为啥activitieDropdownlist的值始终无法获取到

<form method="post">
          <?php echo CHtml::activeDropDownList($m_subjectlist,'subject_name',$sb_list);?>
 <div class="row submit">
     <?php echo CHtml::submitButton('显示我所选择的专业'); ?>
 </div>
 <?php  echo 'test'.$result.'111';?>
 </form>

	public function actionDbdplist()
	{
    $m_subjectlist = new SubjectList();
    $m_gradelist = new Grade;
    $this->subject_list = SubjectList::model()->findAll(); 
    $this->grade_list = Grade::model()->findAll();                                         
	//使用findAll 得到的结果是数据对象所以这样小小处理一下吧他们转换成数组:
    foreach($this->subject_list as $item)
	{
         $sb_list[$item->subject_id] = $item->subject_name;
    }
    foreach($this->grade_list as $item)
	{
         $gd_list[$item->grade_id] = $item->grade_name;
    }



	$result=$_POST['subject_name'];
	

    $this->render('dbdplist',array(
               'm_subjectlist' => $m_subjectlist,
               'm_gradelist' => $m_gradelist,
               'subject_list'=>$this->subject_list,
               'grade_list'=>$this->grade_list,
               'sb_list'=>$sb_list,
               'gd_list'=>$gd_list,
               'result'=>$result,
		'model'=>$model,));
			  
 }