Rails Migrations

Recently, I’ve been playing around with Rails and one of the things that has really impressed me is the support for migrations. If you haven’t come across them before, it’s a well defined way of doing agile development with your databases. Your database contains a table schema_info with a column version. And then you define an ordered series of scripts which each contain up and down methods to migrate between versions.

If that sounds complicated, check out the example from the documentation:

  class AddSsl < ActiveRecord::Migration
    def self.up
      add_column :accounts, :ssl_enabled, :boolean, :default => 1
    end

    def self.down
      remove_column :accounts, :ssl_enabled
    end
  end

You’ll notice that we’re not doing any SQL here. That means that as a side effect of falling into line with Rails, you get a level of database portability for free. Marvellous!

All this was prompted by looking at the recipe on migrations in the new Rails Recipes book.

This is not to say that migrations are a panacea, however. As always, by trading SQL for a domain language, you lose access to some features. Foreign key relationships aren’t easy to express in this manner, for instance. But that doesn’t mean they can’t be used—for an example see robby’s post on Rails Migrations and PostgreSQL Constraints. It just ties you to a particular platform. Which is probably no big deal for most people, as they’re likely only using one database anyway.

So far, I’ve had two major tripups with migrations.

  1. The SQLite adaptor doesn’t appear to support some things properly. I think it managed to nuke the contents of a column I was playing with. But I need to look further at this.
  2. Purely my own fault. I had set up a load of migrations in my development environment and I forgot to transfer them to the test environment. I got very confused for a while until I remembered to do that… Again, it’s my own fault because I was running the individual test files by hand, instead of using rake.

But still, compared to everything else I’ve seen, migrations in rails are superb. Particularly after going through similar pains at work in the last week! Rails once again hits the sweet spot, providing just enough framework to get things done easily, without getting in the way too much.

P.S. If you are playing with migrations, I recommend installing Jamis Buck’s verbose_migrations plugin. It gives you a little more feedback on what’s going on. Normally, I like the Unix way of “no feedback until it goes wrong”, but whilst I’m still learning to trust migrations, this plugin is helpful.

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