invoiceninja/app/Ninja/Transformers/InvoiceTransformer.php

<?php namespace App\Ninja\Transformers;

use App\Models\Invoice;
use League\Fractal;
use League\Fractal\TransformerAbstract;

class InvoiceTransformer extends TransformerAbstract
{
    protected $defaultIncludes = [
        'invoice_items',
    ];

    public function includeInvoiceItems($invoice)
    {
        return $this->collection($invoice->invoice_items, new InvoiceItemTransformer);
    }

    public function transform(Invoice $invoice)
    {
        return [
            'id' => (int) $invoice->public_id,
            'invoice_number' => $invoice->invoice_number,
            'amount' => (float) $invoice->amount,
            'balance' => (float) $invoice->balance,
        ];
    }
}