ruby-on-rails – Rails委托更新调用

class User < ActiveRecord::Base
   has_one :profile   
   delegate :name,:name=,:to => :profile   
end

class Profile < ActiveRecord::Base
  belongs_to :user
end

铁轨

u = User.new 
u.build_profile         #=> init Profile
u.name = 'foo'
u.email = 'some@ema.il'
u.save                  #=> both User and Profile are saved

u.name = 'bar'
u.save                  #=> true,but changes in Profile were not saved!

u.email = 'new@ema.il'
u.save                  #=> true,new User email was saved,Profile - still not!

u.name                  #=> 'bar',but in database it's 'foo'

为什么未更新配置文件(仅首次保存)？如何解决这个问题？