private function initializeMapper() {
    // set the mapper
    $persistenceFacade = ObjectFactory::getInstance('persistenceFacade');
    if ($persistenceFacade->isKnownType($this->type)) {
      $this->mapper = $persistenceFacade->getMapper($this->type);
    }
    else {
      // initialize null mapper if not done already
      if (self::$nullMapper == null) {
        self::$nullMapper = new NullMapper();
      }
      $this->mapper = self::$nullMapper;
    }
  }