protected function conn($db_config_name = '')
    {
        $db_config_name = $db_config_name ? $db_config_name : $this->db_config_name;
        if (! isset(self::$_db_handle[$db_config_name])) {
            if (true == C('sql_log')) {
                wlog('SQL-Log', '#'.$db_config_name);
            }
            $dbdriver = 'db_'.C('dbdriver');
            include_once FW_PATH.'/dbdrivers/'. $dbdriver .'.class.php';
            self::$_db_handle[$db_config_name] = $this->db = new $dbdriver($db_config_name);
        } else {
            $this->db = self::$_db_handle[$db_config_name];
        }
        return $this->db;
    }