Merge branch 'discussions_nplusone' into 'master'

Fix N+1 issue in note discussions

See merge request gitlab-org/gitlab!57374

app/serializers/discussion_entity.rb

@@ -2,7 +2,13 @@
 
 class DiscussionEntity < BaseDiscussionEntity
   expose :notes do |discussion, opts|
-    request.note_entity.represent(discussion.notes, opts.merge(with_base_discussion: false))
+    request.note_entity.represent(
+      discussion.notes,
+      opts.merge(
+        with_base_discussion: false,
+        discussion: discussion
+      )
+    )
   end
 
   expose :positions, if: -> (d, _) { display_merge_ref_discussions?(d) } do |discussion|

app/serializers/note_entity.rb

@@ -36,7 +36,8 @@ class NoteEntity < API::Entities::Note
     end
 
     expose :can_resolve_discussion do |note|
-      note.discussion.resolvable? && note.discussion.can_resolve?(current_user)
+      discussion = options.fetch(:discussion, nil) || note.discussion
+      discussion.resolvable? && discussion.can_resolve?(current_user)
     end
   end
 

changelogs/unreleased/discussions_nplusone.yml

+---
+title: Fix N+1 issue when loading merge request comments
+merge_request: 57374
+author:
+type: performance

spec/requests/projects/merge_requests_discussions_spec.rb

@@ -26,6 +26,10 @@ RSpec.describe 'merge requests discussions' do
 
     # https://docs.gitlab.com/ee/development/query_recorder.html#use-request-specs-instead-of-controller-specs
     it 'avoids N+1 DB queries', :request_store do
+      send_request # warm up
+
+      create(:diff_note_on_merge_request, noteable: merge_request,
+             project: merge_request.project)
       control = ActiveRecord::QueryRecorder.new { send_request }
 
       create(:diff_note_on_merge_request, noteable: merge_request,