code: public function extract($object)
      {
          if (!$object instanceof VisitInterface) {
              throw new Exception\InvalidArgumentException(
                  '$object must be an instance of Zf2Forum\Model\Visit\VisitInterface'
              );
          }
          $data = parent::extract($object);
  
          $thread = $object->getThread();
          $data['thread_id'] = (int)$thread->getThreadId();
          unset($data['thread']);
          $data['visit_time'] = $data['visit_time']->format('Y-m-d H:i:s');
          return $data;
      }
docs: extract
  
  @param unknown_type $object
  @throws Exception\InvalidArgumentException
  @return unknown