Welcome to Multiple Migration Paths

NOTE: this has only been tested with edge rails, post 2.0

Installation

Make sure you are edge rails, then:

  script/plugin install git://github.com/zilkey/multiple_migration_paths.git

You may also need to add it to the front of your plugins path like so:

  config.plugins = [ :multiple_migration_paths, :all ]

Summary

This allows you to store migrations in multiple directories, and they will all be run together, as if they were in the same directory. No changes have been made to any rails rake tasks, no additional tables are needed to migrate.

Take the following directory structure:

  db
  |-- migrate
  |   |-- 20080427101407_first.rb
  |   |-- 20080427101408_second.rb
  |   `-- 20080427101409_third.rb
  |-- schema.rb
  `-- your_plugin_migrations
      |-- 20080427101308_fourth.rb
      `-- 20080427101408_third.rb

You would add your custom directory by adding the following at the bottom or environment.rb or in an initializer or plugin:

  ActiveRecord::Migration.migration_paths << File.join(RAILS_ROOT,"db","your_plugin_migrations")

In this case, migrations would be run in the following order:

your/plugin_migrations/20080427101308_fourth.rb (it‘s the first chronologically, even though it‘s named "fourth")
migrate/20080427101407_first.rb
migrate/20080427101408_second.rb
migrate/20080427101409_third.rb <- note that this comes from the migrate directory because the migrate directory comes before the your_plugin_migrations directory

db/migrate is hard-coded to come before anything you add.

What happens if you add a migration that comes chronologically before the last one? Rails automatically picks it up for you - so no worries!

Notice how it takes a full path - so your migrations don‘t even need to be in your app (think migrations in gems)! Also, because you get to set the paths, you can have migrations from multiple gems/plugins all at the same time, and have any kind of directory structure - like:

  db
  |-- migrate
  |   |-- some_sub_folder
  |   |   `-- 20080427101409_third.rb
  |   `-- 20080427101308_fourth.rb

Important note on ordering

Rails will treat migrations in order (I believe) of the version - not in the order that they were applied to the database.

This means that if you add migration X, then add migration Y with a prior date, then migrate down and migrate up again, it will migrate down X then Y.

Also, if you add X, then a prior Y before you migrate, Y will migrate before X.

This is all standard rails behavior and would hold true for any migrations that were checked into version control at different times. I‘m just reiterating it here because there is a slightly higher risk of conflicts with this plugin. Backup your database before running them.

Rake Tasks

Since all kinds of mayhem can break loose with migrations, you may want to see what migrations are in the database. You can do so with:

  rake db:migrations:files
  rake db:migrations:paths

If I have time, I can make this a bit more human readable by presenting the dates, but that‘s for later…

Behind the scenes