invoiceninja/app/Models/Invoice.php

<?php

namespace App\Models;

use App\Models\Filterable;
use App\Utils\Traits\MakesHash;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\SoftDeletes;

class Invoice extends BaseModel
{
    use MakesHash;
    use SoftDeletes;
    use Filterable;

	protected $guarded = [
		'id',
	];

    const STATUS_DRAFT = 1;
    const STATUS_SENT =  2;
    const STATUS_VIEWED = 3;
    const STATUS_PARTIAL = 5;
    const STATUS_PAID = 6;
    const STATUS_OVERDUE = -1;
    const STATUS_UNPAID = -2;

    public function company()
    {
        return $this->belongsTo(Company::class);
    }

    public function user()
    {
        return $this->belongsTo(User::class);
    }

    public function invitations()
    {
        $this->morphMany(Invitation::class, 'inviteable');
    }
}