mongo_relations.rb

# Lets you define relations between ActiveRecord and Mongo objects

module MongoRelations
  module InstanceMethods
    # get an object representing the MongoMapper equivalent of this object
    # for the purpose of access relations on the object
    fattr(:mongo_obj) do
      res = klass.mongo_class.new
      my_id = id
      res.class_eval do
        define_method(:id) do
          my_id
        end
      end
      res
    end
  end
  module ClassMethods
    # get an anonymous class representing the MongoMapper equivalent of this class
    fattr(:mongo_class) do
      Class.new do
        include MongoMapper::Document
      end
    end
    def mongo_has_many(name,ops={},&b)
      define_method(name) do
        mongo_obj.send(name)
      end
      mongo_class.send(:has_many, name, ops, &b)
    end
  end
  def self.included(mod)
    mod.send(:include,InstanceMethods)
    mod.extend(ClassMethods)
  end
end

class User < ActiveRecord::Base
  mongo_has_many :workspaces, :foreign_key => :user_ids
end

class Workspace
  include MongoMapper::Document
  key :user_ids
  key :name
end

User.first.workspaces.find('4bd0c170d489e6af74000130')