public function initByConfig($key, $config)
    {
        if(empty($config))
        {
            throw new Exception('db config is empty');
        }
        $driver = 'Li\\'.ucfirst($config['driver']);

        if(!(isset($this->$key) && $this->$key instanceof $driver))
        {
            $this->$key =  new $driver($config);
        }      

    }