Symfony: [Serializer] Empty object + ObjectNormalizer + JSON encoder = empty JSON array

[meyerbaptiste]

| Q | A
| ---------------- | -----
| Bug report? | yes
| Feature request? | no
| BC Break report? | no
| RFC? | no
| Symfony version | >= 2.7

Let us take this small example with Melody:

<?php
<<<CONFIG
packages:
    - "symfony/serializer: ^3.3"
    - "symfony/property-access: ^3.3"
CONFIG;

$encoders = [new Symfony\Component\Serializer\Encoder\JsonEncoder()];
$normalizers = [new Symfony\Component\Serializer\Normalizer\ObjectNormalizer()];
$serializer = new Symfony\Component\Serializer\Serializer($normalizers, $encoders);

var_dump($serializer->serialize(new class {}, 'json')); // string(2) "[]"

As you can see the result is an empty array ([]) instead of an empty object ({}). IMO this behavior is an important problem and it should be considered as a bug.

This problem comes from the ObjectNormalizer class which returns an empty array so the JsonEncoder class returns an empty JSON array too:

var_dump(json_encode([])); // string(2) "[]"

One solution would be to pass an empty ArrayObject instance to the JsonEncoder class:

var_dump(json_encode(\new ArrayObject)); // string(2) "{}"

To arrive to this result, we have to change the contract of the NormalizerInterface::normalize() method to return an iterable (or a scalable as currently) instead of an array.

In addition, as discussed with @dunglas, this change would allow us to use generators with normalizers.

Then we would be able to adapt the ObjectNormalizer class to deal with an ArrayObject instance instead of an array. As you can imagine, it's a huge BC break but I don't have a better solution to fix this issue...

WDYT?

[chalasr]

Couldn't a flag/context option be passed to the object normalizer, enabling normalization of empty objects as JSON objects? It would be set to false by default with a deprecation notice saying that it'll be default true in 4.0, and maybe remove it at the end?

[simonwelsh]

This is still an issue in 4.1, and using an empty ArrayObject no longer works.

Using JSON_FORCE_OBJECT is not an option as it prevents you from ever returning a JSON array. A trimmed down example we have is:

[
  'details' => new \stdClass(),
  'prices' => [
    ['amount' => 12.00, 'symbol' => '$'],
  ],
]

This should encode to {"details":{},"prices":[{"amount":12,"symbol":"$"}]}

[javiereguiluz]

@chalasr about this comment:

Couldn't a flag/context option be passed to the object normalizer, enabling normalization of empty objects as JSON objects?

I'm probably missing something ... but the current behaviour looks like a bug to me. Empty PHP objects should generate empty JSON objects, not empty JSON arrays. The last example showed by @simonwelsh explains it well.