private function fetchOutstanding()
    {
        // continuation
        $response = $this->_connection->put($this->url() . '/' . $this->_id, '', []);
        ++$this->_fetches;

        $data = $response->getJson();

        $this->_hasMore = (bool) $data[self::ENTRY_HASMORE];
        $this->add($data[self::ENTRY_RESULT]);

        if (!$this->_hasMore) {
            // we have fetched the complete result set and can unset the id now
            $this->_id = null;
        }

        $this->updateLength();
    }