ActiveRecord有一个composed_of()方法用来声明组件关系，如:

class Person < ActiveRecord::Base
  composed_of :address,
              :class_name => "Address",
              :mapping => [%w(address_street street),
                           %w(address_city city),
                           %w(address_state state),
                           %w(address_country country)]

class Address
  attr_accessor :street, :city, :state, :country
  def initialize(street, city, state, country)
    @street = street
    @city = city
    @state = state
    @country = country
  end
end

这样我们就声明Person composed_of Address的关系，这将映射people表的address_street，address_city，address_state和address_country域到Address类，允许你将address作为first-class对象来操作，但是Address类没有继承ActiveRecord::Base，它只是用来结构化数据和面向对象操作。
需要注意的是

person.address.city = "LA"
person.save

这不会更新person的city字段，因为composed_of()的值对象被认为是immutable的，而只有new一个Address对象才能保存

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