code: public function extract($object)
      {
          if (!$object instanceof TopicInterface) {
              throw new Exception\InvalidArgumentException('$object must be an instance of Zf2Forum\Model\Topic\TopicInterface');
          }
          $data = parent::extract($object);
          if ($data['id'] == "") {
              unset($data['id']);
          }
          unset($data['message_count']);
          unset($data['visit_count']);
          unset($data['last_post']);
          return $data;
      }
docs: Extract values from an object
  
  @param  object $object
  @return array
  @throws Exception\InvalidArgumentException