composed_of,model,data object

http://hideto.javaeye.com/blog/77459
http://rorataspire.wordpress.com/2008/07/10/rails-composed_of/
http://api.rubyonrails.org/classes/ActiveRecord/Aggregations/ClassMethods.html#M001663
http://opensoul.org/2006/11/16/making-code-composed_of-code-more-useful

composed_of 感覺就是用來把妳一些 field 在弄成一個 object, 當然也就可以拿這個 value object  產生出新的 object, 也可以用在find 裡面當條件用

   1. class Person < ActiveRecord::Base
  2.   composed_of :address,
  3.               :class_name => "Address",
  4.               :mapping => [%w(address_street street),
  5.                            %w(address_city city),
  6.                            %w(address_state state),
  7.                            %w(address_country country)]
  8. 
  9. class Address
 10.   attr_accessor :street, :city, :state, :country
 11.   def initialize(street, city, state, country)
 12.     @street = street
 13.     @city = city
 14.     @state = state
 15.     @country = country
 16.   end
 17. end

person.address = Address.new(...)  
person.save
person.address.city = "LA"  
person.save
這不會更新person的city字段，因為composed_of()的值對象被認為是immutable的，而只有new一個Address對象才能保存


class UserData
 attr_accessor :name

 def initialize(name)
   @name = name
 end
end

class User < ActiveRecord::Base
  composed_of :user_data, :mapping =>%w(name name)   <= 其實是這樣 ["name", "name"]
end

現在就可以用這樣了
>> u = UserData.new('ilake')
=> #<UserData:0xb7197f28 @name="ilake">
>> User.find(:all, :conditions => {:user_data => u})
=> [#<User id: 5, name: "ilake"]