You should not expose ArrayCollection but use ArrayCollection->getValues instead, which will get you a real array (also fixes an index issue if you delete somthing from this ArrayCollection). ArrayCollection is an object and it's correct that it gets normalized to an object imo. See also api-platform/docs#504 and related.

@soyuka this also affected us somehow when orm paginator from api-platform returned zero results we started to get an empty object instead of empty array (for an application/json request).

I'd like to reproduce this @norkunas because it is indeed a bad behavior. I still believe that #36601 is legit or we should rollback this which actually causes the issue in the first place with \ArrayObject. I mean we can't on one hand allow empty objects to be normalized as empty objects (as they should) but have some kinds of objects be normalized as array (ArrayCollection/Paginator). According to the unit tests, an empty array is still encoded as empty array and the error should be best handled by the developer.

I agree that merged PR is a good one, but then api-platform should do the work to serialize paginator as empty array :)

@soyuka it's BC! And this is unexpected behavior and completely unusable in real world.
Symfony can give a setting for managing the behavior.

for example:

• Doctrine Entity has ArrayCollection property (for relations)
• Controller serialize the Entity with the property
• Frontend expects an array

In symfony 4.4.8 it's works as expected.
But in 4.4.9 not.

DoctrineCollections should be implement JsonSerializable. But without it this change breaks a lot of production code.

What works as expected? Because normalizing an empty ArrayObject doesn't pre-4.4.9. I need to check if it worked pre-#28363 which then is also BC :/. I think that we should just deliberate on what we want the ObjectNormalizer to do on empty objects as pre-4.4.9 this is also "unexpected behavior":

use Doctrine\Common\Collections\ArrayCollection;
use Symfony\Component\Serializer\Encoder\JsonEncoder;
use Symfony\Component\Serializer\Serializer;

$serializer = new Serializer([], [new JsonEncoder()]);
$arr = new ArrayObject();

echo $serializer->serialize(['foo' => $arr], 'json');

Does #37049 fix the issue?

Does #37049 fix the issue?

Indeed, having the PRESERVE_EMPTY_OBJECTS to false keeps these objects as arrays. 👍 on the fix. (is this the default value btw?)

@xabbuh Yes, it works.

IMHO we should explicitly whitelist the old behavior for ArrayObject and instances of Doctrine Collections because they are designed to store lists (not maps) and it makes sense to always serialize them as JSON arrays (not objects).