Monday, June 16, 2008

Applying individual timestamp migrations

I posted in my previous entry (Timestamp based migrations in Rails 2.1) about the new UTC timestamp based migrations in Rails 2.1. After working with them a little more, I found how to apply or remove individual migrations. Simply run rake db:migrate:up VERSION=YYYYMMDDHHMMSS to apply a single specific migration, or rake db:migrate:down VERSION=YYYYMMDDHHMMSS to remove a specific migration.

However, I found a bug with db:migrate:down as noted at When you run this, the self.down function of the migration is called and the database is changed. However, the entry in schema_migrations for this migration is not removed. So if you run rake db:migrate later, this migration will not be applied because Rails thinks that the migration has already been applied. My work around for this is to manually delete the row in schema_migrations after I run db:migrate:down.

Wednesday, June 11, 2008

Timestamp based migrations in Rails 2.1

One really cool feature added to Rails 2.1 is UTC timestamp based migrations. Now, when you generate a new migration, instead of the migration number being sequential, it's a UTC timestamp. That way if you're working on different branches of code, you don't need to coordinate between the braches who gets what version number.

But the best part of this is how the migrations are tracked. Instead of just keeping a current database schema number like before, Rails keeps track of each migration that has been run. Then when you run rake db:migrate, Rails only applies the migrations that haven't been run, they don't have to be in order.

As an example, say I added a migration two days ago called add_new_column with script/generate migration. 20080610012942_add_new_column.rb gets generated. My co-worker John is working off of another code branch, and yesterday he added a new migration called change_accounts. 20080610225643_change_accounts.rb gets generated. Then today I add a new migration called delete_state. 20080612020139_delete_state.rb gets generated. I have two migrations, and John only has his one. When I run rake db:migrate, my two migrations get applied, and when John runs it his one migration gets applied. When we sync our code branches, we get each others migration files. Even though John's migration was created AFTER the first one that I created, when he runs migrate, both of my migrations will get added, and his migration that was already added will not get run again. And when I run migrate, only John's migration will get added, even though it was generated before the last migration that I ran.

The way Rails does this is that the first time you run a migration with Rails 2.1, a new table gets created called schema_migrations. An entry is added to this table for each migration that gets run. The old schema_info table is deleted. That way any time you run a migration, it checks each migration file against the database to see if it's been applied to your database yet.

This feature is already proving very useful for me. I no longer have to have migrations completely syncronized between myself and the other developer working on my project. We each work on our own branch, and we merge our migrations together when we're ready to merge all of our code.

A good Railscast video showing this is up at

New vacation pics on Flickr

Check out my Flickr page at I just posted pictures, mapped ofcourse, from my recent vacation to Spain!