某些规则仅在Yii中更新时生效

我做这个功能的地方，用户可以更新描述和图像，但我也想允许用户如果不选择图像，前一个图像将保持不变。它告诉我，我不能空着，现在我知道这是因为规则，但我试图制定规则只是为了更新，但它似乎不起作用。

以下是模型中的规则

return array(
        array('description,imageUrl', 'required', 'on'=>'create'),
        array('imageUrl','file','types'=>'jpg,gif,png'),
        array('description','required', 'on'=>'update'),
        // The following rule is used by search().
        // @todo Please remove those attributes that should not be searched.
        array('id, description', 'safe', 'on'=>'search'),
    );

在我的actionUpdate控制器中，我有以下内容。

    $model=$this->loadModel($id);
    $model->setScenario('update');

    // Uncomment the following line if AJAX validation is needed
    // $this->performAjaxValidation($model);
    if(isset($_POST['DestinationWedding']))
    {
        $model->attributes=$_POST['DestinationWedding'];
        if(!empty($_POST['DestinationWedding']['imageUrl'])){
            //Generating a new file name
            $fileName = mt_rand();
            //Assging the super gloabn array file
            $uploadedFile = CUploadedFile::getInstance($model,'imageUrl');
            //Check if the name is taken or not
            $checkName = DestinationWedding::model()->findAllByAttributes(array('imageUrl'=>$fileName));
            while(!empty($checkName)){
                $fileName = mt_rand();
                $checkName = DestinationWedding::model()->findAllAttributes(array('imageUrl'=>$fileName));
            }
            //Removeing the old image
            unlink(Yii::getPathOfAlias('webroot').'/images/upload/destinationWeddings/'.$model['imageUrl']);
            $model->imageUrl = $fileName;
        }
        $valid = $model->validate();
        if($valid){
            if($model->save()){
                //Moveing the uploaded file
                $uploadedFile->saveAs('images/upload/destinationWeddings/'.$fileName);
                //Seeting a new session showing the user successful message
                Yii::app()->user->setFlash('succcess','Wedding is updated successfully');
                $this->redirect(array('admin'));
            }else{
                //Setting a new session shwoing the user error message
                Yii::app()->user->setFlash('error','An error has occured');
            }
        }
    }
    $this->render('update',array(
        'model'=>$model,
    ));

我的操作创建是以下

$model=new DestinationWedding;
        $model->setScenario = 'create';
        // Uncomment the following line if AJAX validation is needed
        // $this->performAjaxValidation($model);
        if(isset($_POST['DestinationWedding']))
        {
            $model->attributes=$_POST['DestinationWedding'];
            //Assinging the super golbal array File
            $uploadedFile = CUploadedFile::getInstance($model,'imageUrl');
            do{
                //Generating a new file name
                $fileName = mt_rand();
                //Check if the name is taken or not
                $checkName = DestinationWedding::model()->findAllByAttributes(array('imageUrl'=>$fileName));
            }while(!empty($checkName));
            $model->imageUrl = $fileName;
            if($model->save()){
                //Moveing the upload file
                $uploadedFile->saveAs("images/upload/destinationWeddings/".$fileName);
                //Setting a new session showing the user successful message
                Yii::app()->user->setFlash('success',"File uploaded Successfully");
            }else{
                //Setting a new session showing the user errror message
                Yii::app()->user->setFlash('eroor','File was not uploaded Successfully');
            }
        }
        $this->render('create',array(
            'model'=>$model,
        ));

所以我想要extly是

• 用户上传图像时可以更改图像
• 验证文件
• 如果保留为空，则不会出现错误，因为使用了以前的图像