基于switch case动态加载模型时，CakePHP保存不起作用

Question

这是我的示例代码：

switch($topic_type) {
    case 1: //post
        $model = 'Userposts';
        $field_name = 'user_post_id';
        $select_name = 'post_title';
        break;
}
$model = ClassRegistry::init($model);
$model->alias = 'TP';
$model->id = $topic_id;
$result = $model->find('first',
            array(
                'conditions'=>array($field_name=>$topic_id),
                'fields'=>array($select_name.' as topic',$field_name.' as id'),
                ));
 if(!empty($result)){
    $this->updata[$model_name][$select_name] = $modified_topic;
    $this->updata[$model_name][$field_name] = $topic_id;
        if($model->save($this->updata)){
            $respArr['token'] = $token; 
            $respArr['status'] = 1; 
            $respArr['msg'] = 'Success'; 
            echo json_encode(array('token' => $token, 'status' => 1, 'msg' => 'Success'));exit;
        }
 }

$ model-＆gt; find正在工作并获取结果，其中$ model-＆gt;在find找不到后保存。

field_name是主键，select_name是我想在修改标题后更新的保存。