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diff --git a/docs/source/devel/migrations.rst b/docs/source/devel/migrations.rst new file mode 100644 index 00000000..16c02b04 --- /dev/null +++ b/docs/source/devel/migrations.rst @@ -0,0 +1,62 @@ +.. MediaGoblin Documentation + + Written in 2011, 2012 by MediaGoblin contributors + + To the extent possible under law, the author(s) have dedicated all + copyright and related and neighboring rights to this software to + the public domain worldwide. This software is distributed without + any warranty. + + You should have received a copy of the CC0 Public Domain + Dedication along with this software. If not, see + <http://creativecommons.org/publicdomain/zero/1.0/>. + +========== +Migrations +========== + +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 + <http://code.google.com/p/sqlalchemy-migrate/>`_. + See `their docs <https://sqlalchemy-migrate.readthedocs.org/>`_. +- `Alembic <https://bitbucket.org/zzzeek/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! |