ActiveRecord有一個(gè)composed_of()方法用來(lái)聲明組件關(guān)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

這樣我們就聲明Person composed_of Address的關(guān)系，這將映射people表的address_street，address_city，address_state和address_country域到Address類，允許你將address作為first-class對(duì)象來(lái)操作，但是Address類沒(méi)有繼承ActiveRecord::Base，它只是用來(lái)結(jié)構(gòu)化數(shù)據(jù)和面向?qū)ο蟛僮鳌?
需要注意的是

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

這不會(huì)更新person的city字段，因?yàn)閏omposed_of()的值對(duì)象被認(rèn)為是immutable的，而只有new一個(gè)Address對(duì)象才能保存

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