Improve the performance for MR discussions loadosw-improve-discussions-query

It considerably improves the query for preloading MR
discussion diffs for:

1. MR diff comments (made at Diffs tab; presented at the main MR page)
2. Comments on commits being listed at the MR

Additionally, it also prevents a second similar query that was
unnecessarily being made.

1 files changed, 4 insertions, 9 deletions

diff --git a/app/models/diff_note.rb b/app/models/diff_note.rb
index 861185dc222..bae442481af 100644
--- a/app/models/diff_note.rb
+++ b/app/models/diff_note.rb
@@ -108,13 +108,10 @@ class DiffNote < Note
   end
 
   def fetch_diff_file
+    return note_diff_file.raw_diff_file if note_diff_file
+
     file =
-      if note_diff_file
-        diff = Gitlab::Git::Diff.new(note_diff_file.to_hash)
-        Gitlab::Diff::File.new(diff,
-                               repository: repository,
-                               diff_refs: original_position.diff_refs)
-      elsif created_at_diff?(noteable.diff_refs)
+      if created_at_diff?(noteable.diff_refs)
         # We're able to use the already persisted diffs (Postgres) if we're
         # presenting a "current version" of the MR discussion diff.
         # So no need to make an extra Gitaly diff request for it.
@@ -126,9 +123,7 @@ class DiffNote < Note
         original_position.diff_file(repository)
       end
 
-    # Since persisted diff files already have its content "unfolded"
-    # there's no need to make it pass through the unfolding process.
-    file&.unfold_diff_lines(position) unless note_diff_file
+    file&.unfold_diff_lines(position)
 
     file
   end