Using Datamapper with existing rails application

Question

I have an existing Rails 3 application using ActiveRecord, and I want to switch to Datamapper. The instructions given in the dm-rails page only talk about creating a new application. Does anyone know how to throw away all activerecord dependancies and migrate to datamapper?

Thanks!

Annuity

just out of curiosity, why would you want to replace AR with DM? – Polymath

@jpartogi, probably because the application is built around an existing database schema. AR is a complete PITA if your schema isn't the way AR wants it. We're in the same boat. – Batwing

Answer 1

It's realtively straightforward, but there are a couple of things you need to do.

In your Gemfile, remove "rails" and instead require the following.

gem 'activesupport',      RAILS_VERSION, :require => 'active_support'
gem 'actionpack',         RAILS_VERSION, :require => 'action_pack'
gem 'actionmailer',       RAILS_VERSION, :require => 'action_mailer'
gem 'railties',           RAILS_VERSION, :require => 'rails'

Where RAILS_VERSION is the version of Rails you want to use (e.g. ~> 3.1). This is basically all of rails except ActiveRecord.

At the top of config/application.rb, remove the require for rails (I forget what the original require looks like) and replace it with specific requires for the railties you need.

require "action_controller/railtie"
require "action_mailer/railtie"

I think the only other one is a Test::Unit railtie, but we're not using Test::Unit, so we don't include it.

Finally, if you want to use the identity map (I suggest you do, but it's not needed), place in your ApplicationController's class body:

use Rails::DataMapper::Middleware::IdentityMap

That should be everything; the rest is just configuring your database.yml according to the README (it's pretty much cross-compatible with a standard rails one anyway).

For reference, take a look at what the generators does:

-zsh$  curl http://datamapper.org/templates/rails.rb
apply 'http://datamapper.org/templates/rails/gemfile.rb'
apply 'http://datamapper.org/templates/rails/application.rb'

If you look at the contents of those two files you'll see the extra stuff you'd get if you had used the generator.

Batwing

Thanks! Your instructions worked. The links to the file were invaluable! – Annuity