Django-m2m_changed信号未保存对相关模型的更改

Question

我创建了一个在m2m_changed信号上运行的函数。完成各种操作后，我需要更新BMGUser对象上的字段。

本质上：我向BMGUser订阅了一个AWS Simple Notification Service主题，并将该arn存储在BMGUser对象中。

问题：当我直接更新BMGUser对象上的Resorts字段时，代码起作用。当我从Resort对象更新bmg_users字段时，该函数被触发并运行，但是对该函数中用户对象的更改不会持久。

代码：

class Resort(models.Model):
    """
    Resort model
    """

class BMGUser(models.Model):
    """
    Extend the User model to include a few more fields
    """
    resorts = models.ManyToManyField(Resort, related_name='bmg_users')
    sub_arn = models.CharField("AWS SNS Subscription arns", max_length=1000, blank=True, null=True)

@receiver(m2m_changed, sender=BMGUser.resorts.through)
def subscribe_sns_topic(instance: Union[BMGUser, Resort], action: str, reverse: bool, pk_set: Set[int],
                        **kwargs) -> None:
    """
    Subscribe or unsubscribe the user to the relevant resort SNS topic, if resort added to their obj

    :param instance: BMGUser or Resort object being updated
    :param action: type of update on relation
    :param reverse: False if BMGUser is being modified directly; True if Resort object is being modified
    :param pk_set: set of primary keys being added or removed to the m2m field
    """
    # Instance -> BMGUser
    # This works
    if action == 'post_add' and not reverse:
        sub_arns = subscribe_user_to_topic(instance, client)
        instance.sub_arn = json.dumps(sub_arns)
        instance.save()

    # Instance -> BMGUser
    # This works
    elif action == 'pre_remove' and not reverse:
        for id in pk_set:
            resort = Resort.objects.get(pk=id)
            sub_arns = unsubscribe_user_to_topic(instance, client, resort)

            instance.sub_arn = json.dumps(sub_arns)
            instance.save()

    # Instance -> Resort
    # This fails
    elif action == 'post_add' and reverse:

        for indx, user in enumerate(instance.bmg_users.all()):
            if user.pk in pk_set:
                sub_arns = subscribe_user_to_topic(user, client)

                user.sub_arn = json.dumps(sub_arns)
                user.save()

    # Instance -> Resort
    # Haven't been able to test this yet
    elif action == 'pre_remove' and reverse:
        users = BMGUser.objects.filter(pk__in=pk_set)

        for user in users:
            sub_arns = unsubscribe_user_to_topic(user, client, instance)

            user.sub_arn = json.dumps(sub_arns)
            user.save()

关于为什么不保留对用户对象的更新的任何建议？