# app/models/artist.rb
class Artist < ActiveRecord::Base
  # Relationships
  has_many  :releases
  has_many  :songs, :through => :releases
  has_many  :featured_songs,  :through => :releases,
                              :class_name => "Song",
                              :source => :song,
                              :conditions => { 'releases.featured', true }

end

ruby-1.9.2-p180 :004 > a = Artist.first
ruby-1.9.2-p180 :005 > a.featured_songs.create(:title => "Title", :user => User.first)

ruby-1.9.2-p180 :004 > a = Artist.first
ruby-1.9.2-p180 :005 > a.featured_songs.create(:title => "Title", :user => User.first)
  User Load (0.9ms)  SELECT `users`.* FROM `users` LIMIT 1
  SQL (1.0ms)  BEGIN
  SQL (5.5ms)  INSERT INTO `songs` (`created_at`, `title`, `updated_at`, `user_id`) VALUES (?, ?, ?, ?)  [["created_at", Thu, 11 Aug 2011 18:30:34 UTC +00:00], ["title", "Title"], ["updated_at", Thu, 11 Aug 2011 18:30:34 UTC +00:00], ["user_id", 1]]
  SQL (1.2ms)  INSERT INTO `releases` (`album_id`, `artist_id`, `created_at`, `featured`, `song_id`, `updated_at`) VALUES (?, ?, ?, ?, ?, ?)  [["album_id", nil], ["artist_id", 1], ["created_at", Thu, 11 Aug 2011 18:30:34 UTC +00:00], ["featured", nil], ["song_id", 6], ["updated_at", Thu, 11 Aug 2011 18:30:34 UTC +00:00]]
   (0.1ms)  COMMIT

class Song < ActiveRecord::Base
  has_many :releases
  has_many :artists, :through => :releases
  accepts_nested_attributes_for :releases
end

a.songs.create(:title => 'Title', :user => User.first, :releases_attributes => [{ :id => 123, :featured => true}])

class User < ActiveRecord::Base
  belongs_to :address

  accepts_nested_attributes_for :address
end

class Address < ActiveRecord::Base
  has_one :user
end

u = User.new
u.address.build({:city => "bozoville", :zip => "22222", ...})
u.save!