PHP关联表添加数据 为什么添加进去从表的mt_web_newssort.sortid的内容显示不了

Question

/*
* 新闻保存处理
*/
public function edit(){
$db = M('WebNews');
$news_id = intval($_GET['id']);
if ($this->isAjax()){
if ($this->isPost()){
if ($news_id){
$db->data($_POST)->where(array('mt_web_news.news_id'=>$news_id))->save();
}else{
$_POST['up_time'] = time();
$_POST['add_time'] =time();

                    $db->data($_POST)->join('mt_web_newssort ON mt_web_news.sort = mt_web_newssort.sortid')->add();

                }
                $this->success("保存成功");
            }else{
                if ($news_id>0){
                    $data = $db->where(array('news_id'=>$news_id))->find();
                    echo json_encode($data);
                }
            }
        }else{
            $this->display('newsedit');
        }
}