How to make a drop down list in yii2?

How to make a dropdown in yii2 using an activeform and a model? Since all the methods has changed in yii2,how it is done in the new one?

It is like

<?php
use yii\helpers\ArrayHelper;
use backend\models\Standard;
?>

<?= Html::activeDropDownList($model, 's_id',
      ArrayHelper::map(Standard::find()->all(), 's_id', 'name')) ?>

ArrayHelper in Yii2 replaces the CHtml list data in Yii 1.1.[Please load array data from your controller]

EDIT

Load data from your controller.

Controller

$items = ArrayHelper::map(Standard::find()->all(), 's_id', 'name');
...
return $this->render('your_view',['model'=>$model, 'items'=>$items]);

In View

<?= Html::activeDropDownList($model, 's_id',$items) ?>

It seems you've found your answer already but since you mentioned the active form I'll contribute with one more, even if it differs only ever so slightly.

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

    echo $form->field($model, 'attribute')
        ->dropDownList(
            $items,           // Flat array ('id'=>'label')
            ['prompt'=>'']    // options
        );

    ActiveForm::end();
?>

There are some good solutions above, and mine is just a combination of two (I came here looking for a solution).

@Sarvar Nishonboyev's solution is good because it maintains the creation of the form input label and help-block for error messages.

I went with:

<?php
use yii\helpers\ArrayHelper;
use app\models\Product;
?>
<?=
$form->field($model, 'parent_id')
     ->dropDownList(
            ArrayHelper::map(Product::find()->asArray()->all(), 'parent_id', 'name')
            )
?>

Again, full credit to: @Sarvar Nishonboyev's and @ippi