[Fixed]-Django 3 – Model.save() when providing a default for the primary key

Consider this example. Suppose we have a simple model as

CONSTANT = 10


def foo_pk_default():
    return CONSTANT


class Foo(models.Model):
    id = models.IntegerField(primary_key=True, default=foo_pk_default)
    name = models.CharField(max_length=10)

The main thing I have done in this example is, I did set a default callable function for Primary Keys. Also, I returned only a single value from the function, for the sake of demonstration.

## Django 2.2
In [5]: foo_instance_1 = Foo(name='foo_name_1')

In [6]: foo_instance_1.save()

In [7]: print(foo_instance_1.__dict__)
{'_state': , 'id': 10, 'name': 'foo_name_1'}

In [8]: foo_instance_2 = Foo(name='foo_name_2')

In [9]: foo_instance_2.save()

In [10]: print(foo_instance_2.__dict__)
{'_state': , 'id': 10, 'name': 'foo_name_2'}

## Django 3.X
In [6]: foo_instance_1 = Foo(name='foo_name_1')

In [7]: foo_instance_1.save()

In [8]: print(foo_instance_1.__dict__)
{'_state': , 'id': 10, 'name': 'foo_name_1'}

In [9]: foo_instance_2 = Foo(name='foo_name_2')

In [10]: foo_instance_2.save()
# Raised "IntegrityError: UNIQUE constraint failed: music_foo.id"

Conclusion

In Django<3.0, the Model.save() will do an update or insert operation if there is a PK value associated with the model instance whereas in Django>=3.0, only perform an insert operation hence the UNIQUE constraint failed exception.

Impact of this change in the current project

Since this Django change is only applicable when a new instance is created and we usually don’t set any default value functions for Primary Keys.

In short, this change will not make any problem unless you are providing default value during model instance creation.