validation in Datamapper orm - codeigniter-2

Question

hello below is my 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 this is my 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 is that it can't show error message when validation failed...
it returns following message.
Unable to access an error message corresponding to your rule name: required.
Unable to access an error message corresponding to your rule name: required.
please tell me whats problem is there..?