baoshi-was/app/BillingModel.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class BillingModel extends Model
{

    protected $fillable=[
        'carrier_id','province_id','city_id','unit_id','range_min','range_max','unit_price','base_fee','initial_weight'
    ];

    protected $appends=[
        'carrier_name',
        'province_name',
        'city_name',
        'unit_name',
    ];

    public function carrier(){
        return $this->belongsTo('App\Carrier','carrier_id','id');
    }

    public  function province(){
        return $this->belongsTo('App\Province','province_id','id');
    }

    public  function city(){
        return $this->belongsTo('App\City','city_id','id');
    }

    public function unit(){
        return $this->belongsTo('App\Unit','unit_id','id');
    }

    public function getCarrierNameAttribute(){
        return $this['carrier']? $this['carrier']['name']:null;
    }
    public function getProvinceNameAttribute(){
        return $this['province']? $this['province']['name']:null;
    }
    public function getCityNameAttribute(){
        return $this['city']? $this['city']['name']:null;
    }
    public function getUnitNameAttribute(){
        return $this['unit']? $this['unit']['name']:null;
    }
}