public function getEtags()
    {
        $etag = '';

        if ($this->data instanceof ArrayableInterface) {
            $this->data = $this->data->toArray();
        }

        foreach ( $this->data as $item ) {
            $item = new Resource($item);
            $etag .= $item->getEtag();
            unset($item);
        }

        return md5( $etag );
    }