travel/service/app/model/Qas.php

<?php

namespace app\model;

class Qas extends base
{

    public function qaCitys()
    {
        return $this->hasOne(QaCitys::class,'city_id','city_id');
    }

    public function qaQuestions()
    {
        return $this->hasMany(QaQuestions::class,'qa_id','id')->order('sort desc');
    }

}