code: public function extract($object)
      {
          if (!$object instanceof MessageInterface) {
              throw new Exception\InvalidArgumentException('$object must be an instance of Zf2Forum\Model\Message\MessageInterface');
          }
          $data = parent::extract($object);
  
  
          $thread = $object->getThread();
          $data['forum_topic_id'] = (int)$thread->getId();
          unset($data['topic']);
          $data['timestamp_created'] = $data['timestamp_created']->format('Y-m-d H:i:s');
          return $data;
      }
docs: Extract values from an object
  
  @param  object $object
  @return array
  @throws Exception\InvalidArgumentException