如何使用 post_save 通过 Django 信号填充模型字段？

Question

After creating a model object I would like to use a post_save signal to populate another model field in the same object with the ID and other values.

But the real question here is, how do i access the field from the instance? nothing worked

Example: User creates an object -> id of the object is 93 -> I would like to add #00093 as value of another field in the same object.

models.py

class Ticket(models.Model):
    user = models.ForeignKey(User, on_delete=models.SET_NULL, null=True)
    designation = models.CharField(max_length=255, blank=True, null=True)
    summary = models.CharField(max_length=255)

signals.py

@receiver(post_save, sender=Ticket)
def ticket_designation(sender, instance, **kwargs):
    instance.designation = "#" + "000" + str(instance.id)
    print("Ticket" + instance.designation + " was created.")

Answer 1

I think you really want to do this:

@receiver(post_save, sender=Ticket)
def ticket_designation(sender, instance, created, **kwargs):
    if created:
        designation = "#" + "000" + str(instance.id)
        Ticket.filter(pk=instance.pk).update(designation=designation)
        print("Ticket" + instance.designation + " was created.")

Explanation:

why use if created : designation is only created once when the new Ticket is created. It's not updated every time you save the same object.

why use Ticket.objects.filter(..).update(..) : we cannot do instance.save() as it'll then call this post_save again and recursion occurs. .update on queryset does not call signals so we're safe.

---

If you want to use instance.save you might want to look at how to prevent post_save recursion in django.