在post_save信号中获取字段的先前值 [英] Get previous value of a field in post_save signal

问题描述

我在Django中有一个发布信号，我需要在其中访问字段的先前值:

I have a post signal in django, where I need to access previous value of a field:

post_save.connect(callback_function_postsave, sender=Media)

我理想地知道我应该为此使用pre_save:

I know ideally I should use pre_save for this:

pre_save.connect(callback_function_presave, sender=Media)

def callback_function_presave(sender, instance,*args,**kwargs):
try:
    old_value = sender.objects.get(pk=instance.pk).field
except sender.DoesNotExist:
    return

但是，必须在 post_signal 中获取 old_value ，因为基于此，我必须决定是否进行第3方api调用.我无法在 pre_save 中进行api调用，因为api使用的是同一数据库，并且期望提交更新的值.

However, its imperative to get the old_value in post_signal, because based on it, I have to decide whether to make a 3rd party api call or not. I cant make the api call in pre_save as the api is using the same database and expects updated value committed.

我想到的一种可能的方法是将old_value添加到实例本身，然后可以通过post_save进行访问:

One possible way I can think of is add the old_value to instance itself which can then be accessed by post_save:

def callback_function_presave(sender, instance,*args,**kwargs):
try:
    instance.old_value = sender.objects.get(pk=instance.pk).field
except sender.DoesNotExist:
    return

def callback_function_postsave(sender, instance,*args,**kwargs):
try:
    old_value = instance.old_value
except:
    print "This is a new entry"

有没有更好的方法来实现这一目标.

Is there any better way to achieve this.

推荐答案

不幸的是，post_save信号并没有给您旧值(

Unfortunately the post_save signal doesn't give you the old values (post_save). So storing the old value on the model seems to be a good solution.

我会这样写pre_save:

I would have written the pre_save like this:

def save_old_value(sender, instance, *args, **kwargs):
    if instance.id:
        instance.old_value = instance.__class__.objects.get(id=instance.id).old_value

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