为什么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,));			   }