code: public function getEntityIdentifier($entity)
      {
          if (!($this->doctrine instanceof Registry)) {
              return null;
          }
  
          $ids = $this
              ->doctrine
              ->getManager()
              ->getClassMetadata(get_class($entity))
              ->getIdentifierValues($entity);
  
          return $ids ? self::IDENTIFIER_PREFIX.implode(self::IDENTIFIER_SEPARATOR, $ids) : null;
      }
docs: @param object $entity
  
  @return string|null