1 files changed, 28 insertions, 0 deletions
diff --git a/lib/gitlab/background_migration/set_merge_request_diff_files_count.rb b/lib/gitlab/background_migration/set_merge_request_diff_files_count.rb
new file mode 100644
index 00000000000..9f765d03d62
--- /dev/null
+++ b/lib/gitlab/background_migration/set_merge_request_diff_files_count.rb
@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # Sets the MergeRequestDiff#files_count value for old rows
+    class SetMergeRequestDiffFilesCount
+      COUNT_SUBQUERY = <<~SQL
+        files_count = (
+          SELECT count(*)
+          FROM merge_request_diff_files
+          WHERE merge_request_diff_files.merge_request_diff_id = merge_request_diffs.id
+        )
+      SQL
+
+      class MergeRequestDiff < ActiveRecord::Base # rubocop:disable Style/Documentation
+        include EachBatch
+
+        self.table_name = 'merge_request_diffs'
+      end
+
+      def perform(start_id, end_id)
+        MergeRequestDiff.where(id: start_id..end_id).each_batch do |relation|
+          relation.update_all(COUNT_SUBQUERY)
+        end
+      end
+    end
+  end
+end