Its called 'scenarios': http://www.yiiframework.com/doc-2.0/guide-structure-models.html#scenarios
in model:
public function rules()
{
return [
['title', 'required', 'on' => 'create']
];
}
in controller:
public function actionCreate()
{
$model = new Item();
$model->scenario = 'create';
...
}
You'd better define scenario as a constant though.
source : http://stackoverflow.com/questions/29449019/yii2-validate-only-create-action