记laravel-admin Form表单 BelongsTo关系模型在保存前修改的注意事项

            $form->hidden('user.foo');

            $form->saving(function (Form $form) {
                /** 错的 
                报间接修改重载属性错误
                Indirect modification of overloaded property
                **/
                $form->user['foo'] = $form->baz;
                //正确写法
                $form->user = ['foo' => $form->baz];
            });