public function getNgRestPrimaryKey()
    {
        if ($this->_ngRestPrimaryKey === null) {
            $keys = static::primaryKey();
            if (!isset($keys[0])) {
                throw new InvalidConfigException("The NgRestModel '".__CLASS__."' requires at least one primaryKey in order to work.");
            }
            
            return (array) $keys;
        }

        return $this->_ngRestPrimaryKey;
    }