Use linear root_ancestor when possible

Changelog: performance

app/models/namespaces/traversal/linear.rb

@@ -71,15 +71,21 @@ module Namespaces
         traversal_ids.present?
       end
 
-      def root_ancestor
-        return super if parent.nil?
-        return super unless persisted?
+      def use_traversal_ids_for_root_ancestor?
+        return false unless Feature.enabled?(:use_traversal_ids_for_root_ancestor, default_enabled: :yaml)
 
-        return super if traversal_ids.blank?
-        return super unless Feature.enabled?(:use_traversal_ids_for_root_ancestor, default_enabled: :yaml)
+        traversal_ids.present?
+      end
+
+      def root_ancestor
+        return super unless use_traversal_ids_for_root_ancestor?
 
         strong_memoize(:root_ancestor) do
-          Namespace.find_by(id: traversal_ids.first)
+          if parent.nil?
+            self
+          else
+            Namespace.find_by(id: traversal_ids.first)
+          end
         end
       end
 

spec/models/namespace_spec.rb

@@ -974,6 +974,14 @@ RSpec.describe Namespace do
     end
   end
 
+  shared_examples 'disabled feature flag when traversal_ids is blank' do
+    before do
+      namespace.traversal_ids = []
+    end
+
+    it { is_expected.to eq false }
+  end
+
   describe '#use_traversal_ids?' do
     let_it_be(:namespace, reload: true) { create(:namespace) }
 
@@ -985,6 +993,8 @@ RSpec.describe Namespace do
       end
 
       it { is_expected.to eq true }
+
+      it_behaves_like 'disabled feature flag when traversal_ids is blank'
     end
 
     context 'when use_traversal_ids feature flag is false' do
@@ -996,6 +1006,30 @@ RSpec.describe Namespace do
     end
   end
 
+  describe '#use_traversal_ids_for_root_ancestor?' do
+    let_it_be(:namespace, reload: true) { create(:namespace) }
+
+    subject { namespace.use_traversal_ids_for_root_ancestor? }
+
+    context 'when use_traversal_ids_for_root_ancestor feature flag is true' do
+      before do
+        stub_feature_flags(use_traversal_ids_for_root_ancestor: true)
+      end
+
+      it { is_expected.to eq true }
+
+      it_behaves_like 'disabled feature flag when traversal_ids is blank'
+    end
+
+    context 'when use_traversal_ids_for_root_ancestor feature flag is false' do
+      before do
+        stub_feature_flags(use_traversal_ids_for_root_ancestor: false)
+      end
+
+      it { is_expected.to eq false }
+    end
+  end
+
   describe '#use_traversal_ids_for_ancestors?' do
     let_it_be(:namespace, reload: true) { create(:namespace) }
 
@@ -1007,6 +1041,8 @@ RSpec.describe Namespace do
       end
 
       it { is_expected.to eq true }
+
+      it_behaves_like 'disabled feature flag when traversal_ids is blank'
     end
 
     context 'when use_traversal_ids_for_ancestors? feature flag is false' do