config/initializers/mongo.rb it setups mongomapper to be used locally and in heroku with MongoHQ

mongo.rb

MongoMapper.config = { 
  Rails.env => { 'uri' => ENV['MONGOHQ_URL'] || 
                          "mongodb://localhost/rubyconf_simple_app-#{Rails.env}-1" } }

MongoMapper.connect(Rails.env)

if defined?(PhusionPassenger)
   PhusionPassenger.on_event(:starting_worker_process) do |forked|
     MongoMapper.connection.connect if forked
   end
end