Adding a backend test for the feature flag

app/controllers/projects/merge_requests_controller.rb

@@ -452,7 +452,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     params = request.query_parameters
     params[:view] = cookies[:diff_view] if params[:view].blank? && cookies[:diff_view].present?
 
-    if Feature.enabled?(:default_merge_ref_for_diffs, @project)
+    if Feature.enabled?(:default_merge_ref_for_diffs, project)
       params = params.merge(diff_head: true)
     end
 

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -87,6 +87,22 @@ RSpec.describe Projects::MergeRequestsController do
         end
       end
 
+      context 'with `default_merge_ref_for_diffs` feature flag enabled' do
+        before do
+          stub_feature_flags(default_merge_ref_for_diffs: true)
+          go
+        end
+
+        it 'adds the diff_head parameter' do
+          expect(assigns["endpoint_metadata_url"]).to eq(
+            diffs_metadata_project_json_merge_request_path(
+              project,
+              merge_request,
+              'json',
+              diff_head: true))
+        end
+      end
+
       context 'when diff is missing' do
         render_views