protected static function getMapper($type, $strict=true) {
    $persistenceFacade = ObjectFactory::getInstance('persistenceFacade');
    $mapper = $persistenceFacade->getMapper($type);
    if ($strict && !($mapper instanceof RDBMapper)) {
      throw new PersistenceException('Only PersistenceMappers of type RDBMapper are supported.');
    }
    return $mapper;
  }