public function subscribe(Forum $forum, User $user, $selfActivation = true)
    {
        $this->om->startFlushSuite();
        $notification = new Notification();
        $notification->setUser($user);
        $notification->setForum($forum);
        $notification->setSelfActivation($selfActivation);
        $this->om->persist($notification);
        $this->dispatch(new SubscribeForumEvent($forum));
        $this->om->endFlushSuite();
    }