south migration: “database backend does not accept 0 as a value for AutoField” (mysql)

Question

I\'m new to django and trying to have a Foreign key back to users for an assignee and reporter. But when i\'m trying to apply the change with South i get the error

Answer 1

I wasn't using South, but I recently upgraded from Django 1.4 to 1.6 (with MySQL as db backend for both), and was getting the same ValueError when trying to save some models. I tracked it down to a field that was a recursive ForeignKey. So I had:

class Foo(models.Model):
    ...
    duplicate = models.ForeignKey('self', blank=True, null=True)
    ...

Somewhere along the line—unfortunately I'm not sure where—many of my objects had gotten the value of 0 for duplicate_id.

>>> Foo.objects.filter(duplicate_id=0).count()
2078

This didn't occur for any of my other ForeignKey fields, only the self-referential one. So I set the values of that field back to None, and this fixed the error.

>>> Foo.objects.filter(duplicate_id=0).update(duplicate=None)
2078L

Because this particular error doesn't point you to a specific field that's causing problems, in general you can check if a ForeignKey field fieldname has any 0 values:

>>> Foo.objects.filter(fieldname_id=0).count()

If this gives a non-zero result, that field probably needs to be fixed.