Why doesn't the activeForm custom rule work?

Question

Yii

There is a form with a field (there are other fields, but it does not apply to the topic):

<?=$form->field($model,'date', ['enableAjaxValidation' => true, 'enableClientValidation' => false])->input('date');?>

and there is a test custom rule, which, in theory, should always give an error.

public function rules()
    {
        return [
            ['date', 'date', 'format' => 'php:Y-m-d'],
            ['date', 'checkDate'],
        ];
    }

    public function checkDate($attribute, $param) {
        $this->addError('date', 'Значение: '.$this->$attribute);
    }

When the ajax field changes, it works, errors of other fields appear in it, but the error of the date field does not appear.
What could be the problem?

Answer 1

In the controller action add

if (Yii::$app->request->isAjax && $model->load(Yii::$app->request->post())) {
            Yii::$app->response->format = Response::FORMAT_JSON;
            return ActiveForm::validate($model);
        }

Although for ajax validation it is better to do a separate action, even a separate controller.
Here it is shown what's what.