Doctrine\ODM\OrientDB\ConfigurationException::missingKey PHP Method

    public static function missingKey($key)
    {
        return new self(sprintf('%s must be set in the configuration.', $key));
    }

 public function getProxyDirectory()
 {
     if (!isset($this->options['proxy_dir'])) {
         throw ConfigurationException::missingKey('proxy_dir');
     }
     return $this->options['proxy_dir'];
 }