I have read http://www.yiiframework.com/wiki/24/creating-a-dependent-dropdown/ (Dependen Dropdown Yii1), but I cant implementation in Yii2 because Yii2 not built-in AJAX functionality hem.. I am search about it and get this statement

qiangxue commented on Aug 28, 2013 In 2.0 we removed most in-page js code. You should write explicit js code in external js files to achieve similar result as in 1.1. The code is very trivial though.

So we must create own code hehe oh no we can use great extension http://demos.krajee.com/widget-details/depdrop, this right but some one dislike use third party too much.. Oke I try to explain You about this classic case :)

Create Dropdown ¶

First, You should know how to make dropdown in Yii2, I use activefield

echo $form->field($model, 'name_field')->dropDownList(
		[items],
		[options]
	);

The Table ¶

In this case, I have 2 table table category

• id int primary key auto increment
• name varchar(255)

table category

• id int primary key auto increment
• title varchar(255)
• content text
• category_id int

The View ¶

use yii\helpers\ArrayHelper;
$dataCategory=ArrayHelper::map(\common\models\Category::find()->asArray()->all(), 'id', 'name');
	echo $form->field($model, 'category_id')->dropDownList($dataCategory, 
	         ['prompt'=>'-Choose a Category-',
			  'onchange'=>'
				$.post( "'.Yii::$app->urlManager->createUrl('post/lists?id=').'"+$(this).val(), function( data ) {
				  $( "select#title" ).html( data );
				});
			']); 
	
	$dataPost=ArrayHelper::map(\common\models\Post::find()->asArray()->all(), 'id', 'title');
	echo $form->field($model, 'title')
        ->dropDownList(
            $dataPost,           
            ['id'=>'title']
        );

Yii2 have change chtml list be ArrayHelper::map.. so You should use library Array helper

use yii\helpers\ArrayHelper;

The Controller ¶

public function actionLists($id)
    {
		$countPosts = \common\models\Post::find()
				->where(['category_id' => $id])
				->count();
				
        $posts = \common\models\Post::find()
				->where(['category_id' => $id])
				->orderBy('id DESC')
				->all();
				
		if($countPosts>0){
			foreach($posts as $post){
				echo "<option value='".$post->id."'>".$post->title."</option>";
			}
		}
		else{
			echo "<option>-</option>";
		}
		
    }

Reference ¶

membuat dependent dropdown tanpa extension tambahan