[Rails] Masalah dengan 1 form yang menambah row di 2 tabel

Halo semua..saya lagi belajar Rails dan masalah dikit. Mohon bantuan
teman2.

Saya punya tabel users dan addresses. 1 user hanya punya 1 address.

di models/user.rb
class User < ActiveRecord::Base
has_one :address

validates_presence_of :username, :firstname, :passwd, :message => "can't
empty";
validates_format_of :email, :with =>
/\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i
validates_confirmation_of :passwd
enddi models/address.rb
class Address < ActiveRecord::Base
has_many :province
belongs_to :user
enddi views/user/new.rb (sebagian), form ini akan post ke user/create
...
<div>
<label for="user_gender">Gender</label>
<%= select :user, :gender, {"Male" => "m", "Female" => "f"} -%>
</div>
<div>
<label for="user_birthdate">Birth date</label>
<%= date_select :user, :birthdate, :order => [:day, :month, :year],
:include_blank => true -%>
</div>
<div>