Document model._state.adding (since UUIDField sets value before save)

[Mattia Procopio]

When using a pk I expect the behaviour we had in the past meaning that inside .save() method checking for self.pk gives me a clue of creation vs update (documentation confirms that ​https://docs.djangoproject.com/en/1.8/ref/models/instances/#how-django-knows-to-update-vs-insert). This is not working when using UUIDField since when we enter the save method the pk is already set to the default value.

to reproduce

import uuid

from django.db import models

class AwesomeModel(models.Model):
    id = models.UUIDField(default=uuid.uuid4, primary_key=True)
    x = models.CharField(max_length=10)

    def save(self, *args, **kwargs):
        if self.pk:
            print("we have already a pk set")
        return super(AwesomeModel, self).save(*args, **kwargs)


a = AwesomeModel(x='some')
a.save()
we have already a pk set

[Tim Graham]

We should likely document model._state.adding. See #24377 for a patch where we had to adapt the admin.

[Mariusz Felisiak]

Duplicate of #31502.