datamapper_example.rb

class User
  include DataMapper::Resource

  property :id, Serial
  property :name, String
  has n, :subscriptions
  has n, :feeds, :through => :subscriptions, :mutable => true
end

class Subscription
  include DataMapper::Resource

  property :id, Serial
  belongs_to :user
  belongs_to :feed
end

class Feed
  include DataMapper::Resource

  property :id, Serial
  property :uri, String
  property :name, String

  has n, :subscriptions
  has n, :users, :through => :subscriptions
end

user.subscriptions # => returns an Array of subscriptions
user.feeds         # => it's a shortcut for user.subscriptions.feeds
user.feeds << feed # => it adds the feed to the collection
user.save          # => *booom* it should save the user and the associated collections