RUBY

rails has_many through source 1

class Pet < ActiveRecord::Base
  has_many :dogs
end

class Dog < ActiveRecord::Base
  belongs_to :pet
  has_many :breeds
end

class Dog::Breed < ActiveRecord::Base
  belongs_to :dog
end

rails has_many through source 2

class User
  has_many :subscriptions
  has_many :newsletters, :through => :subscriptions
end

class Newsletter
  has_many :subscriptions
  has_many :users, :through => :subscriptions
end

class Subscription
  belongs_to :newsletter
  belongs_to :user
end

class Newsletter
  has_many :subscriptions
  has_many :subscribers, :through => :subscriptions, :source => :user
end