def METHOD_NAME(self):
    new_watcher_emails = set(self._watchers)
    old_watcher_emails = set(self.object.get_watchers().values_list("email", flat=True))
    adding_watcher_emails = list(new_watcher_emails.difference(old_watcher_emails))
    removing_watcher_emails = list(old_watcher_emails.difference(new_watcher_emails))
    User = get_user_model()
    adding_users = User.objects.filter(email__in=adding_watcher_emails)
    removing_users = User.objects.filter(email__in=removing_watcher_emails)
    for user in adding_users:
        notifications_services.add_watcher(self.object, user)
    for user in removing_users:
        notifications_services.remove_watcher(self.object, user)
    self.object.watchers = [user.email for user in self.object.get_watchers()]