How to update two records?

I have problem to update personaldata accessing with/from @user.
@debug @user’ shows that @user.personaldata attributes are properlly
modified, but ‘@user.save’ saves only table users but not personaldatas
Is a problem with my associations? How to save personaldata with
@user.save ?

class User < ActiveRecord::Base
has_one :personaldata
end

class Personaldata < ActiveRecord::Base
belongs_to :user
end

class UserController < ApplicationController
def update
@user = User.find_by_username(params[:id], :include =>
[:personaldata])
@user.attributes = params[:personaldata] # <- doesn’t work
#@user.personaldata.attributes = params[:personaldata] # <- this way
it doesn’to work too

if @user.save
flash[:notice] = ‘Updated’
redirect_to :action => ‘list’
else
render :action => ‘new’
end
end
end