public function addConnection($uriString)
    {
        if (!isset(self::$connectionList[$uriString])) {
            self::$connectionList[$uriString] = Factory::getDbRelationalInstance($uriString);

            if (self::$transaction) {
                self::$connectionList[$uriString]->beginTransaction();
            }
        }

        return self::$connectionList[$uriString];
    }