Test project namespace is destroyed with project_namespace.rb

Changelog: added

app/models/namespace.rb

@@ -57,7 +57,7 @@ class Namespace < ApplicationRecord
   has_one :admin_note, inverse_of: :namespace
   accepts_nested_attributes_for :admin_note, update_only: true
 
-  validates :owner, presence: true, unless: ->(n) { n.type == "Group" }
+  validates :owner, presence: true, unless: ->(n) { n.type == "Group" || n.type == 'Namespaces::ProjectNamespace' }
   validates :name,
     presence: true,
     length: { maximum: 255 }

spec/factories/namespaces/project_namespace.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :project_namespace, class: 'Namespaces::ProjectNamespace' do
+    project
+    sequence(:name) { |n| "project_namespace#{n}" }
+    path { name.downcase.gsub(/\s/, '_') }
+    type { 'Namespaces::ProjectNamespace' }
+    owner { nil }
+    project_creation_level { ::Gitlab::Access::MAINTAINER_PROJECT_ACCESS }
+  end
+end

spec/models/project_spec.rb

@@ -185,6 +185,17 @@ RSpec.describe Project, factory_default: :keep do
       end
     end
 
+    context 'when destroying project' do
+      let_it_be(:project) { create(:project) }
+      let_it_be(:project_namespace) { create(:project_namespace, project: project) }
+
+      it 'also destroys the associated ProjectNamespace' do
+        project.destroy!
+
+        expect { project_namespace.reload }.to raise_error(ActiveRecord::RecordNotFound)
+      end
+    end
+
     context 'when creating a new project' do
       let_it_be(:project) { create(:project) }