Yii2 Ajax验证未验证一个字段的唯一性

public function rules()
{
    return [
        // 其他验证规则...
        ['username', 'unique', 'message' => '该用户名已被使用。'],
    ];
}

public function actionCheckUsername()
{
    $model = new User();
    if (Yii::$app->request->isAjax && $model->load(Yii::$app->request->post())) {
        Yii::$app->response->format = Response::FORMAT_JSON;
        return ActiveForm::validate($model, ['username']);
    }
}

<?php $form = ActiveForm::begin(['id' => 'user-form']); ?>

<?= $form->field($model, 'username')->textInput(['maxlength' => true]) ?>

<?php ActiveForm::end(); ?>

<?php
$script = <<< JS
    $('#user-form').on('beforeSubmit', function () {
        var form = $(this);
        $.ajax({
            url: form.attr('action'),
            type: 'post',
            data: form.serialize(),
            success: function (response) {
                if (response.hasOwnProperty('username')) {
                    form.yiiActiveForm('updateAttribute', 'user-username', [response.username]);
                } else {
                    // 验证通过，继续其他操作
                }
            }
        });
        return false;
    });
JS;

$this->registerJs($script);
?>