How To Create Multilevel Dependent Dropdownlist In Yii

Hi, I want to create multilevel dependent DropDownList.
This are my tables form database.
----------------------------------------------------------
class{id,class_name}
subjects{id,class_id,name}
sub_section{id,class_id,sub_id,section_name}
sub_topic{id,class_id,sub_id,sub_section_id,topic_name}
----------------------------------------------------------

While creating records for sub_topic table,
I want to display available classes from class table in a DropDownlist (I am able to do this) and then on selecting a class, I want to display the corresponding subjects(name) from subjects table in second DropDownList ,where subjects.class_id = selected id in the class dropdown (I am also able to do this),
Now on selecting a subject, I want to display the corresponding section(section_name) from sub_section table in third DropDownList ,
where sub_section.sub_id = selected id in the subjects dropdown.

here i am getting third dropdown empty.


This is my code

View file

	<div class="row">
		<?php echo $form->labelEx($model,'class_id'); ?>
		<?php
                  $classArray = CHtml::listData(Classes::model()->findAll(),'id','class_name');
                   echo $form->DropDownList($model,'class_id',$classArray,
                            array(
							    'prompt'=>'Select Class',
                                'ajax' => array(
                                'type'=>'POST',
                                'url'=>CController::createUrl('SubTopic/dynamicSubjects'),
                                'update'=>'#'.CHtml::activeId($model,'sub_id')
                                 )));
		?>
        <?php echo $form->error($model,'class_id'); ?>
	</div>

	<div class="row">
		<?php echo $form->labelEx($model,'sub_id'); ?>
		<?php echo $form->dropDownList($model,'sub_id',array(
																	'prompt'=>'Select Subject',
	    															'ajax' => array(
																	'type'=>'POST',
																	'url'=>CController::createUrl('SubTopic/dynamicSubSection'),
																	'update'=>'#'.CHtml::activeId($model,'sub_section_id')	
																))); ?>
		<?php echo $form->error($model,'sub_id'); ?>
	</div>

	<div class="row">
		<?php echo $form->labelEx($model,'sub_section_id'); ?>
		<?php echo $form->dropDownList($model,'sub_section_id',array()); ?>
		<?php echo $form->error($model,'sub_section_id'); ?>
	</div>

	<div class="row">
		<?php echo $form->labelEx($model,'topic_name'); ?>
		<?php echo $form->textField($model,'topic_name',array('size'=>60,'maxlength'=>255)); ?>
		<?php echo $form->error($model,'topic_name'); ?>
	</div>

My Controller file SubTopicController.php

public function actionDynamicSubjects() {
            $class_id = $_POST['SubTopic']['class_id'];
            $data=Subjects::model()->findAll('class_id=:class_id',
                    array(':class_id'=> $class_id));

            $data=CHtml::listData($data,'id','name');
            foreach($data as $value=>$name)  {
                echo CHtml::tag('option',
                   array('value'=>$value),CHtml::encode($name),true);
            }
        }
		
	public function actionDynamicSubSection() {
            $sub_id = $_POST['SubTopic']['sub_id'];
            $data=SubSection::model()->findAll('sub_id=:sub_id',
                    array(':sub_id'=> $sub_id));

            $data=CHtml::listData($data,'id','section_name');
            foreach($data as $value=>$section_name)  {
                echo CHtml::tag('option',
                   array('value'=>$value),CHtml::encode($section_name),true);
            }
        }