为何activitieDropdownlist的值始终无法获取到

为什么activitieDropdownlist的值始终无法获取到？
视图层view

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

控制器层

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

------解决方案--------------------
看一下 CHtml::activeDropDownList 的定义