Model validation allows nullable primary key field.

[JustinTArthur]

If I have a primary key field that is also nullable, the model validation code is currently allowing it:

class MyModel(models.Model):
    other_model = OneToOneField(OtherModel, primary_key=True, null=True)

This behavior seems like a defect given two circumstances:

• No SQL implementation I know of allows a primary key column to be NULL in the schema.
• Django's CASCADE deletion will None/NULL out any null=True foreign keys (see source:/trunk/django/db/models/deletion.py@15927#L19),
  • While a SQL backend might correctly map the None to a 0 instead of NULL in the UPDATE statement, after a second deletion against the same table, you would have more than one row with primary key of 0, thus violating the uniqueness constraint intrinsic to a primary key field.

[JustinTArthur]

The correct link for the CASCADE interest point is actually source:/django/trunk/django/db/models/deletion.py@15927#L19

[JustinTArthur]

Patch to add validation error when a model field is both a primary key and nullable.

[Julie Pichon]

I'm attaching a patch with a test for the new validation behaviour -- the patch also includes JustinTArthur's diff as there was a missing double-quote around the field name.

[Malcolm Tredinnick]

In [16678]:

Teach "django-admin.py validate" to forbid nullable primary keys.

Fixes #15884, with thanks to JustinTArthur and Julie Pichon.

[Aymeric Augustin]

The patch breaks with Oracle, see #16694.