topnotch_website/app/Models/PackingSlip.php

<?php

namespace App\Models;

use Database\Factories\PackingSlipFactory;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsTo;
use Illuminate\Database\Eloquent\SoftDeletes;

class PackingSlip extends Model
{
    /** @use HasFactory<PackingSlipFactory> */
    use HasFactory, SoftDeletes;

    protected $fillable = [
        'order_id',
        'customer_id',
        'date_received',
        'amount',
        'contents',
    ];

    /**
     * @return BelongsTo<Customer, self>
     */
    public function customer(): BelongsTo
    {
        return $this->belongsTo(Customer::class);
    }
}