protected function prepareUrl($url)
    {
        if (empty($url)) {
            throw new \RuntimeException(sprintf('Empty URL at item with id: "%s"', $this->getId()));
        }

        if (stripos($url, '://') !== false) {
            throw new \RuntimeException(sprintf('Malformed relative URL at item with id: "%s"', $this->getId()));
        }

        if (stripos($url, '/') !== 0) {
            throw new \RuntimeException(sprintf('Relative URL must start with "/" at item with id: "%s"', $this->getId()));
        }

        return $url;
    }