yii2-mongodb: How i can make the has many or has one relationship between two collection -

i using 2 collection state , city , want relationship between both of them , using below code these:

common\models\state.php

return $this->hasmany(city::classname(), ['state_id'=>'_id']);

common\models\city.php

return $this->hasone(state::classname(), ['_id' =>'state_id']);

when run below code, running , return blank result cities. $model = \common\models\state::find()->all();

when change state_id type in collection objectid returns result. please suggest me how can typecast state_id in relation.

for example; comment associated single user, in comment class

public function getuser() {     return $this->hasone(user::classname(), ['_id' => 'user_id']); } 

in controller want retrieve comments , user objects

    $query = comment::find()->orderby('created_at desc');     $dataprovider = new activedataprovider([         'pagination' => ['pagesize' =>5],         'query' => $query,     ]);      return $this->render('index', [         'dataprovider' => $dataprovider,     ]); 

you can access user objects comments view

$model->user['profile_url']