willaengine/app/ShippingGroup.php

<?php
namespace App;

use Illuminate\Database\Eloquent\Model;

class ShippingGroup extends Model
{
    protected $fillable = ['name', 'description', 'business_id', 'min_value_for_free'];

    public $timestamps = false;

    public function shipping_rules()
    {
        return $this->hasMany('App\ShippingRule');
    }

    public function categories()
    {
        return $this->belongsToMany('App\Category');
    }

    public function shipping_methods()
    {
        return $this->belongsToMany('App\ShippingMethod');
    }
}