code: static public function create($generatorType)
      {
          switch ($generatorType) {
              case ClassMetadata::IDGENERATOR_ASSIGNED:
                  $instance = new AssignedIdGenerator();
                  break;
              case ClassMetadata::IDGENERATOR_UUID:
                  $instance = new CouchUUIDGenerator();
                  break;
              default:
                  throw new \Exception("ID Generator does not exist!");
          }
          return $instance;
      }
docs: @param int $generatorType
  
  @return IdGenerator
  @throws \Exception