screen/application/admin/model/Ad.php

<?php

namespace app\admin\model;

use app\admin\model\Admin;
use think\Model;

/**
 */
class Ad extends \app\common\model\Ad
{
    public function admin(){
        return $this->belongsTo(Admin::class,'admin_id','id',[],'left')->setEagerlyType(0);
    }
}