Symfony 4: Updating an existing entity through a deserialized entity ends up with a new related item or with an ORMInvalidArgumentException

Question

That's my first question here on Stackoverflow.com and before I'll write to much. First the controller function:

/**
 * @Rest\Patch("/identifiers/v2/{id}")
 *
 * @ParamConverter("identifier")
 * @ParamConverter("identifierPatch", converter="fos_rest.request_body")
 */
public function patchAction(Identifier $identifier, Identifier $identifierPatch)
{
    $identifier->setLandingPage($identifierPatch->getLandingPage());
    $identifier->setIdentifier($identifierPatch->getIdentifier());
    $identifier->setIsUsed($identifierPatch->getIsUsed());

    $this->entityManager->flush();

    /**
     * Just for debugging...
     */
    $view = $this->view([
        'identifier' => $identifier,
        'identifierPatch' => $identifierPatch
    ]);

    return $this->handleView($view);
}

When i try to UPDATE an existing entity this way I get an ORMInvalidArgumentException with a message "A new entity was found through the relationship (...)"

When I set cascade={"persist"} on the related entity:

/**
 * @ORM\ManyToOne(targetEntity="App\Entity\LandingPage", inversedBy="identifiers")
 * @ORM\JoinColumn(nullable=false)
 * @Assert\NotNull()
 * @Serializer\Type("App\Entity\LandingPage")
 */
private $landing_page;

... the related entity will be inserted as new entity and that's not what I am looking for.

I could use $this->entityManager->merge($identifier) but that's not what I am looking for aswell, because I'll need to do some manual validations in future and I would like to return the entity as response (the related entity will be null when not updated) and $this->entityManager->merge() will be deprecated in Doctrine 3.

Question: Is there any way to update the given entity with the deserialized entity?

Greetings, Danny Endert

---

EDIT (1): Well, I guess i found a solution regarding this "issue".

services.yaml

jms_serializer.object_constructor:
    alias: jms_serializer.doctrine_object_constructor
    public: false

Now I'm not getting any exception and the related entity will not be inserted as new entity.