Merge branch 'fix/use-fewer-queries-for-mr-notes' into 'master'

Reduce number of queries made for merge_requests/:id/diffs

## What does this MR do?
It reduces the number of DB queries made while processing and rendering MR notes.

## Are there points in the code the reviewer needs to double check?
N/A

## Why was this MR needed?
For `https://staging.gitlab.com/gitlab-org/gitlab-ce/merge_requests/3142/diffs.json`, for each note we make number of DB queries, almost all of them are handled by the AR caching layer, but they seem to add up a few seconds. Testing on staging, calling `merge_requests/3142/diffs.json` was reduced to ~5.5 seconds from ~8 seconds.

## What are the relevant issue numbers?
N/A

## Screenshots (if relevant)
N/A

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [ ] ~~API support added~~
- ~~Tests~~
  - [ ] ~~Added for this feature/bug~~
  - [ ] ~~All builds are passing~~
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5551

4 files changed, 12 insertions, 3 deletions

diff --git a/CHANGELOG b/CHANGELOG
index 8759bb04dec..2704f89ccda 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -28,6 +28,7 @@ v 8.11.0 (unreleased)
   - Add commit stats in commit api. !5517 (dixpac)
   - Make error pages responsive (Takuya Noguchi)
   - Change requests_profiles resource constraint to catch virtually any file
+  - Reduce number of queries made for merge_requests/:id/diffs
 
 v 8.10.3 (unreleased)
 
diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 23252fa59cc..47c21a18b33 100644
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/app/controllers/projects/merge_requests_controller.rb
@@ -407,7 +407,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     }
 
     @use_legacy_diff_notes = !@merge_request.support_new_diff_notes?
-    @grouped_diff_discussions = @merge_request.notes.grouped_diff_discussions
+    @grouped_diff_discussions = @merge_request.notes.inc_author_project_award_emoji.grouped_diff_discussions
 
     Banzai::NoteRenderer.render(
       @grouped_diff_discussions.values.flat_map(&:notes),
diff --git a/app/models/concerns/issuable.rb b/app/models/concerns/issuable.rb
index acb6f5a2998..cbae1cd439b 100644
--- a/app/models/concerns/issuable.rb
+++ b/app/models/concerns/issuable.rb
@@ -17,7 +17,7 @@ module Issuable
     belongs_to :assignee, class_name: "User"
     belongs_to :updated_by, class_name: "User"
     belongs_to :milestone
-    has_many :notes, as: :noteable, dependent: :destroy do
+    has_many :notes, as: :noteable, inverse_of: :noteable, dependent: :destroy do
       def authors_loaded?
         # We check first if we're loaded to not load unnecessarily.
         loaded? && to_a.all? { |note| note.association(:author).loaded? }
diff --git a/app/models/legacy_diff_note.rb b/app/models/legacy_diff_note.rb
index 04a651d50ab..865712268a0 100644
--- a/app/models/legacy_diff_note.rb
+++ b/app/models/legacy_diff_note.rb
@@ -25,6 +25,14 @@ class LegacyDiffNote < Note
     @discussion_id ||= self.class.build_discussion_id(noteable_type, noteable_id || commit_id, line_code)
   end
 
+  def project_repository
+    if RequestStore.active?
+      RequestStore.fetch("project:#{project_id}:repository") { self.project.repository }
+    else
+      self.project.repository
+    end
+  end
+
   def diff_file_hash
     line_code.split('_')[0] if line_code
   end
@@ -34,7 +42,7 @@ class LegacyDiffNote < Note
   end
 
   def diff_file
-    @diff_file ||= Gitlab::Diff::File.new(diff, repository: self.project.repository) if diff
+    @diff_file ||= Gitlab::Diff::File.new(diff, repository: project_repository) if diff
   end
 
   def diff_line