A lot of code checks if an object is saved as an update, or created with a check of the truthiness of instance.pk, for example:

from django.db import models

class MyModel(models.Model):
    # ...

    def save(self, *args, **kwargs):
        if self.pk:
            print('updating')
        else:
            print('creating')
        return super().save(*args, **kwargs)

Why is it a problem?

Because if the primary key field constructs a default value itself, self.pk will contain that value, and therefore the check no longer works.

A popular example of this is when MyModel has UUIDField as primary key. For example:

from django.db import models
import uuid

class MyModel(models.Model):
    id = models.UUIDField(
        primary_key=True,
        default=uuid.uuid4,
        editable=False
    )
    # …

If we now create an object without saving it in the database, it will already have a UUID:

>>> my_model = MyModel()
>>> my_model.id
UUID('c4e8412a-7ec0-419d-b34c-c0bafb67ff49')

so now the check will fail.

What can be done to resolve the problem?

We can work with instance._state.adding instead, like:

from django.db import models

class MyModel(models.Model):
    # …

    def save(self, *args, **kwargs):
        if self._state.adding:
            print('creating')
        else:
            print('updating')
        return super().save(*args, **kwargs)

This will determine how we got to this MyModel object: from the database, or by creating a model object. This, however, does not say that a record with the primary key does not exist at the database level.

Indeed, MyModel(pk=1)._state.adding will also be True even if a record with id=1 already exists in the database. To confirm existence, the only option is to query the database.