Fix N+1 issue in note discussions

When rendering discussions, we refer back to note.discussion, but
because this association is not set, we find discussion by id
again for each note. TO avoid this, note's discussion is preset
when discussion instance is created.

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,