house_and_rooms.rb

class House < ActiveRecord::Base
  has_many :rooms
end

class Room < ActiveRecord::Base
  belongs_to :house
  default_scope :order => 'dimensions DESC'
  attr_accessor :color, :dimensions
end

###
h = House.new
h.rooms << Room.new(:dimensions => '50')
h.rooms << Room.new(:dimensions => '500')
h.save
h.rooms #=> I get [#<Room :dimensions => '50'>, #<Room :dimensions => '500'>]