Nullify project_namespace_id reference

Because project namespaces are not backfilled for projects yet
and we may need to rollback created project namespaces) it would
be dangerous to use cascade deletion for projects.

For now we just nullify project_namespace_id reference, when backfilling
is done, we can restore cascade deletion.

Changelog: changed

db/post_migrate/20220112090556_remove_cascade_delete_from_project_namespace_foreign_key.rb

+# frozen_string_literal: true
+
+class RemoveCascadeDeleteFromProjectNamespaceForeignKey < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  TARGET_COLUMN = :project_namespace_id
+
+  def up
+    with_lock_retries do
+      remove_foreign_key_if_exists(:projects, column: TARGET_COLUMN)
+    end
+
+    add_concurrent_foreign_key(:projects, :namespaces, column: TARGET_COLUMN, on_delete: :nullify)
+  end
+
+  def down
+    with_lock_retries do
+      remove_foreign_key_if_exists(:projects, column: TARGET_COLUMN)
+    end
+
+    add_concurrent_foreign_key(:projects, :namespaces, column: TARGET_COLUMN, on_delete: :cascade)
+  end
+end

db/schema_migrations/20220112090556

+e78e11a47017c67130fe88fa538578553b7015c18cf222b5e7fb7f503254dc6d
\ No newline at end of file

db/structure.sql

@@ -29344,7 +29344,7 @@ ALTER TABLE ONLY protected_branch_push_access_levels
     ADD CONSTRAINT fk_7111b68cdb FOREIGN KEY (group_id) REFERENCES namespaces(id) ON DELETE CASCADE;
 
 ALTER TABLE ONLY projects
-    ADD CONSTRAINT fk_71625606ac FOREIGN KEY (project_namespace_id) REFERENCES namespaces(id) ON DELETE CASCADE;
+    ADD CONSTRAINT fk_71625606ac FOREIGN KEY (project_namespace_id) REFERENCES namespaces(id) ON DELETE SET NULL;
 
 ALTER TABLE ONLY integrations
     ADD CONSTRAINT fk_71cce407f9 FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE;

spec/models/namespaces/project_namespace_spec.rb

@@ -17,11 +17,11 @@ RSpec.describe Namespaces::ProjectNamespace, type: :model do
     let_it_be(:project) { create(:project) }
     let_it_be(:project_namespace) { project.project_namespace }
 
-    it 'also deletes the associated project' do
+    it 'keeps the associated project' do
       project_namespace.delete
 
       expect { project_namespace.reload }.to raise_error(ActiveRecord::RecordNotFound)
-      expect { project.reload }.to raise_error(ActiveRecord::RecordNotFound)
+      expect(project.reload.project_namespace).to be_nil
     end
   end
 end