Fix failing spec with orphaned namespace

lib/gitlab/background_migration/backfill_project_repositories.rb

@@ -101,7 +101,7 @@ module Gitlab
 
       # Route model
       class Route < ActiveRecord::Base
-        belongs_to :source, inverse_of: :route
+        belongs_to :source, inverse_of: :route, polymorphic: true
       end
 
       # Namespace model

spec/lib/gitlab/background_migration/backfill_project_repositories_spec.rb

@@ -95,7 +95,7 @@ describe Gitlab::BackgroundMigration::BackfillProjectRepositories do
           project.route.destroy
           subgroup.route.destroy
 
-          expect { project.disk_path }
+          expect { project.reload.disk_path }
             .to raise_error(Gitlab::BackgroundMigration::BackfillProjectRepositories::OrphanedNamespaceError)
         end
       end