Yii2 Dependant drop down not working when we open update page - drop-down-menu

Question

Yii2 Dependant drop down not working when we open update page.My depend drop down value came from database. When I create entry It works Very well with the condition. but I when go for Update then Pre selectect value does not show. i.e. Which I save while creating. This is my _form.php
$arrCountry = \yii\helpers\ArrayHelper::map($modelCountry, 'country_id', 'country');
echo $form->field($model, 'country_id')->dropDownList($arrCountry,
['prompt'=>'-Choose a country-',
'onchange'=>'
$.post( "'.Yii::$app->urlManager->createUrl('country/lists?id=').'"+$(this).val(), function( data ) {
$( "select#city_name" ).html( data );
});
']);
$dataPost=\yii\helpers\ArrayHelper::map(City::find()->asArray()->all(), 'city_id', 'city_name');
// print_r($dataPost);die;
echo $form->field($model, 'city_id')
->dropDownList(
$dataPost,
['id'=>'city_name']
);
2.Controller
public function actionLists($id)
{
$countCity =City::find()
->where(['city_id' => $id,'status'=>'not assign'])
->count();
$arrCity = City::find()
->where(['city_id' => $id,'status'=>'not assign'])
->orderBy('city_id DESC')
->all();
if($countCity>0){
foreach($arrCity as $arrCity){
echo "<option value='".$arrCity->city_id."'>".$arrCity->city_name."</option>";
}
}
else{
echo "<option>-</option>";
}
}