Question

I’ve got an existing production Ruby/Rails app that I want to migrate to MongoDB over time, as time permits, because it’s not an option to just rewrite it all at one time. I’d love to be able to just deprecate my old classes as I get to them. I plan to use MongoMapper. I can’t find an example where anyone explains how to set up the database config files to allow connection to both data stores within one app.

FWIW, I’m using Rails 3. I appreciate the help.

Answer 1

Include your mongo_mapper gem in you Gemfile. Then in the models that you slowly want to start migrating over to MongoMapper, you just include this in your model:

include MongoMapper::Document

here is an example of a Mongo publisher model

class Publisher
  include MongoMapper::Document

  key :_id, String
  key :mtd_uniques, Integer
  key :mtd_demo_uniques, Integer
  key :archive, Array
  key :ignore, Boolean
end

My user model (postgres):

class User < ActiveRecord::Base
  validates_presence_of :first_name, :last_name, :email, :type
  acts_as_authentic

  def self.inherited(child)
    child.instance_eval do
      def model_name
        User.model_name
      end
    end
    super
  end
end

The nice thing about this is that all of your other models still use ActiveRecord so you can use 2 different databases till everything is migrated over to Mongo. This is an example from what I’m using. Large data aggregations using MongoMapper, and User model using postgres (app hosted on Heroku)

For my setup I dumped config stuff in my config.yml

development:
  adapter: MongoDB
  host: localhost
  database: my-dev-db

test:
  adapter: MongoDB
  host: localhost
  database: my-dev-db

staging:
  adapter: MongoDB
  host: remote-host (for me amazon ec2)
  database: my-staging-db

production:
  adapter: MongoDB
  host: remote-host (for me amazon ec2)
  database: my-production-db

and created an initializer that differentiates between the 2 DBs:

/initializers/database.rb

# load YAML and connect
database_yaml = YAML::load(File.read("#{Rails.root}/config/config.yml"))
puts "Initializing mongodb"
if database_yaml[Rails.env] && database_yaml[Rails.env]['adapter'] == 'MongoDB'
  mongo_database = database_yaml[Rails.env]
  MongoMapper.connection = Mongo::Connection.new(mongo_database['host'], 27017, :pool_size => 5, :timeout => 5)
  MongoMapper.database =  mongo_database['database']
end