[Serializer] Serialization of empty ArrayCollection to JSON returns '{}' instead '[]'

[yauhenko]

Symfony version(s) affected: 5.0.9, 5.1.0

Description
Entity User

class User implements UserInterface {

	/**
	 * @ORM\Id()
	 * @ORM\GeneratedValue()
	 * @ORM\Column(type="integer", options={"unsigned": true})
	 * @Groups("main")
	 */
	private int $id;
       // ...
	/**
	 * @ORM\ManyToMany(targetEntity=Department::class)
	 * @ORM\JoinColumn(onDelete="CASCADE")
	 * @Groups("main")
	 */
	private $departments;

My controller

	public function getUser(User $id) {
            return $this->json($user, 200, [], ['groups' => ['main']);
	}

Expected:

{
    "id": 2,
   // ...
    "departments": []
  }

Actual result:

{
    "id": 2,
   // ...
    "departments": {}
  }

How to reproduce
Just upgraded symfony/serializer to 5.0.9

[yceruto]

Duplicated of #37041? and already fixed in #37049.

Thanks for reporting it !

[SergeyYesin]

This thing is not yet fixed in 5.0 and 5.1 branches.

[guillaumesmo]

This thing is not yet fixed in 5.0 and 5.1 branches.

it will be in the next patch releases (4.4.10, 5.0.10 and 5.1.1) usually at the end of the month

[arisrais]

Thanks, I am waiting for it. I will bypass serializer v4.4.9 (my CI is down)