Datamapper orm

Question

你好，下面是我的模特

 <?php
 class Orm_demo_model extends DataMapper{

     var $table= 'orm_demo';
     var $has_many=array('post_model');

     var $validation =array(
         'name'=>array(
             'label'=>'Name',
             'rules'=>array('required','trim'),
         ),
         'description'=>array(
             'label'=>'Description',
             'rules'=>array('required','trim'),
         )
     );

 }

 //End Of Orm_demo .php model ..

这是我的控制器

 <?php
 class OrmCtrl extends CI_Controller{

     function __construct() {
         parent::__construct();
     }

    function index(){
        $data['title']='Add data';
        $this->template->load('templates/template','ormDemoForm',$data);       
    } 

    function save(){
        $obj = new Orm_demo_model();
        $obj->name=$this->input->post('name');
        $obj->description= $this->input->post('description');       
        if($obj->save()){
            echo '<script>alert("Record Added..");</script>';
            redirect('ormCtrl/','refresh');
        }
        else{
            echo $obj->error->name;
            echo $obj->error->description;           
          //  echo $obj->error->string;
        }
    }

 }

 //end of ormCtrl.php file

现在的问题是，当验证失败时，它无法显示错误消息......

它返回以下消息。

  Unable to access an error message corresponding to your rule name: required.

无法访问与您的规则名称对应的错误消息：required。

请告诉我，那里有什么问题..？