South migration of parent class with mulitple child classes in same app - django

Question

I'm having an issue migrating a Django class which is a parent of two child classes. All classes are in the same app. Whenever I try to migrate the parent class, South complains that a table already exists. Here are my classes, simplified:
class ParentClass(models.Model):
my_field_in_both = models.DateField(null=True, blank=True)
class Meta():
abstract = True
The two child classes:
class ChildOne(ParentClass, AnotherMixin):
child_field = models.DateField(null=True, blank=True)
class ChildTwo(ParentClass, YetAnotherMixin):
another_child_field = models.DateField(null=True, blank=True)
Now, I can migrate either AnotherMixin or YetAnotherMixin classes no problem. But adding a field on the ParentClass, and then running:
python manage.py schemeamigration <appname> --auto
produces the migration file, but then running:
python manage.py migrate <appname>
gives:
django.db.utils.DatabaseError: table "_south_new_<appname>_ChildTwo" already exists
What am I doing wrong?

Answer 1

It says it already exists because the table already exists. It sounds like you messed up your migrations at some point in the past, but don't worry! Everyone does that at some point.
There are two routes you could take. If one fails, you can still do the other, so Im just going to list them in the order that I'd try them. The easier (and potentially riskier depending on the size of your project and how far you are along) of the two is to try to fake the migration.:
./manage.py migrate ChildTwo --fake
It's going to pretend it did everything correctly, but if there are conflicts with the actual structure of the database and the faked structure of the database, you're going to have to fix things manually. But it sounds like you're not very far in your project, so Im going to go out on a limb here and say that wont be that big of a deal. These are the things you're going to manually delete:
the migrations/ folder in your app.
the <app_name>_childtwo table in your database
any rows in the south_migrationshistory table corresponding to the ChildTwo model. If you're using mysql:
DELETE FROM <database name>.south_migrationhistory WHERE app_name ='<app_name>';
But it may be easier to just delete all migrations corresponding to the entire app then start over with:
./manage.py schemamigration app_name --initial
Like any reasonable person, I despise writing SQL, so I suggest using the phpmyadmin interface if you have it.
I'll stop editing soon, but one last thing, if your app isn't too far along, you could also do ./manage.py reset <app_name> and just start over. You'll still have to get rid of the south migrations folder and db entries