invoiceninja/app/Transformers/InvoiceHistoryTransformer.php

<?php
/**
 * Invoice Ninja (https://invoiceninja.com).
 *
 * @link https://github.com/invoiceninja/invoiceninja source repository
 *
 * @copyright Copyright (c) 2022. Invoice Ninja LLC (https://invoiceninja.com)
 *
 * @license https://www.elastic.co/licensing/elastic-license
 */

namespace App\Transformers;

use App\Models\Activity;
use App\Models\Backup;
use App\Utils\Traits\MakesHash;

class InvoiceHistoryTransformer extends EntityTransformer
{
    use MakesHash;

    protected $defaultIncludes = [
        // 'activity',
    ];

    protected $availableIncludes = [
        'activity',
    ];

    public function transform(?Backup $backup)
    {
        if(!$backup){
            
            return [
                'id' => '',
                'activity_id' => '',
                'json_backup' => (string) '',
                'html_backup' => (string) '',
                'amount' => (float) 0,
                'created_at' => (int) 0,
                'updated_at' => (int) 0,
            ];

        }

        return [
            'id' => $this->encodePrimaryKey($backup->id),
            'activity_id' => $this->encodePrimaryKey($backup->activity_id),
            'json_backup' => (string) '',
            'html_backup' => (string) '',
            'amount' => (float) $backup->amount,
            'created_at' => (int) $backup->created_at,
            'updated_at' => (int) $backup->updated_at,
        ];
    }

    public function includeActivity(Backup $backup)
    {
        $transformer = new ActivityTransformer($this->serializer);

        return $this->includeItem($backup->activity, $transformer, Activity::class);
    }
}