JMS Serializer add groups by some condition - symfony

Question

I have entity Notification and when my entity contain some condition I want add additional groups for serializer, I create event serializer.pre_serialize for Notification and after check condition add additional groups in serialized context, but in response it's not works. Example I want add some nested to my Notification it would be some another Entity, example Question and this entity have some groups, and after set it when I create context SerializationContext::create() everything work, but when I try add additional group in serializer.pre_serialize it's not work
this my entity
class Notifications
{
use TraitTimestampable;
const GROUP_POST_NOTIFICATION = 'post_notifications';
const GROUP_GET_NOTIFICATIONS = 'get_notifications';
/**
* #ORM\Column(type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
* #Annotation\Groups({
* "get_notifications"
* })
*/
private $id;
/**
* #var object
*
* #Annotation\Groups({
* "get_notifications"
* })
*/
private $providerEntity;
and another entity which I adding in event
class Questions implements NotificationInterface
{
use TraitTimestampable;
const GROUP_POST = 'post_question';
const GROUP_GET = 'get_question';
/**
* #ORM\Column(type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
* #Annotation\Groups({
* "get_question"
* })
*/
private $id;
/**
* #return array
*/
public static function getGetGroup()
{
return [self::GROUP_GET];
}
but in response I have
public function onPreSerializeNotifications(PreSerializeEvent $event)
{
/** #var Notifications $notifications*/
$notifications = $event->getObject();
$provider = $notifications->getProvider(); //this is Questions::class
$this->providerEntity = $this->entityManager->getRepository($provider)
->findOneBy(['id' => $notifications->getProviderId()]);
$notifications->setProviderEntity($this->providerEntity);
$attr = $event->getContext()->attributes;
$groups = array_merge($attr->all()['groups'], $provider::getGetGroup());
$attr->set('groups', $groups);
}
but in response I have
"notifications": [
{
"id": 2,
"provider_entity": {}
}
]
why ?