I am trying to populate dropdown list according to other dropdown list using ajax prototype.js framework.But its seems ajax observField function is not picking the change in first dropdown list .

I have two tables named mobiles and moddels
moddels table has an coloumn mobile_id for association.
my models are as follows
mobile.php

<?
class Mobile extends AppModel {
 var $name = 'Mobile';
var $hasMany = array('Moddel');
 }

?>

moddel.php

<?
class Moddel extends AppModel {
 var $name = 'Moddel';
var $belongsTo = array('Mobile');
 }

?>

controllers are as follows
moddel_controller.php

<?
 class ModdelsController extends AppController {
 var $name = 'Moddels';

     function beforeFilter()//executed before any controller action logic
    {   
    if(isset($this->Security) && $this->RequestHandler->isAjax() && $this->action = 'ajax_modd'){
    $this->Security->enabled = false;
    }
    }

 function index()
 {

  $mob = $this->Moddel->Mobile->find('list',array('fields' => array('brand') ) );
        $this->set(compact('mob'));

 }

 }
 function ajax_modd() {

    $this->set('options',
        $this->Moddel->find('list',
            array(
                'conditions' => array(
                    'Moddel.mobile_id' => $this->data['Mobile']['id']
                )
            )
        )
    );
    $this->render('/moddels/ajax_dropdown');
}
 ?>

mobile_controller.php

<?
 class mobilesController extends AppController {
 var $name = 'Mobiles';
 function index()
 {

  $mob = $this->Mobile->find('list',array('fields' => array('brand') ) );
        $this->set(compact('mob'));

 }

 }

index.ctp

<head>
<?  $javascript->link('prototype',false);?>
</head>
<h1> its not working</h1>
<?=$ajax->form('/','post',array('update'=>'comments'));?>
<?

echo $this->Form->input('Mobile.id', array('label' => 'Mobile-brand','options'=>$mob,'empty' => '-- Select a mobile --'));
echo $this->Form->input('Moddel.mobile_id', array('label' => 'Brand', 'empty' => '-- Select a model  --'));

 echo $this->Ajax->observeField('MobileId', array('url' => '/moddels/ajax_modd', 'update' => 'ModdelMobileId'));

?>
<?=$form->end('Add Comment');?>

ajax_dropdown.ctp

<?php foreach($options as $key => $val) { ?>
<option value="<?php echo $key; ?>"><?php echo $val; ?></option>
<?php } ?>

it seems that in above code ajax observeField ...