Ignore empty MR diffs when migrating to external storage

app/models/merge_request_diff.rb

@@ -49,13 +49,14 @@ class MergeRequestDiff < ApplicationRecord
   scope :by_commit_sha, ->(sha) do
     joins(:merge_request_diff_commits).where(merge_request_diff_commits: { sha: sha }).reorder(nil)
   end
+  scope :has_diff_files, -> { where(id: MergeRequestDiffFile.select(:merge_request_diff_id)) }
 
   scope :by_project_id, -> (project_id) do
     joins(:merge_request).where(merge_requests: { target_project_id: project_id })
   end
 
   scope :recent, -> { order(id: :desc).limit(100) }
-  scope :files_in_database, -> { where(stored_externally: [false, nil]) }
+  scope :files_in_database, -> { has_diff_files.where(stored_externally: [false, nil]) }
 
   scope :not_latest_diffs, -> do
     merge_requests = MergeRequest.arel_table

changelogs/unreleased/36646-ignore-empty-diffs-for-external-diffs-migration.yml

+---
+title: Ignore empty MR diffs when migrating to external storage
+merge_request: 20296
+author:
+type: fixed

spec/models/merge_request_diff_spec.rb

@@ -98,6 +98,12 @@ describe MergeRequestDiff do
       end
 
       it { is_expected.to contain_exactly(outdated.id, latest.id, closed.id, merged.id, closed_recently.id, merged_recently.id) }
+
+      it 'ignores diffs with 0 files' do
+        MergeRequestDiffFile.where(merge_request_diff_id: [closed_recently.id, merged_recently.id]).delete_all
+
+        is_expected.to contain_exactly(outdated.id, latest.id, closed.id, merged.id)
+      end
     end
 
     context 'external diffs are enabled for outdated diffs' do