Symfony JMSSerializer Context : Invalid argument supplied for foreach()

Question

I am currently taking a course on OpenClassroom to learn how to develop an API in Symfony.

I am using Symfony 5.4.13 and php 7.4.9

I arrived at the time of the course we must make our API self-discoverable and we must therefore use JMSSerializer.

So I had to change the way I define the groups when I serialize an object.

In this function for example:

/**
 * Renvoie les informations de tout les livres
 * @Route("/api/books", name="all_book", methods={"GET"})
 */
public function getAllBooks(BookRepository $bookRepository, SerializerInterface $serializer, Request $request, TagAwareCacheInterface $cache): JsonResponse
{
    $page = $request->get('page', 1);
    $limit = $request->get('limit', 3);
 
    $idCache = "getAllBooks-" . $page . "-" . $limit;
     
    $jsonBookList = $cache->get($idCache, function (ItemInterface $item) use ($bookRepository, $page, $limit, $serializer) {
        echo ("L'ELEMENT N'EST PAS ENCORE EN CACHE !\n");
        $item->tag("booksCache");
        $bookList = $bookRepository->findAllWithPagination($page, $limit);
        
        $context = SerializationContext::create()->setGroups(array('getBooks'));
        return $serializer->serialize($bookList, 'json', $context);
    });
 
    return new JsonResponse($jsonBookList, Response::HTTP_OK, [], true);
}

My Entity Book.php:

    <?php
 
namespace App\Entity;
 
use Assert\lenght;
use Doctrine\ORM\Mapping as ORM;
use App\Repository\BookRepository;
use JMS\Serializer\Annotation\Groups;
use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Component\Validator\Mapping\ClassMetadata;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\ParamConverter;
use Heatoas\Configuration\Annotation as Hatoas;
 
 
/**
 * @Hateoas\Relation(
 *      "self",
 *      href = @Hateoas\Route(
 *          "detailBook",
 *          parameters = { "id" = "expr(object.getId())" }
 *      ),
 *      exclusion = @Hateoas\Exclusion(groups="getBooks")
 * )
 *
 */
 
/**
 * @ORM\Entity(repositoryClass=BookRepository::class)
 */
class Book
{
    /**
     * @ORM\Id
     * @ORM\GeneratedValue
     * @ORM\Column(type="integer")
     * @Groups("getBooks", "getAuthors")
     */
    private $id;
 
    /**
     * @ORM\Column(type="string", length=255)
     * @Groups("getBooks", "getAuthors")
     * @Assert\NotBlank(message="Le titre du livre est obligatoire")
     * @Assert\Length(min=1, max=255, minMessage="Le titre doit faire au moins {{limit}} caractères", maxMessage=" le titre ne doit pas faire plus de {{limit}} caractères")
     */
    private $title;
 
    /**
     * @ORM\Column(type="text", nullable=true)
     * @Groups("getBooks", "getAuthors")
     */
    private $coverText;
 
    /**
     * @ORM\ManyToOne(targetEntity=Author::class, inversedBy="books")
     * @ORM\JoinColumn(onDelete="CASCADE")
     * @Groups("getBooks")
     *
     */
    private $author;
 
    public function getId(): ?int
    {
        return $this->id;
    }
 
    public function getTitle(): ?string
    {
        return $this->title;
    }
 
    public function setTitle(string $title): self
    {
        $this->title = $title;
 
        return $this;
    }
 
    public function getCoverText(): ?string
    {
        return $this->coverText;
    }
 
    public function setCoverText(?string $coverText): self
    {
        $this->coverText = $coverText;
 
        return $this;
    }
 
    public function getAuthor(): ?Author
    {
        return $this->author;
    }
 
    public function setAuthor(?Author $author): self
    {
        $this->author = $author;
 
        return $this;
    }
}

The error :

{
    "status": 500,
    "message": "Warning: Invalid argument supplied for foreach()"
}

If anyone has a clue, thank you in advance :)