code: protected function loadMetadata($className)
      {
          if (class_exists($className)) {
              return parent::loadMetadata($className);
          }
          throw MappingException::classNotFound($className);
      }
docs: Loads the metadata of the class in question and all it's ancestors whose metadata
  is still not loaded.
  
  @param string $className The name of the class for which the metadata should get loaded.
  @return array
  @throws MappingException