This shows A User/Idea/Rating model using MongoMapper

idea.rb

class Idea
  include MongoMapper::Document
  key :brilliance, String, :default => "Dark Matter Sucks!"
  many :ratings
  belongs_to :user
  key :user_id, ObjectId

  def to_s
    text = "#{user.name} had this IDEA: #{brilliance}\n\tRATINGS:\n"
    ratings.each do |r|
      text += "\t#{r.to_s}"
      text += " [VAIN?]" if user == r.user
      text += "\n"
    end unless ratings.empty?
    text
  end
end

rating.rb

class Rating
  include MongoMapper::Document
  key :value, Integer, :default => 5

  belongs_to :user
  key :user_id, ObjectId
  
  belongs_to :idea
  key :idea_id, ObjectId

  def to_s
    "#{value} by #{user.name}"
  end
end

results

Jon had this IDEA: Ducks walk like ducks. Duh
	RATINGS:
	2 by Fred
	6 by Sally

Jon had this IDEA: Ducks talk like ducks. Duh
	RATINGS:
	3 by Fred
	1 by Sally
	1 by Jon [VAIN?]

scratchpad.rb

require File.expand_path(File.join(File.dirname(__FILE__),'..','config','environment'))

require 'rubygems'
require 'mongo'

connection = Mongo::Connection.new()
db = connection.db("mm-#{Rails.env}")
print "Connected to #{db.name}\n"

db.collection("User").drop # doesn't work????'
User.all.each {|r| r.destroy}
Idea.all.each {|r| r.destroy}
Rating.all.each {|r| r.destroy}

jon = User.create(:name => 'Jon')
fred = User.create(:name => 'Fred')
sally = User.create(:name => 'Sally')

jon_idea1 = Idea.create(:brilliance => "Ducks walk like ducks. Duh", :user => jon)
jon_idea2 = Idea.create(:brilliance => "Ducks talk like ducks. Duh", :user => jon)

rt1 = Rating.create( :idea => jon_idea1, :user => fred, :value => 2)
rt2 = Rating.create( :idea => jon_idea1, :user => sally, :value => 6 )

rt1 = Rating.create( :idea => jon_idea2, :user => fred, :value => 3)
rt2 = Rating.create( :idea => jon_idea2, :user => sally, :value => 1 )
rt2 = Rating.create( :idea => jon_idea2, :user => jon, :value => 1 )

Idea.all.each { |s| puts "#{s.to_s}\n" }

user.rb

class User
  include MongoMapper::Document
  key :name, String, :default => "Albert"

  many :ratings
  many :ideas

  def to_s
    name
  end
end

i ran the above from a rails project where the only changes needed were:

1. add 'this to environment.rb
   config.gem "mongo_mapper"
2. and create a simple one line mongo_config.rb under intializers
   MongoMapper.database = "todo-#{Rails.env}"
   Then my "scratchpad.rb" was under the test directory and it executed.