new.html.haml

#new-user
  - form_for @user, :url => users_path do |f|
    .email
      %label{:for => "email"} email
      = f.text_field :email
      = f.error_message_on :email
    .password
      %label{:for => "password"} password
      = f.password_field :password
      = f.error_message_on :password
    .password-confirmation
      %label{:for => "password_confirmation"} confirm password
      = f.password_field :password_confirmation
      = f.error_message_on :password_confirmation
    .user-profile
      - f.fields_for @user.user_profile do |p|
        .gender
          %label{:for => "gender"} gender
          = p.select :gender, select_opts(:gender)
          = p.error_message_on :gender
        .year
          %label{:for => "year"} year
          = p.select :year, select_opts(:year_of_birth)
          = p.error_message_on :year
    %div
      = submit_tag 'join the urtak project'

user.rb

class User < ActiveRecord::Base
  # ...
  has_one   :user_profile
  # ...
end

users_controller.rb

class UsersController < ApplicationController
  
  before_filter :login_required, :except => [:new, :create]
  
  def new
    @user = User.new
    @user.build_user_profile
  end

  def create
    begin
      @user = User.new(params[:user])
      @user.build_user_profile(params[:user][:user_profile])
      
      
      @user.save!
      self.current_user = @user
      flash[:notice] = "Thanks for signing up!"
      redirect_to home_path
    rescue
      flash[:warning] = "Please fix the errors below."
      render :action => 'new'
    end
  end
end