city user model

<?php

use yii\db\ActiveRecord;

class City extends ActiveRecord {

    public function rules()
    {
        return [
            ['name', 'string']
        ];
    }
}

class User extends ActiveRecord {

    public function rules()
    {
        return [
            ['city_id', 'exist', 'targetClass' => City::className(), 'targetAttribute' => 'id']
        ];
    }

    public function getCity()
    {
        return $this->hasOne(City::className(), ['id' => 'city_id']);
    }
}