Error A circular reference has been detected on symfony 3 - symfony

Question

i do this steps:
1.) I call my api endpoint: http://localhost:8000/api/addrole
2.) This is the controller called:
/**
* #Rest\Post("/addrole")
*/
public function addRoleAction(Request $request)
{
$userid = $request->get('userid');
$assignedRole = $request->get('role');
$assignedRoleName = $request->get('role_name');
// Obtain the User
$em = $this->getDoctrine()->getManager();
$user = $em->getRepository('AppBundle:User')
->find($userid);
// If the user gives not exists, throw error
if (!$user) {
throw new HttpException (400,"No se ha encontrado el usuario solicitado: " .$userid);
}
// obtain present user roles
$presentRoles = $user->getRoles();
$role_length = count($presentRoles);
$role_list = array();
for ($i=0; $i <$role_length ; $i++) {
array_push($role_list,$presentRoles[$i]->getRole());
}
if(!in_array($assignedRole,$role_list)){
$role = $em->getRepository('AppBundle:Role')
->findOneBy(array('role' => $assignedRole));
$user->addRole($role);
$em->persist($user); // persisting only the user.
$em->flush();
$data = array(
'result' => 'Rol asignado',
'user' => $user,
'assignedRole' => $assignedRole
);
return $data;
} else {
throw new HttpException (400,"El usuario ya posee el rol solicitado");
}
}
3.) The data on this step:
$user->addRole($role);
$em->persist($user); // persisting only the user.
$em->flush();
is saving the data on the DB.
4.) this is the response:
{
"error": {
"code": 500,
"message": "Internal Server Error",
"exception": [
{
"message": "A circular reference has been detected (configured limit: 1).",
"class": "Symfony\\Component\\Serializer\\Exception\\CircularReferenceException",...
5.) if i comment this line:
//$em->flush();
the data obviosly don't save, but the error is not throwed.
Any clue about this?

Answer 1

Finally i found the error.
The problem is on this code:
...
post $em->flush();
this: $data = array('result' => 'Rol asignado', 'user' => $user, 'assignedRole' => $assignedRole );
return $data;
After the flush i called the $user object. This generate the problem.
Thanks to all.