Django: how can I tell if the post_save signal triggers on a new object?

问题

I need to do some background post-processing on newly created objects in Django. This post-processing should only run on new objects, not objects that are just updated.

I know that in pre_save I can check if the object has an id, if it has not then it's a new object. But the problem is that in the post-processing I need access to the id (so that I can save the results back to the database).

How can I do this in a clean way?

回答1:

Have a look at docs: https://docs.djangoproject.com/en/stable/ref/signals/#post-save

There is a created named argument which will be set to True if it's a new object.

回答2:

As Docs stated and @seler pointed out, but with an example:

def keep_track_save(sender, instance, created, **kwargs):
    action = 'save' if created else 'update'
    save_duplicate((instance.id, instance.__class__.__name__, action))

post_save.connect(keep_track_save, sender=Group)

回答3:

I just leave it here, maybe it'll be helpful for someone.

from django.db import models
from django.db.models.signals import post_save
from django.dispatch import receiver


class Deal(models.Model):
    name = models.CharField(max_length=255)


@receiver(post_save, sender=Deal)
def print_only_after_deal_created(sender, instance, created, **kwargs):
    if created:
        print(f'New deal with pk: {instance.pk} was created.')

来源：https://stackoverflow.com/questions/10672572/django-how-can-i-tell-if-the-post-save-signal-triggers-on-a-new-object