How to rename a foreignkey field with South?

Question

Renaming a simple charfield etc seems easy (Django - How to rename a model field using South?)

However when I try using the same on a ForeignKey field I get an error:

_mysql_exceptions.OperationalError: (1091, "Can't DROP '[new_fkey_field_name]'; check that column/key exists")

Which stems from the migration trying to run the backwards for some reason (as evidenced in the trace).

Any ideas?

Answer 1

First, you need to use the db column name not the one in the model. Eg: foobar_id not foobar.

Then you need to drop the fk constraints and recreate them after renaming:

db.drop_foreign_key('app_model', 'old_id')
db.rename_column('app_model', 'old_id', 'new_id')
db.alter_column('app_model', 'new_id', models.ForeignKey(to=orm['app.OtherModel']))

If your fk is nullable you need to use change it to:

db.alter_column('app_model', 'new_id', models.ForeignKey(null=True, to=orm['app.OtherModel']))

Answer 2

MySQL users ought to be aware of this bug in south, if indeed it still applies:

http://south.aeracode.org/ticket/697

The workaround is to conduct the migration in 3 steps:

1) Add new field

2) data migrate the data to the new field

3) delete the old field

Answer 3

When renaming a ForeignKey, remember to add _id to the end of the field name you use in Django. E.g.

db.rename_column('accounts_transaction', 'operator_id', 'responsible_id')

And not

db.rename_column('accounts_transaction', 'operator', 'responsible')

But I have only tested this on sqlite (which don't actually have the ALTER_TABLE at all), so I don't know if it will actually work on mysql/postgres.