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;

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

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

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