Add use_traversal_ids_for_ancestors feature flag

app/models/namespaces/traversal/linear.rb

@@ -66,6 +66,7 @@ module Namespaces
 
       def ancestors(hierarchy_order: nil)
         return super() unless use_traversal_ids?
+        return super() unless Feature.enabled?(:use_traversal_ids_for_ancestors, root_ancestor, default_enabled: :yaml)
 
         return self.class.none if parent_id.blank?
 

config/feature_flags/development/use_traversal_ids_for_ancestors.yml

+---
+name: use_traversal_ids_for_ancestors
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/issues/324749
+rollout_issue_url:
+milestone: '13.12'
+type: development
+group: group::access
+default_enabled: false

spec/models/group_spec.rb

@@ -455,10 +455,18 @@ RSpec.describe Group do
 
       describe '#ancestors' do
         it { expect(group.ancestors.to_sql).to include 'traversal_ids <@' }
-      end
 
-      it 'hierarchy order' do
-        expect(group.ancestors(hierarchy_order: :asc).to_sql).to include 'ORDER BY "depth" ASC'
+        it 'hierarchy order' do
+          expect(group.ancestors(hierarchy_order: :asc).to_sql).to include 'ORDER BY "depth" ASC'
+        end
+
+        context 'ancestor linear queries feature flag disabled' do
+          before do
+            stub_feature_flags(use_traversal_ids_for_ancestors: false)
+          end
+
+          it { expect(group.ancestors.to_sql).not_to include 'traversal_ids <@' }
+        end
       end
     end
   end