How to use Transformer for nested relationships?

I have the following code, but nested PrototypeTransformer returns always empty array:

$objects = Object::all();

$resource = new Fractal\Resource\Collection($objects, function(Object $obj) {

    $r = new FractalCollection($obj->prototypez(), new PrototypeTransformer);

    $c = $this->fractal->createData($r)->toArray();

    //dd($c);

    return [
        'id'    => (int) $obj->id,
        'title' => $obj->prefix,
        'prototypes' => $c
    ;
});

via Chebli Mohamed