app-api/app/Models/Express.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Express extends Model{

    protected $table='express';

    public function get_express(){
        return $this->hasMany(Express::class,'number','number');
    }

    public function get_order(){
        return $this->hasOne(Ordertest::class,'id','order_id');
    }
}