public static function splitKey($key)
    {
        if (strpos($key, self::FIELD_KEY_SEPARATOR) === false) {
            return null;
        }
        list($sub, $fieldId) = explode(self::FIELD_KEY_SEPARATOR, $key, 2);

        return array('sub' => $sub, 'gtf_id_field' => $fieldId);
    }