路由中间件验证 - Route Middleware validation

我有一条路线，它是：

Route::get('/{slug}',['as' => 'index', 'middleware' => 'player_group', 'uses' => 'PlayerController@index']);

我要做的是检查经过身份验证的用户是否在｛slug｝组中。

public function handle($request, Closure $next)
{
    $user = Auth::user()->id;
    $user_grupo = DB::select( DB::raw("SELECT id FROM user JOIN group ON group.id_ = user.id_group WHERE user.id = ".$user." AND group.slug = '".$slug."'") );;
    if (!$user_group) {
       //ERROR 
    } else { 
       return $next($request); 
    }
}

但这不起作用。。。它正在消除一个错误：

Undefined variable: slug

要从路由中获取参数，请使用$request->route('slug');