In South, can I copy the value of an old column to a new one?

One of my Django models is a subclass and I want to change its superclass to one that is very similar 开发者_Go百科to the original one. In particular, the new superclass describes the same object and has the same primary key. How can I make South create the new OneToOne field and copy the values from the old one to the new one?

In south, there are two kinds of migrations: schema migrations and data migrations.

After you've created the schemamigration, create a corresponding data migration:

./manage.py datamigration <app> <migration_name>

Do not run the migration (yet). Instead, open up the migration file you just created.

You'll find the method named forwards(). Into this you define the procedure by which values from old tables get copied to new tables.

If you're changing the structure of a given table to a more complex layout, a common method is to have two schema migrations around a data migration: the first schema migration adds fields, the data migration translates the old fields to the new fields, and the second schema migration deletes the old fields. You can do just about anything with the database with the forwards() method, so long as you keep track of which schema (previous or current) you're accessing. Generally, you only read from the orm.-related, and write to the traditional Django accessors.

The South Data Migration Tutorial covers this in some detail. It shows you how to use South's orm reference to access the database using the schema prior to the schema migration and gives access to the database without Django complaining about fields it doesn't understand.

If you're renaming a class, that can be tricky-- it involves creating the new table, migrating from one to the other, and deleting the old table. South can do it, but it might take more than one pass through shifting schemas and data migrations.

South also has the backwards() method, which allows you to return your database tables to a previous step. In some cases, this may be impossible; the new table may record information that will be lost in a downgrade. I recommend using throwing an exception in backwards() if you're not in DEBUG mode.