South, how to migrate from CharField to ForeignKey?

Question

The model:

class ListContext(models.Model):
    content = models.CharField(max_length=200, blank=True)

I use south to manage schema migrations.

Now I change the previous model to this one:

from django.contrib.contenttypes.models import ContentType

class ListContext(models.Model):
    content = models.ForeignKey(ContentType, blank=True, null=True)

For the migration:

$ python manage.py schemamigration --auto page 
 ~ Changed field content on page.ListContext
 + Added index for ['content'] on page.ListContext
Created 0010_auto__chg_field_listcontext_content.py. You can now apply this migration with: ./manage.py migrate page

Everything is fine until this point:

$ python manage.py migrate page 
Running migrations for page:
 - Migrating forwards to 0010_auto__chg_field_listcontext_content.
 > page:0010_auto__chg_field_listcontext_content
FATAL ERROR - The following SQL query failed: ALTER TABLE "page_page_listcontext" ALTER
COLUMN "content_id" TYPE integer, ALTER COLUMN "content_id" DROP NOT NULL, ALTER COLUMN
"content_id" DROP DEFAULT;

The error was: column "content_id" cannot be cast to type integer

Error in migration: page:0010_auto__chg_field_listcontext_content

I can guess the error happens during the cast from string to int, but how can I avoid this and get the migration done?

Could it make any difference, I don't care to preserve the data stored in the table.

Answer 1

If you manually edit the forwards function:

Rename the column:

db.rename_column('sometable', 'content', 'content_old')

Then add your column back:

db.add_column('sometable', 'content', self.gf('django.db.models.fields.IntegerField')(default=0))

Then execute a query that updates the new field by looking up the id.

db.execute("""
  UPDATE sometable SET content =
  (SELECT FKTable.id FROM FKTable
  WHERE (FKTable.content = sometable.content_old AND
  sometable.content_old != '')
  OR (FKTable.content = 'none' AND sometable.content_old = '')) --Maybe cut the OR out
""")

You would then have to do some fancy stuff to make backwards work properly.