Friday, November 6, 2009

Django South Fork

I really like South (the database migration app for Django). I even wrote about it a little bit on Stack Overflow (#1590944). In short, South lets you create python-based migrations to change the database schema. The translation is that you can have a Django model, make a change to it, and then have south change the database schema for you. It's generally awesome.

But there's this one catch to it: It doesn't seem to be able to order migrations on a project-wide basis. This is a biggie for me.

Consider this scenario:
app1.model has a foreign key to app2.model. App2 is a 3rd party app and you decide to extend it. So you create app3.model which inherits from app2.model. Then, you update the foreign key on app1.model to point to app3.model. In order for the app1 migration to run successfully, the app3 migration that creates the table app3_model needs to have run first. How do you control this?

So I forked South. Yeah, bitbucket and all.

My fork works by creating a manifest file (in JSON format for easy human and VCS editing). Each time a 'startmigration' is created, a new entry is added to the manifest. The manifest entries can be re-ordered whenever without any real consequence. When a ' migrate' is run, if no app or migration number are given, the migrate command will default to running the migrations in the order of the manifest. If an app or a migration number are given, 'migrate' will run as it normally does.

I also added another command ' create_manifest' if, for whatever reason, you need to manually create a new manifest file.

For the most part my fork is a drop-in replacement. You can use it, gaining the manifest functionality, and not have to modify any of the way you work.

The only caveat I've found so far is in creating a new manifest file. When a few manifest is created, it generates its list from the applied manifests in the south_migrationhistory table. What it doesn't do is add manifests that are waiting to be applied to the new list. It's a limitation. At some point I'll probably develop a fix for it, but in the mean time you can add entries to the manifest file manually if they need to be applied (but have already been generated by 'startmigration').

Happy migrating.


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