exchange/app/Http/Controllers/Api/ContactController.php

<?php

namespace App\Http\Controllers\Api;

use App\Models\Contact;
use App\Transformers\ContactTransformer;

class ContactController extends ApiController
{
    public function index()
    {  
        $contacts = Contact::orderBy('lft', 'asc')->get();
        return $this->respondWithCollection($contacts, new ContactTransformer($this->locale));
    }
}