Symfony3 :: Handle ManyToMany relations - symfony

Question

I'm trying to save my ManyToMany relations between users and categories. Actually I'm trying to save my category with given users, but this doesn't work.
Form
$builder->add('name')
->add('users', EntityType::class, array(
'label' => 'Benutzer',
'class' => 'AppBundle\Entity\User',
'multiple' => true,
'expanded' => true,
'required' => false,
'choice_label' => function (User $user) {
return $user->getUsername();
}
))
->add('submit', SubmitType::class, array(
'label' => 'Speichern'
));
Form Handler
public function onSuccess(Request $request)
{
// Get category from form
$category = $this->getForm()->getData();
// Redirect to parent category when setted
if ($this->parent) {
$category->setParent($this->parent);
$response = new RedirectResponse($this->router->generate('categories.view', [
'category' => $this->parent->getId()
]));
} else {
// Build new redirect response
$response = new RedirectResponse($this->router->generate('categories.view', [
'category' => $category->getId()
]));
}
try {
// Save category in database
$this->em->merge($category);
$this->em->flush();
} catch (ORMException $ex) {
throw $ex;
}
return $response;
}

Answer 1

Maybe you have to unserialize the Entity $category first?
$detachedCategory = unserialize($category);
$this->em->merge($detachedCategory);
$this->em->flush();
I found this link regarding that:
How to manage deserialized entities with entity manager?
Not sure if that's the answer, but you might want to do more research.