database.mongo.yml

defaults: &defaults
  host: 127.0.0.1

development:
  <<: *defaults
  database: test_dev

test:
  <<: *defaults
  database: test_test

production:
  <<: *defaults
  database: test_prod

mongo.rb

File.open(File.join(Rails.root, 'config/database.mongo.yml'), 'r') do |f|
  @settings = YAML.load(f)[Rails.env]
end

Mongoid.configure do |config|
  name = @settings["database"]
  host = @settings["host"] || 'localhost'
  port = @settings["port"] || 27017
  config.master = Mongo::Connection.new(host,port).db(name)
end

if @settings["username"]
  Mongoid.database.authenticate(@settings["username"], @settings["password"])
end