Use a replicaSet with Ruby Ree, Rails 2.3.4, Mongoid 1.9.2 and Mongo 1.0.9. Mongoid.database= Errors needs admin, so connect to admin then swap db after 2 seconds.

config_initializers_mongoid.rb

require 'mongoid'
# MongoDB ENV vars
  File.open(File.join(RAILS_ROOT, 'config/mongodb.yml'), 'r') do |f|
    @settings = YAML.load(f)[RAILS_ENV]
  end
  port = @settings["port"].nil? ? Mongo::Connection::DEFAULT_PORT : @settings["port"]

  host_arr = @settings["host"]
  connect = Mongo::Connection.multi([[host_arr[0], 27017], [host_arr[1], 27017], [host_arr[2], 27017]], :read_secondary => true)
  db = connect.db(@settings["database"])
  admin_db = connect.db("admin")
  admin_db.authenticate(@settings["username"], @settings["password"])
  db.authenticate(@settings["username"], @settings["password"])  
  Mongoid.database = admin_db
  puts "Authenticating mongo..."
  sleep 2
  Mongoid.database = db
  Mongoid.configure do |config|
    config.allow_dynamic_fields = true
  end
  
  

config_mongodb.yml

defaults: &defaults
  host: ["A", "B", "C"]
  username: 'user'
  password: 'iRuser'
  allow_dynamic_fields: true
  parameterize_keys: true
  persist_in_safe_mode: true
  raise_not_found_error: true
  reconnect_time: 3
  use_object_ids: false

development:
  <<: *defaults
  database: 'my_database'

test:
  <<: *defaults
  database: 'my_test_database'

production:
  host: ["A", "B", "C"]
  username: 'user'
  password: 'iRuser'
  port: 27017
  database: my_database
  allow_dynamic_fields: true
  parameterize_keys: true
  persist_in_safe_mode: true
  raise_not_found_error: true
  reconnect_time: 3
  use_object_ids: false

fake_activerecord.rb

class FakeActiverecord
  include Mongoid::Document
  field :fake_mongoid_id, :tyge => Integer, :default => 0

  # This is to fake activerecord association
  def fake_mongoid
    FakeMongoid.find(self.fake_mongoid_id)
  end
end

fake_mongoid.rb

class FakeMongoid < ActiveRecord::Base

  # This is to fake Mongoid association
  def fake_activerecords
    FakeActiverecord.where(:fake_mongoid_id => self.id.to_s)
  end
end