yii中为模型的attributes赋值失败

$menuId = isset($_GET['mId']) ? $_GET['mId'] : 0;
if ($menuId) {
    $menu = MenuTree::model()->findByPk($menuId);
    if(isset($_POST['MenuTree'])){
        var_dump($menu->attributes); //在这里跟踪输出的数据正常，跟表单中填写的一致
        $menu->attributes = $_POST['MenuTree']; //对attributes进行赋值
        var_dump($menu->attributes); //输出$menu模型中的attributes，不正常，结果并不是POST接收到的值，而是数据库原有的值
        if($menu->save()){
            Yii::app()->user->setFlash('success',"恭喜您，修改成功，请继续！");
            $this->redirect(Yii::app()->createUrl('menu/contentEdit',array('mId'=>$menuId)));
        }else{
            throw new CException("修改失败!");
        }
    }
    $this->render("contentEdit", array('menu' => $menu));
} else {
    throw new CHttpException('404');
}

 
public function setAttributes($values,$safeOnly=true)
{
    if(!is_array($values))
        return;
    $attributes=array_flip($safeOnly ? $this->getSafeAttributeNames() : $this->attributeNames());
    foreach($values as $name=>$value)
    {
        if(isset($attributes[$name]))
            $this->$name=$value;
        else if($safeOnly)
            $this->onUnsafeAttribute($name,$value);
    }
}

public function rules()
{
    // NOTE: you should only define rules for those attributes that
    // will receive user inputs.
    return array(
        // more code...
        array('field1 , field2 ,field3', 'safe'), //Modify the fields in here
        // more code...
    );
}