Deprecated
This behavior is available in rails core as of commit e0750d6a5c7f621e4ca12205137c0b135cab444a
http://github.com/rails/rails/commit/e0750d6a5c7f621e4ca12205137c0b135cab444a

Allows creation of objects through associations by assigning hash to association

  class User < ActiveRecord::Base
    has_one :wristband
    has_many :favorites
    has_many :favorite_wristbands, :through => :favorites, :source => :wristband
  end

  class Wristband < ActiveRecord::Base
    belongs_to :user
  end

  class Favorite < ActiveRecord::Base
    belongs_to :user
    belongs_to :wristband
  end

  user = User.new(:wristband => { :color => 'orange' })

  user = User.new(:favorite_wristbands => [ { :color => 'pink' } ] )