What is the best approach to change primary keys in an existing Django app?

Question

I have an application which is in BETA mode. The model of this app has some classes with an explicit primary_key. As a consequence Django use the fields and doesn\'t create

Answer 1

To change primary key with south you can use south.db.create_primary_key command in datamigration. To change your custom CharField pk to standard AutoField you should do:

1) create new field in your model

class MyModel(Model):
    id = models.AutoField(null=True)

1.1) if you have a foreign key in some other model to this model, create new fake fk field on these model too (use IntegerField, it will then be converted)

class MyRelatedModel(Model):
    fake_fk = models.IntegerField(null=True)

2) create automatic south migration and migrate:

./manage.py schemamigration --auto
./manage.py migrate

3) create new datamigration

./manage.py datamigration  fill_id

in tis datamigration fill these new id and fk fields with numbers (just enumerate them)

    for n, obj in enumerate(orm.MyModel.objects.all()):
        obj.id = n
        # update objects with foreign keys
        obj.myrelatedmodel_set.all().update(fake_fk = n)
        obj.save()

    db.delete_primary_key('my_app_mymodel')
    db.create_primary_key('my_app_mymodel', ['id'])

4) in your models set primary_key=True on your new pk field

id = models.AutoField(primary_key=True)

5) delete old primary key field (if it is not needed) create auto migration and migrate.

5.1) if you have foreign keys - delete old foreign key fields too (migrate)

6) Last step - restore fireign key relations. Create real fk field again, and delete your fake_fk field, create auto migration BUT DO NOT MIGRATE(!) - you need to modify created auto migration: instead of creating new fk and deleting fake_fk - rename column fake_fk

# in your models
class MyRelatedModel(Model):
    # delete fake_fk
    # fake_fk = models.InegerField(null=True)
    # create real fk
    mymodel = models.FoeignKey('MyModel', null=True)

# in migration
    def forwards(self, orm):
        # left this without change - create fk field
        db.add_column('my_app_myrelatedmodel', 'mymodel',
                  self.gf('django.db.models.fields.related.ForeignKey')(default=1, related_name='lots', to=orm['my_app.MyModel']),keep_default=False)

        # remove fk column and rename fake_fk
        db.delete_column('my_app_myrelatedmodel', 'mymodel_id')
        db.rename_column('my_app_myrelatedmodel', 'fake_fk', 'mymodel_id')

so previously filled fake_fk becomes a column, that contain actual relation data, and it does not get lost after all the steps above.