Django：在信号中获取M2M相关对象 [英] Django: Getting m2m related objects in signals

问题描述

我有点想看到类似的问题（ Django访问ManyToMany字段，但仍然看不到如何获取更新的相关对象列表。

I was kind of trying to see similar questions (Django accessing ManyToMany fields from post_save signal), but still don't see how to get the updated related objects list.

例如我有以下模型

class User(models.Model):
    username = models.CharField

class Blog(models.Model):
    user = models.ManyToManyField('User')

现在，我要通过django admin将用户添加到给定的博客中。

Now I am adding a user to a given blog, via django admin.

所以我希望下面的信号将打印所有新用户（我刚刚添加的）...但是...我得到了所有的旧列表时间：（

So I expect that the signal below, will print all new users (which I have just added)... but... I am getting the old list all the time :(

@receiver(m2m_changed, sender=Blog.users.through)
def blog_users_change(sender, instance, **kwargs):
     print instance.users.all()

最后一行给出用户的旧列表 instance.users.all（）。例如，此处添加的用户不会得到体现。

The last line gives old list of users instance.users.all(). E.g. users added here are not reflected.

推荐答案

m2m_changed 信号在保存/更新过程的多个阶段被触发，并且有一个操作参数，告诉您它处于哪个阶段。来自文档：

m2m_changed signals are fired at several stages in the save/update process, and there is an action argument supplied to the signal handler that tells you what stage it is in. From the documentation:

动作

一个字符串，指示在关系上完成的更新的类型。以下之一：

A string indicating the type of update that is done on the relation. This can be one of the following:

pre_add
在将一个或多个对象添加到关系之前发送。

"pre_add" Sent before one or more objects are added to the relation.

post_add
在将一个或多个对象添加到关系后发送。

"post_add" Sent after one or more objects are added to the relation.

pre_remove
在从关系中删除一个或多个对象之前发送。

"pre_remove" Sent before one or more objects are removed from the relation.

后删除
从关系中删除一个或多个对象后发送。

"post_remove" Sent after one or more objects are removed from the relation.

pre_clear
在关系清除之前发送。

"pre_clear" Sent before the relation is cleared.

post_clear
关系清除后发送。

"post_clear" Sent after the relation is cleared.

如果捕捉到 pre_remove 操作，则将获得所有之前对象已从关系中删除。这就是为什么您看到的用户列表似乎没有变化的原因。

If you catch the pre_remove action then you will get all the objects before some have been removed from the relationship. That is why you are seeing an apparently unchanged list of users.

您的代码需要先检查操作，然后再确定该怎么办。例如：

Your code needs to check the action before deciding what to do. For example:

@receiver(m2m_changed, sender=Blog.users.through)
def blog_users_change(sender, instance, action, **kwargs):
    if action == 'pre_remove':
         # This will give you the users BEFORE any removals have happened
         print instance.users.all()
    elif action == 'post_remove':
         # This will give you the users AFTER any removals have happened
         print instance.users.all()

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