.. MediaGoblin Documentation
Written in 2011, 2012 by MediaGoblin contributors
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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 <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
- 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
- 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!