MongDB in Rails 3 using mongo_mapper

Using MongoMapper with Rails 3 is easier than ever. Thanks to new features in ActiveSupport, and the new ActiveModel framework (which MongoMapper 0.9+ uses), your app can be up and running on MongoDB in a matter of seconds.

First, if you’re generating a new Rails 3 application, it is recommended to leave out the ActiveRecord dependencies (unless you need them of course). From the console, just run:

rails new my_app --skip-active-record

But, not everyone is starting fresh. If you’re converting an existing Rails 3 application from ActiveRecord (or another ORM), simply openconfig/application.rb and replace

require 'rails/all'

with

require "action_controller/railtie"
require "action_mailer/railtie"
require "active_resource/railtie"
require "rails/test_unit/railtie"
# Uncomment for asset pipelining in Rails 3.1
# require "sprockets/railtie"

Next, add MongoMapper to your Gemfile, and run bundle install:

gem 'mongo_mapper'
gem 'bson_ext'

Now, you’re almost ready to go, but you still need some configuration info. Generate config/mongo.yml by running:

script/rails generate mongo_mapper:config

(Note: in versions of MongoMapper below 0.9+, you were required to implement any configuration files manually using an initializer. This is now resolved, and requires nothing more on your part.

If you want to configure your application with a MongoDB URI (i.e. on Heroku), then you can use the following settings for your production environment:

production:
 uri: <%= ENV['MONGODB_URI'] %>

Technically, you can initialize MongoMapper and use it to store data now. However, I like to configure Rails’ model generator. Inside of the Application class (config/application.rb) I add:

config.generators do |g|
  g.orm :mongo_mapper
end

This will allow you to use the rails generate model command with MongoMapper.

You’re now finished, go forth and create!