screen/application/common/model/UserLevel.php

<?php

namespace app\common\model;

use think\Model;


class UserLevel extends Model
{
    public function user()
    {
        return $this->hasMany(User::class,'level');
    }

    protected static function init()
    {
        self::afterDelete(function (self $level){
            $level->user()->update([
                'level'=>0,
            ]);
        });
    }
}