Denormalize commits count for merge request diffs

For each MR diff an extra 'SELECT COUNT()' is executed
to get number of commits for the diff. Overall time to get counts for
all MR diffs may be quite expensive. To speed up loading of MR info,
information about number of commits is stored in a MR diff's extra column.

Closes #38068

1 files changed, 7 insertions, 4 deletions

diff --git a/app/models/merge_request_diff.rb b/app/models/merge_request_diff.rb
index e35de9b97ee..afab72930c1 100644
--- a/app/models/merge_request_diff.rb
+++ b/app/models/merge_request_diff.rb
@@ -49,6 +49,7 @@ class MergeRequestDiff < ActiveRecord::Base
     ensure_commit_shas
     save_commits
     save_diffs
+    save
     keep_around_commits
   end
 
@@ -56,7 +57,6 @@ class MergeRequestDiff < ActiveRecord::Base
     self.start_commit_sha ||= merge_request.target_branch_sha
     self.head_commit_sha  ||= merge_request.source_branch_sha
     self.base_commit_sha  ||= find_base_sha
-    save
   end
 
   # Override head_commit_sha to keep compatibility with merge request diff
@@ -195,7 +195,7 @@ class MergeRequestDiff < ActiveRecord::Base
   end
 
   def commits_count
-    merge_request_diff_commits.size
+    super || merge_request_diff_commits.size
   end
 
   private
@@ -264,13 +264,16 @@ class MergeRequestDiff < ActiveRecord::Base
       new_attributes[:state] = :overflow if diff_collection.overflow?
     end
 
-    update(new_attributes)
+    assign_attributes(new_attributes)
   end
 
   def save_commits
     MergeRequestDiffCommit.create_bulk(self.id, compare.commits.reverse)
 
-    merge_request_diff_commits.reload
+    # merge_request_diff_commits.reload is preferred way to reload associated
+    # objects but it returns cached result for some reason in this case
+    commits = merge_request_diff_commits(true)
+    self.commits_count = commits.size
   end
 
   def repository