Attach the user in session to the current EntityManager when writing functional tests

I'm writing a functional test for an Action entity having a relationship with the User entity:

<?php

namespace Acme\AppBundle\Entity;

/**
 * Class Action
 *
 * @ORM\Table()
 * @ORM\Entity(repositoryClass="Acme\AppBundle\Repository\ActionRepository")
 */
class Action
{
    /**
     * @var int
     *
     * @ORM\Column(type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var \Acme\AppBundle\Entity\User
     *
     * @ORM\ManyToOne(targetEntity="\Acme\AppBundle\Entity\User", inversedBy="actions")
     * @ORM\JoinColumn(name="user_id", referencedColumnName="id")
     */
    private $createdBy;
}

User:

namespace Acme\AppBundle\Entity;

/**
 * @ORM\Entity
 * @ORM\Table(name="`user`")
 */
class User extends BaseUser
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;

    /**
     * @var ArrayCollection
     *
     * @ORM\OneToMany(targetEntity="Action", mappedBy="createdBy")
     */
    private $actions;
}

And the user is setted in the controller with the following snippet:

<?php

namespace Acme\ApiBundle\Controller;

/**
 *
 * @Route("/actions")
 */
class ActionController extends FOSRestController
{
    public function postAction(Request $request)
    {
        $action = new Action();
        $action->setCreatedBy($this->getUser());

        return $this->processForm($action, $request->request->all(), Request::METHOD_POST);
    }
}

When calling the action with a REST client for example, everything works fine, the relationship between Action and User is persisted correctly.

Now, when testing the action with a functional test, the relationship is not working because of the following error:

A new entity was found through the relationship 'Acme\AppBundle\Entity\Action#createdBy' that was not configured to cascade persist operations for entity: test. To solve this issue: Either explicitly call EntityManager#persist() on this unknown entity or configure cascade persist this association in the mapping for example @ManyToOne(..,cascade={"persist"}).

For my functional test I need to inject a JWT and a session token because my routes are secured by a JWT and I need to have a user in session.

Here is how I inject that:

<?php

namespace Acme\ApiBundle\Tests;

class ApiWebTestCase extends WebTestCase
{
    /**
     * @var ReferenceRepository
     */
    protected $fixturesRepo;

    /**
     * @var Client
     */
    protected $authClient;

    /**
     * @var array
     */
    private $fixtures = [];

    protected function setUp()
    {
        $fixtures = array_merge([
            'Acme\AppBundle\DataFixtures\ORM\LoadUserData'
        ], $this->fixtures);

        $this->fixturesRepo = $this->loadFixtures($fixtures)->getReferenceRepository();

        $this->authClient = $this->createAuthenticatedClient();
    }

    /**
     * Create a client with a default Authorization header.
     *
     * @return \Symfony\Bundle\FrameworkBundle\Client
     */
    protected function createAuthenticatedClient()
    {
        /** @var User $user */
        $user = $this->fixturesRepo->getReference('user-1');

        $jwtManager = $this->getContainer()->get('lexik_jwt_authentication.jwt_manager');
        $token = $jwtManager->create($user);

        $this->loginAs($user, 'api');

        $client = static::makeClient([], [
            'AUTHENTICATION' => 'Bearer ' . $token,
            'CONTENT_TYPE' => 'application/json'
        ]);

        $client->disableReboot();

        return $client;
    }
}

Now, the issue is that the injected UsernamePasswordToken contains a User instance which is detached from the current EntityManager, thus resulting in the Doctrine error above.

I could merge the ûser object in the postAction method into the EntityManager but I don't want to do that because it means I modify my working code to make a test passes. I've also tried directling merging the $user object in my test into the EntityManager like this:

$em = $client->getContainer()->get('doctrine')->getManager()
$em->merge($user);

But it's not working either.

So now, I'm stuck, I really don't know what to do except that I need to attach the user in session back to the current EntityManager.