public function injectEntityManager(EntityManagerInterface $entityManager, ClassMetadata $classMetadata) : void
    {
        if ($entityManager !== self::$entityManager) {
            throw new RuntimeException(
                'Trying to use PersistentObject with different EntityManager instances. ' .
                'Was PersistentObject::setEntityManager() called?'
            );
        }

        $this->cm = $classMetadata;
    }