如何将一个新注册用户设置为已登录状态? [ 新手入门 ]

如下代码:???号处的应该如何赋值呢?

public function actionCreate() {
    $model = new Users('create');
    // Uncomment the following line if AJAX validation is needed
    $this->performAjaxValidation($model);

    if (isset($_POST['Users'])) {
        $model->attributes = $_POST['Users'];
        $model->pwd=md5($model->pwd);//将密码加密后存储
        $model->pwd2=md5($model->pwd2);
        if ($model->save())
            yii::app()->user->username=$model->username;//???
            yii::app()->user->id=$model->id;//???
            Yii::app()->user->setState('identity','user');//设为已登录                
            $this->redirect(array('site/index'));
        }
        $this->render('create', array(
            'model' => $model,
        ));
}