关于laravel-admin form自定义字段的问题

业务场景：封禁用户
user表中没有reason（原因）字段，我想在保存时只想将reason存入缓存中，unset（$form->reason）没有效果，依然报错：
SQLSTATE[42S22]: Column not found: 1054 Unknown column ‘reason’

form = new Form(new User());
        ...
        $form->radio('ban', __('Ban'))
        ->options(['0' => __('Normal'), '1' => __('Time limited ban'), '2' => __('Permanent ban')])
        ->when(1, function (Form $form) {
            $form->datetime('time_limited', __('Time limited'));
            $form->textarea('reason', __("Reason"));
        })->when(2, function (Form $form) {
            $form->textarea('reason', __("Reason"));
        });

 $form->saving(function (Form $form) {
            $user_id = $form->model()->id;
            switch ($form->ban){
                ...
                case 2:
                    if(Redis::exists('ban:'.$user_id)){
                        Redis::del('ban:'.$user_id);
                    }
                    unset($form->reason);
                    //dump($form);exit();
                    Redis::hMSet('ban:'.$user_id,['reason'=>$form->reason]);
                    break;
                ....
            }
        });