How to validate uniqueness of embedded objects

embedded_uniqueness_validations.rb

require 'pp'
require 'rubygems'
require 'mongo_mapper'

MongoMapper.database = 'testing'

class Rating
  include MongoMapper::EmbeddedDocument
  
  key :user_id, ObjectId
  key :score, Integer
  
  validate :uniqueness_of_user_id
  
  def widget
    _root_document
  end
  
  private
    def uniqueness_of_user_id
      if widget.ratings.select { |r| r.user_id == user_id }.size > 1
        errors.add(:user_id, 'has already rated this widget')
      end
    end
end

class Widget
  include MongoMapper::Document
  key :name, String
  key :user_id, ObjectId
  many :ratings
  
  validates_associated :ratings
end
Widget.collection.remove # clear collection before each run

rating = Rating.new(:user_id => Mongo::ObjectID.new)

widget = Widget.new(:ratings => [rating, rating.clone])
puts widget.valid? # false

widget2 = Widget.new(:ratings => [rating, Rating.new(:user_id => Mongo::ObjectID.new)])
puts widget2.valid? # true