Add option to start a new discussion on an MR

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
     });
   };

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') {

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

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!

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
-    preload_max_access_for_authors(@notes, @project)
+    @note     = @project.notes.new(noteable: @issue)
+    @discussions = @issue.discussions
+    @notes = prepare_notes_for_rendering(@discussions.flat_map(&:notes))
     respond_to do |format|
       format.html

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

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)
-        )
-        # 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
+          discussion_html: discussion_html(discussion),
+          # 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

app/controllers/projects/snippets_controller.rb

 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

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 init_collection
-    @notes =
-      if @params[:target_id]
-        on_target(@params[:target_type], @params[:target_id])
+  def target
+    return @target if defined?(@target)
+    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

app/helpers/notes_helper.rb

@@ -24,9 +24,9 @@ module NotesHelper
   end
   def diff_view_data
-    return {} unless @comments_target
-    @comments_target.slice(:noteable_id, :noteable_type, :commit_id)
+    return {} unless @new_diff_note_attrs
+    @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)

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

app/models/commit_discussion.rb

+class CommitDiscussion < Discussion
+  def self.override_discussion_id(note)
+    discussion_id(note)
+  end
+
+  def potentially_resolvable?
+    false
+  end
+end

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

app/models/concerns/noteable.rb

+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

app/models/concerns/resolvable_note.rb

+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

app/models/diff_discussion.rb

+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

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
-    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)
+  %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
+      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?

app/models/discussion.rb

 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,
-            to: :diff_file,
-            allow_nil: true
-  def self.for_notes(notes)
-    notes.group_by(&:discussion_id).values.map { |notes| new(notes) }
+  def self.build(notes, noteable = nil)
+    notes.first.discussion_class(noteable).new(notes, noteable)
+  end
+  def self.build_collection(notes, noteable = nil)
+    notes.group_by { |n| n.discussion_id(noteable) }.values.map { |notes| build(notes, noteable) }
+  end
+  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?
-    @last_resolved_note ||= resolved_notes.sort_by(&:resolved_at).last
+  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
+  # 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
-    # Reset the memoized values
-    @last_resolved_note = @resolvable = @resolved = @first_note = @last_note = nil
+    @notes = notes_relation.fresh.to_a
+    MEMOIZED_VALUES.each do |var|
+      instance_variable_set(:"@#{var}", nil)
+    end
   end
 end

app/models/discussion_note.rb

+class DiscussionNote < Note
+  NOTEABLE_TYPES = %w(MergeRequest).freeze
+
+  validates :noteable_type, inclusion: { in: NOTEABLE_TYPES }
+
+  def discussion_class(*)
+    SimpleDiscussion
+  end
+end

app/models/individual_note_discussion.rb

+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