codeigniter 2 - validation in Datamapper orm -

hello below model

 <?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 .. 

and controler

 <?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 

now problem can't show error message when validation failed...

it returns following message.

  unable access error message corresponding rule name: required. 

unable access error message corresponding rule name: required.

please tell me whats problem there..?