izyim-api/app/Shipmentitem.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Shipmentitem extends Model
{
    protected $fillable = [
        'id',
        'so_id',//shipping order
        'sa_id',//shipment arrangement
    ];

    public function so()
    {
        return $this->belongsTo('App\So');
    }

    public function shipment()
    {
        return $this->belongsTo('App\Shipment');
    }
}