Need a way to do schema migration on the database

[Christopher Allan Webber]

Over time our database format will change, and it'll be important to be able to update peoples' existing databases. We need both a way to specify migrations and to be able to run them.

http://namlook.github.com/mongokit/migration.html contains some info on this.

As far as where the revision info is stored, maybe that could be kept in something like

settings_document = db['app_settings']['mediagoblin']
settings_document['db_revision'] = 'some_identifier'

but I'm not sure. I've been thinking that maybe we'll need to store some user settings in the database eventually once we get some sort of admin interface around. Maybe this is the first thing to start storing.

[Juan José González]

We can use the approach of lazy migration on the article. In order to make work a different version in a specific database we can mark migration function with a number of version. If the software is that version or greater the function should be executed.

I think it could be done just writting the version of the software in some config file and some DocumentMigration models.

[Christopher Allan Webber]

We could store the current version of the migrations in a global variable in models.py maybe? But storing the user's current version of the software in a config file might be iffy, esp since most users run the mediagoblin.ini version in the toplevel of the application which for git's sake shouldn't change.

Thanks for jumping on this Juan, assigned to you!

[Christopher Allan Webber]

In working on http://bugs.foocorp.net/issues/340 I've implemented quite a bit of this. Yeah, using mongokit's migration stuff, which seems to work fine.

[Christopher Allan Webber]

So now we have migrations but not much documentation on how they work, but here's the basics:

• lazy migrations should happen automatically, but we have none currently

• bulk migrations have to be run manually, like so:

  ./bin/gmg migrate

So we should document this properly in the docs, both that in general when people do a git pull that they should do ./bin/gmg migrate to be sure but also how to write migrations (though I guess we can just reference the mongokit docs)

[Christopher Allan Webber]

Ok, updated the docs slightly telling people to migrate every time they run buildout. Pretty happy with this, closing this ticket!

[Will Kahn-Greene]

The original url for this bug was ​http://bugs.foocorp.net/issues/299 .