This is the AutocompleteAjax widget and a Yii 2 enhanced wrapper for the Autocomplete | jQuery UI. A simple way to search model id of the attributes model.

    "require": {
    "keygenqt/yii2-autocomplete-ajax": "*"
}

    <?php $form = ActiveForm::begin(); ?>

<?= $form->field($model, 'autocomplete')->widget(AutocompleteAjax::class, [
    'url' => ['ajax/search'],
    'options' => ['placeholder' => 'Find by user email or user id.']
]) ?>

<?= $form->field($model, 'integer')->textInput()->input('text', ['placeholder' => 'Integer']) ?>

<?= $form->field($model, 'email')->textInput()->input('email', ['placeholder' => 'Email']) ?>

<?= Html::submitButton('Submit', ['class' => 'btn btn-primary']) ?>

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

    class AjaxController extends FrontendController
{
    public function actionSearch($term)
    {
        if (Yii::$app->request->isAjax) {

            sleep(2); // for test

            $results = [];
            if (is_numeric($term)) {
                /** @var Test $model */
                $model = Test::findOne(['id' => $term]);

                if ($model) {
                    $results[] = [
                        'id' => $model['id'],
                        'label' => $model['email'] . ' (model id: ' . $model['id'] . ')',
                    ];
                }
            } else {
                $q = addslashes($term);
                foreach (Test::find()->where("(`email` like '%{$q}%')")->all() as $model) {
                    $results[] = [
                        'id' => $model['id'],
                        'label' => $model['email'] . ' (model id: ' . $model['id'] . ')',
                    ];
                }
            }
            echo Json::encode($results);
        }
    }
}