.original_discussion_id
 
       record(note.noteable, recipient_id, reply_key, attrs)
     end
@@ -89,31 +87,34 @@ class SentNotification < ActiveRecord::Base
     self.reply_key
   end
 
-  def note_attributes
-    {
-      project:        self.project,
-      author:         self.recipient,
-      type:           self.note_type,
-      noteable_type:  self.noteable_type,
-      noteable_id:    self.noteable_id,
-      commit_id:      self.commit_id,
-      line_code:      self.line_code,
-      position:       self.position.to_json
+  def note_params
+    attrs = {
+      noteable_type: self.noteable_type,
+      noteable_id: self.noteable_id,
+      commit_id: self.commit_id
     }
-  end
 
-  def create_note(note)
-    Notes::CreateService.new(
-      self.project,
-      self.recipient,
-      self.note_attributes.merge(note: note)
-    ).execute
+    if self.in_reply_to_discussion_id.present?
+      attrs[:in_reply_to_discussion_id] = self.in_reply_to_discussion_id
+    else
+      attrs.merge!(
+        type: self.note_type,
+
+        # LegacyDiffNote
+        line_code: self.line_code,
+
+        # DiffNote
+        position: self.position.to_json
+      )
+    end
+
+    attrs
   end
 
   private
 
   def note_valid
-    Note.new(note_attributes.merge(note: "Test")).valid?
+    Notes::BuildService.new(self.project, self.recipient, note_params.merge(note: 'Test')).execute.valid?
   end
 
   def keep_around_commit
diff --git a/app/models/simple_discussion.rb b/app/models/simple_discussion.rb
new file mode 100644
index 00000000000..a5ef5c565e7
--- /dev/null
+++ b/app/models/simple_discussion.rb
@@ -0,0 +1,9 @@
+class SimpleDiscussion < Discussion
+  def self.build_discussion_id(note)
+    [*super(note), SecureRandom.hex]
+  end
+
+  def reply_attributes
+    super.merge(discussion_id: self.id)
+  end
+end
diff --git a/app/models/snippet.rb b/app/models/snippet.rb
index 30aca62499c..380835707e8 100644
--- a/app/models/snippet.rb
+++ b/app/models/snippet.rb
@@ -2,6 +2,7 @@ class Snippet < ActiveRecord::Base
   include Gitlab::VisibilityLevel
   include Linguist::BlobHelper
   include CacheMarkdownField
+  include Noteable
   include Participable
   include Referable
   include Sortable
diff --git a/app/services/concerns/issues/resolve_discussions.rb b/app/services/concerns/issues/resolve_discussions.rb
index 297c7d696c3..378967f0231 100644
--- a/app/services/concerns/issues/resolve_discussions.rb
+++ b/app/services/concerns/issues/resolve_discussions.rb
@@ -25,7 +25,7 @@ module Issues
           Array(discussion_or_nil)
         else
           merge_request_to_resolve_discussions_of
-            .resolvable_discussions
+            .discussions_to_be_resolved
         end
     end
   end
diff --git a/app/services/notes/build_service.rb b/app/services/notes/build_service.rb
new file mode 100644
index 00000000000..47ed3e3fc3d
--- /dev/null
+++ b/app/services/notes/build_service.rb
@@ -0,0 +1,29 @@
+module Notes
+  class BuildService < BaseService
+    def execute
+      # TODO: Remove when we use a selectbox instead of a submit button
+      params[:type] = DiscussionNote.name if params.delete(:new_discussion)
+
+      in_reply_to_discussion_id = params.delete(:in_reply_to_discussion_id)
+      if project && in_reply_to_discussion_id.present?
+        discussion =
+          project.notes.find_original_discussion(in_reply_to_discussion_id) ||
+          project.notes.find_discussion(in_reply_to_discussion_id)
+
+        unless discussion
+          note = Note.new
+          note.errors.add(:base, 'Discussion to reply to cannot be found')
+          return note
+        end
+
+        params.merge!(discussion.reply_attributes)
+      end
+
+      note = Note.new(params)
+      note.project = project
+      note.author = current_user
+
+      note
+    end
+  end
+end
diff --git a/app/services/notes/create_service.rb b/app/services/notes/create_service.rb
index 61d66a26932..c08cddcbee5 100644
--- a/app/services/notes/create_service.rb
+++ b/app/services/notes/create_service.rb
@@ -3,10 +3,8 @@ module Notes
     def execute
       merge_request_diff_head_sha = params.delete(:merge_request_diff_head_sha)
 
-      note = Note.new(params)
-      note.project = project
-      note.author  = current_user
-      note.system  = false
+      note = Notes::BuildService.new(project, current_user, params).execute
+      return note unless note.valid?
 
       # We execute commands (extracted from `params[:note]`) on the noteable
       # **before** we save the note because if the note consists of commands
diff --git a/app/services/system_note_service.rb b/app/services/system_note_service.rb
index 35cfcc3682e..c9e25c7aaa2 100644
--- a/app/services/system_note_service.rb
+++ b/app/services/system_note_service.rb
@@ -228,12 +228,10 @@ module SystemNoteService
 
   def discussion_continued_in_issue(discussion, project, author, issue)
     body = "created #{issue.to_reference} to continue this discussion"
+    note_attributes = discussion.reply_attributes.merge(project: project, author: author, note: body)
 
-    note_params = discussion.reply_attributes.merge(project: project, author: author, note: body)
-    note_params[:type] = note_params.delete(:note_type)
-
-    note = Note.create(note_params.merge(system: true))
-    note.system_note_metadata = SystemNoteMetadata.new({ action: 'discussion' })
+    note = Note.create(note_attributes.merge(system: true))
+    note.system_note_metadata = SystemNoteMetadata.new(action: 'discussion')
 
     note
   end
diff --git a/app/views/discussions/_discussion.html.haml b/app/views/discussions/_discussion.html.haml
index 2d78c55211e..38112f67881 100644
--- a/app/views/discussions/_discussion.html.haml
+++ b/app/views/discussions/_discussion.html.haml
@@ -18,19 +18,21 @@
 
           .inline.discussion-headline-light
             = discussion.author.to_reference
-            started a discussion on
+            started a discussion
 
-            - if discussion.for_commit?
+            - if discussion.for_commit? && @noteable != discussion.noteable
+              on
               - commit = discussion.noteable
               - if commit
                 commit
-                = link_to commit.short_id, namespace_project_commit_path(discussion.project.namespace, discussion.project, discussion.noteable, anchor: discussion.line_code), class: 'monospace'
+                - anchor = discussion.line_code if discussion.diff_discussion?
+                = link_to commit.short_id, namespace_project_commit_path(discussion.project.namespace, discussion.project, discussion.noteable, anchor: anchor), class: 'monospace'
               - else
                 a deleted commit
-            - else
+            - elsif discussion.diff_discussion?
+              on
               - if discussion.active?
-                = link_to diffs_namespace_project_merge_request_path(discussion.project.namespace, discussion.project, discussion.noteable, anchor: discussion.line_code) do
-                  the diff
+                = link_to 'the diff', discussion_diff_path(discussion)
               - else
                 an outdated diff
 
diff --git a/app/views/discussions/_notes.html.haml b/app/views/discussions/_notes.html.haml
index 2789391819c..c8fcc37d1ab 100644
--- a/app/views/discussions/_notes.html.haml
+++ b/app/views/discussions/_notes.html.haml
@@ -1,18 +1,19 @@
-%ul.notes{ data: { discussion_id: discussion.id } }
+%ul.notes{ data: { discussion_id: discussion.id, original_discussion_id: discussion.original_id } }
   = render partial: "projects/notes/note", collection: discussion.notes, as: :note
 
 - if current_user
   .discussion-reply-holder
-    - if discussion.diff_discussion?
+    - if discussion.potentially_resolvable?
       - line_type = local_assigns.fetch(:line_type, nil)
 
       .btn-group-justified.discussion-with-resolve-btn{ role: "group" }
         .btn-group{ role: "group" }
           = link_to_reply_discussion(discussion, line_type)
+
         = render "discussions/resolve_all", discussion: discussion
-        - if discussion.for_merge_request?
-          .btn-group.discussion-actions
-            = render "discussions/new_issue_for_discussion", discussion: discussion, merge_request: discussion.noteable
-            = render "discussions/jump_to_next", discussion: discussion
+        
+        .btn-group.discussion-actions
+          = render "discussions/new_issue_for_discussion", discussion: discussion, merge_request: discussion.noteable
+          = render "discussions/jump_to_next", discussion: discussion
     - else
       = link_to_reply_discussion(discussion)
diff --git a/app/views/discussions/_resolve_all.html.haml b/app/views/discussions/_resolve_all.html.haml
index e30ee1b0e05..689a22acd27 100644
--- a/app/views/discussions/_resolve_all.html.haml
+++ b/app/views/discussions/_resolve_all.html.haml
@@ -1,9 +1,8 @@
-- if discussion.for_merge_request?
-  %resolve-discussion-btn{ ":discussion-id" => "'#{discussion.id}'",
-      ":merge-request-id" => discussion.noteable.iid,
-      ":can-resolve" => discussion.can_resolve?(current_user),
-      "inline-template" => true }
-    .btn-group{ role: "group", "v-if" => "showButton" }
-      %button.btn.btn-default{ type: "button", "@click" => "resolve", ":disabled" => "loading", "v-cloak" => "true" }
-        = icon("spinner spin", "v-show" => "loading")
-        {{ buttonText }}
+%resolve-discussion-btn{ ":discussion-id" => "'#{discussion.id}'",
+    ":merge-request-id" => discussion.noteable.iid,
+    ":can-resolve" => discussion.can_resolve?(current_user),
+    "inline-template" => true }
+  .btn-group{ role: "group", "v-if" => "showButton" }
+    %button.btn.btn-default{ type: "button", "@click" => "resolve", ":disabled" => "loading", "v-cloak" => "true" }
+      = icon("spinner spin", "v-show" => "loading")
+      {{ buttonText }}
diff --git a/app/views/projects/commit/show.html.haml b/app/views/projects/commit/show.html.haml
index d5fc283aa8d..0d11da2451a 100644
--- a/app/views/projects/commit/show.html.haml
+++ b/app/views/projects/commit/show.html.haml
@@ -10,6 +10,7 @@
   - else
     .block-connector
   = render "projects/diffs/diffs", diffs: @diffs, environment: @environment
+
   = render "projects/notes/notes_with_form"
   - if can_collaborate_with_project?
     - %w(revert cherry-pick).each do |type|
diff --git a/app/views/projects/notes/_form.html.haml b/app/views/projects/notes/_form.html.haml
index b561052e721..80db16ea578 100644
--- a/app/views/projects/notes/_form.html.haml
+++ b/app/views/projects/notes/_form.html.haml
@@ -4,12 +4,18 @@
   = hidden_field_tag :view, diff_view
   = hidden_field_tag :line_type
   = hidden_field_tag :merge_request_diff_head_sha, @note.noteable.try(:diff_head_sha)
+  = hidden_field_tag :in_reply_to_discussion_id
+
   = note_target_fields(@note)
-  = f.hidden_field :commit_id
-  = f.hidden_field :line_code
-  = f.hidden_field :noteable_id
   = f.hidden_field :noteable_type
+  = f.hidden_field :noteable_id
+  = f.hidden_field :commit_id
   = f.hidden_field :type
+
+  -# LegacyDiffNote
+  = f.hidden_field :line_code
+
+  -# DiffNote
   = f.hidden_field :position
 
   = render layout: 'projects/md_preview', locals: { preview_class: "md-preview", referenced_users: true } do
@@ -23,6 +29,11 @@
 
   .note-form-actions.clearfix
     = f.submit 'Comment', class: "btn btn-nr btn-create append-right-10 comment-btn js-comment-button"
+
+    - if @note.can_be_discussion_note?
+      = submit_tag 'Start discussion', name: 'new_discussion', class: "btn btn-nr append-right-10 btn-inverted js-note-new-discussion"
+
     = yield(:note_actions)
+
     %a.btn.btn-cancel.js-note-discard{ role: "button", data: {cancel_text: "Cancel" } }
       Discard draft
diff --git a/app/views/projects/notes/_note.html.haml b/app/views/projects/notes/_note.html.haml
index 6c0e6d48d6c..097dd17c094 100644
--- a/app/views/projects/notes/_note.html.haml
+++ b/app/views/projects/notes/_note.html.haml
@@ -31,7 +31,7 @@
             - if note.resolvable?
               - can_resolve = can?(current_user, :resolve_note, note)
               %resolve-btn{ "project-path" => project_path(note.project),
-                  "discussion-id" => note.discussion_id,
+                  "discussion-id" => note.discussion_id(@noteable),
                   ":note-id" => note.id,
                   ":resolved" => note.resolved?,
                   ":can-resolve" => can_resolve,
diff --git a/app/views/projects/notes/_notes.html.haml b/app/views/projects/notes/_notes.html.haml
index 022578bd6db..50809068453 100644
--- a/app/views/projects/notes/_notes.html.haml
+++ b/app/views/projects/notes/_notes.html.haml
@@ -1,7 +1,7 @@
 - if @discussions.present?
   - @discussions.each do |discussion|
-    - if discussion.for_target?(@noteable)
-      = render partial: "projects/notes/note", object: discussion.first_note, as: :note
+    - if discussion.render_as_individual_notes?
+      = render partial: "projects/notes/note", collection: discussion.notes, as: :note
     - else
       = render 'discussions/discussion', discussion: discussion
 - else