_address_form.html.haml

%ul
  =f.hidden_field :tag
  %li
    =f.label :line_1
    =f.text_field :line_1
  %li
    =f.label :line_2
    =f.text_field :line_2
  %li
    =f.label :city
    =f.text_field :city
  %li
    =f.label :county
    =f.text_field :county
  %li
    =f.label :postcode
    =f.text_field :postcode

_form.html.haml

-form_for @customer do |customer_form|
  %fieldset
    %legend User Fields
    =render :partial => 'users/new_form', :locals => {:f => customer_form}
  -customer_form.fields_for :account_address do |account_address_form|
    %fieldset
      %legend Address Fields
      =render :partial => 'addresses/form', :locals => {:f => account_address_form}
  - if @customer.is_business?
    %fieldset
      %legend Business Fields
      =render :partial => 'users/business_fields', :locals => {:f => customer_form}
  %fieldset
    %legend
    = customer_form.submit 'Register'

customers_controller.rb

class CustomersController < ApplicationController
  
  before_filter :new_customer, :only => [:new]
  before_filter :find_customer, :only => [:edit, :update]

  before_filter :already_logged_in, :only => [:new]
  
  def show
    @customer = Customer.find(params[:id])
  end
  
  def new
    # debugger
  end
  
  def create
    @customer = Customer.new(params[:customer])
    respond_to do |format|
      if @customer.save_without_validation
        format.html { redirect_to(edit_customer_path(@customer)) }
        format.xml  { render  :xml      => @customer,
                              :status   => :created,
                              :location => :customer
                    }
      else
        format.html { render  :action => :new }
        format.xml  { render  :xml    => @customer.errors,
                              :status => :unprocessable_entity
                    }
      end
    end
  end
  
  def edit
    # @customer                 = Customer.find(params[:id])
    # # @customer.account_address = Address.new(:tag => 'account-address')
  end
  
  def update
    debugger
    @customer.attributes  = params[:customer]
    # @customer.account_address.attributes  = params[:customer][:account_address_attributes]
    respond_to do |format|
      
      if @customer.save!
        flash[:notice] = 'Customer was successfully saved.'
        format.html { redirect_to(customer_path(@customer)) }
        format.xml  { head :ok }
      else
        format.html { render  :action => "edit" }
        format.xml  { render  :xml => @customer.errors,
                              :status => :unprocessable_entity }
      end
    end
  end
  
  private
    
    def already_logged_in
      if logged_in?
        flash[:notice] = "Already Logged In!"
        render :template => 'customers/new_when_logged_in'
      end
    end
    
    def new_customer
      @customer = Customer.new
      build_customer
    end
    
    def find_customer
      @customer = Customer.find(params[:id])
      build_customer
    end
    
    def build_customer
      @customer ||= Customer.new
      @customer.build_addresses                                  unless @customer.addresses
      @customer.build_account_address(:tag => 'account-address') unless @customer.account_address
    end
    
end