test_poly_mm.rb

require 'mongo'
require 'mongo_mapper'

MongoMapper.connection = Mongo::Connection.new('localhost', nil)

MongoMapper.database = 'sandbox_poly'

class Commentable
  include MongoMapper::Document
  many :comments, :as => :commentable
end

class Article < Commentable
  key :title
end

class Product < Commentable
  key :sku
end

class Comment
  include MongoMapper::Document
  key :text
  belongs_to :commentable
end