Reduce DB queries when loading root_ancestor

In the most common case where the record is persisted, we do not need to
load `parent`

Changelog: performance

app/models/namespaces/traversal/linear.rb

@@ -74,7 +74,7 @@ module Namespaces
         return super unless use_traversal_ids_for_root_ancestor?
 
         strong_memoize(:root_ancestor) do
-          if parent.nil?
+          if parent_id.nil?
             self
           else
             Namespace.find_by(id: traversal_ids.first)

app/models/namespaces/traversal/recursive.rb

@@ -7,12 +7,12 @@ module Namespaces
       include RecursiveScopes
 
       def root_ancestor
-        return self if parent.nil?
-
-        if persisted?
+        if persisted? && !parent_id.nil?
           strong_memoize(:root_ancestor) do
-            recursive_self_and_ancestors.reorder(nil).find_by(parent_id: nil)
+            recursive_ancestors.reorder(nil).find_by(parent_id: nil)
           end
+        elsif parent.nil?
+          self
         else
           parent.root_ancestor
         end

spec/models/namespace_spec.rb

@@ -1377,6 +1377,13 @@ RSpec.describe Namespace do
         expect { root_group.root_ancestor }.not_to exceed_query_limit(0)
       end
 
+      it 'returns root_ancestor for nested group with a single query' do
+        nested_group = create(:group, parent: root_group)
+        nested_group.reload
+
+        expect { nested_group.root_ancestor }.not_to exceed_query_limit(1)
+      end
+
       it 'returns the top most ancestor' do
         nested_group = create(:group, parent: root_group)
         deep_nested_group = create(:group, parent: nested_group)