So, about migrations. Every time we change the way the database structure works, we need to add a migration so that people running older codebases can have their databases updated to the new structure when they run ./bin/gmg dbupdate.

The first time ./bin/gmg dbupdate is run by a user, it creates the tables at the current state that they’re defined in models.py and sets the migration number to the current migration... after all, migrations only exist to get things to the current state of the db. After that, every migration is run with dbupdate.

There’s a few things you need to know:

  • We use sqlalchemy-migrate. See their docs.
  • Alembic might be a better choice than sqlalchemy-migrate now or in the future, but we originally decided not to use it because it didn’t have sqlite support. It’s not clear if that’s changed.
  • SQLAlchemy has two parts to it, the ORM and the “core” interface. We DO NOT use the ORM when running migrations. Think about it: the ORM is set up with an expectation that the models already reflect a certain pattern. But if a person is moving from their old patern and are running tools to get to the current pattern, of course their current database structure doesn’t match the state of the ORM!
  • How to write migrations? Maybe there will be a tutorial here in the future... in the meanwhile, look at existing migrations in mediagoblin/db/migrations.py and look in mediagoblin/tests/test_sql_migrations.py for examples.
  • Common pattern: use inspect_table to get the current state of the table before we run alterations on it.
  • Make sure you set the RegisterMigration to be the next migration in order.
  • What happens if you’re adding a totally new table? In this case, you should copy the table in entirety as it exists into migrations.py then create the tables based off of that... see add_collection_tables. This is easier than reproducing the SQL by hand.
  • If you’re writing a feature branch, you don’t need to keep adding migrations every time you change things around if your database structure is in flux. Just alter your migrations so that they’re correct for the merge into master.

That’s it for now! Good luck!

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