EntityChangeSet in preUpdate not containing relations? - symfony

Question

I am trying to do something when a User joins a Group. I am trying to use the preUpdate event for it and then check if the corresponding relations have changed. Unfortunately in my Group the 'users" relation is never in the changeset, as well as in my User the Usergroup is never in the changeset.
Here the two listeners:
public function preUpdate(PreUpdateEventArgs $args){
if($args->hasChangedField('users')){
$old = $args->getOldValue('users');
$new = $args->getNewValue('users');
}
}
public function preUpdate(PreUpdateEventArgs $args){
if($args->hasChangedField('userGroups')){
$old = $args->getOldValue('userGroups');
$new = $args->getNewValue('userGroups');
}
}
Thats my TestCase:
$group->addUser($user);
$em->beginTransaction();
$em->persist($group);
$em->flush();
$em->rollback();
Both listeners are called, but the relation is never in the changeset.
I want to add the user into a redis table, where I manage some specific data. Maybe the onFlush or some other events are better, since I don't need to modify the saved data. I just want to know if a there is a new Entry in my User-UserGroup Relation. I thought the easiest way to check this would be the changeset within the preUpdate function.

Answer 1

I solved it via the onFlush event:
public function onFlush(OnFlushEventArgs $args){
$em = $args->getEntityManager();
$uow = $em->getUnitOfWork();
foreach ($uow->getScheduledCollectionUpdates() as $col) {
if($this->isUserGroupUserAssociation($col->getMapping())){
$userGroup = $col->getOwner();
foreach($col->getInsertDiff() as $user){
$this->container->get('strego_user.user_manager')->triggerJoined($userGroup,$user);
}
}
}
}
protected function isUserGroupUserAssociation($association){
return($association['fieldName'] == "users" &&
$association['sourceEntity'] == "Strego\UserBundle\Entity\UserGroup"
);
}