How to add one to one relation field in a existing model through South migration

Question

I already have a model,

class ModelA( models.Model ):
    name = models.CharField ( max_length = 255, blank = False )

and i have many entries in it. Now i want to add a field in it, which is

user = models.OneToOneField( User )

How do i add this field into ModelA? Is there any solution other than deleting all previous entries?

Answer 1

I would use this pattern:

1. Add "user = models.OneToOneField(User, null=True)" to your Model (don't remove the "name" field)
2. run 'manage.py schemamigration --auto'. And apply the migration. Now there are two columns in your table.
3. Now create a datamigration. Edit the file: you need to loop over all objects in your model and set the user field.
4. Remove the "name=models.CharField" from the model.py file. And remove the null=True from the user field.
5. run 'manage.py schemamigration --auto'. And apply the migration

BTW, if you use OneToOneField() without null=True, you can set primary_key=True on this field, since it must be unique. But I don't know if south can handle this migration.