gistfile1.rb

require 'mongoid'

Mongoid.configure do |config|
  config.master = Mongo::Connection.new.db('testing')
end

class Product
  include Mongoid::Document
  field :name
  references_many :categories, :stored_as => :array
  collection.remove
end

class Category
  include Mongoid::Document
  field :name
  collection.remove
end

category = Category.create(:name => 'Books')
product = Product.create(:name => 'Enduring Love')
product.categories << category

product.attributes
# => {"name"=>"Enduring Love", "_id"=>"4c48659bf7e2da1388000002", "category_ids"=>["4c48659bf7e2da1388000001"]}
#
category.attributes
# => {"name"=>"Books", "_id"=>"4c48659bf7e2da1388000001"}