ActiveRecord为什么插入的数据都是NULL？ [ 求助交流 ]

User继承于yii\db\ActiveRecord，

class User extends \yii\db\ActiveRecord
{
    public $username;
    public function attributes()
    {
        return ['username'];
    }
}

$user = new \frontend\models\User();
$user->username = 'x';
$user->save();