带有CI的动态列表

我正在尝试使用codeigniter创建一个多级列表。该列表看起来像:

State1

city1
city2

state2

 city3

等。

但是当我运行代码时，我得到了错误

            Severity: Notice
            Message: Undefined property: stdClass::$city
           Filename: views/testview.php
         Line Number: 32

在第32行，我有:

         <?php  foreach($state->city as $city):?>

我的控制器:

        //....
   $data['states'] = $this->state_model->get_cities();
   $this->load->view('testview', $data);

我的模型:

   function get_cities(){
  $states = $this->db->get('state');
  foreach ($states->result() as $state){
$cities = $this->db->get_where('city', array('state_id'=>$state->id));
$state->cities = $cities->result();
}
 return $states;

}

视图:

   <?php foreach($states as $state):?>
 <h4><?php echo $state->statename;?></h4>
 <?php  foreach($state->city as $city):?>
<?php echo $city->cityname; ?>
 <?php endforeach;?>
<?php endforeach;?>