new.html.slim

= simple_form_for @sign_up, url: sign_up_path, html: { class: "form-horizontal" } do |form|
  = form.simple_fields_for @sign_up.customer do |fields|
    = fields.input :first_name
    = fields.input :last_name, label: false
  = form.simple_fields_for @sign_up.user do |fields|
    = fields.input :email
    = fields.input :password
  = form.simple_fields_for @sign_up.customer do |fields|
    = fields.input :zip_code
  .form-actions
    = form.button :submit, class: "btn btn-primary"

sign_up.rb

class SignUp
  include ActiveModel::Model

  def customer
    @customer ||= Customer.new
  end

  def customer=(params = {})
    @customer = Customer.new(params)
  end

  def save
    user.with_transaction_returning_status do
      user.save
      customer.user_id = user.id
      customer.save

      @user.persisted? && @customer.persisted?
    end
  end

  def save!
    user.save!
    customer.user_id = user.id
    customer.save!
  end

  def user
    @user ||= User.new
  end

  def user=(params = {})
    @user = User.new(params)
  end

end

sign_ups_controller.rb

class SignUpsController < ApplicationController
  respond_to :html

  def new
    @sign_up = SignUp.new
    respond_with @sign_up
  end

  def create
    @sign_up = SignUp.new(sign_up_params)

    if @sign_up.save
      flash[:notice] = t("flash.sign_up.create.notice")
      respond_with @sign_up, location: root_url
    else
      flash.now[:alert] = t("flash.sign_up.create.alert")
      render action: "new"
    end
  end

private

  def sign_up_params
    params.require(:sign_up).permit(
      customer: [
        :first_name,
        :last_name,
        :zip_code ],
      user: [
        :email,
        :password ] )
  end

end