some helper methods for handling guest user in Rails (with devise) ecommerce website

guest_user.rb

#
# User Stories:
#   To start shopping easily,
#   guest user wants to add items to shopping cart without signing up
#
#
# Code Explanation:
#   - a guest_user is created when new user arrives to your website, so that he could have his own shopping cart, and adding items to shopping cart.
#   - when the guest_user sign up, all items in the shopping cart should be transferred to the new signed up user account
#

module GuestUser
  extend ActiveSupport::Concern

  def current_or_guest_user
    if current_user
      if session[:guest_user_id]
        transfer_guest_user_records_to_logged_in_user
        guest_user.destroy
        session[:guest_user_id] = nil
      end
      current_user
    else
      guest_user
    end
  end

  def guest_user
    @cached_guest_user ||= User.find(session[:guest_user_id] ||= create_guest_user.id)
  rescue ActiveRecord::RecordNotFound # if session[:guest_user_id] invalid
     session[:guest_user_id] = nil
     guest_user
  end

  private

  def transfer_guest_user_records_to_logged_in_user
    if current_user.shopping_cart
      guest_user_shopping_cart = guest_user.shopping_cart
      guest_user_shopping_cart.shopping_cart_items.each do |shopping_cart_item|
        shopping_cart_item.shopping_cart_id = current_user.shopping_cart.id
        shopping_cart_item.save!
      end
      guest_user_shopping_cart.destroy
    else      
      shopping_cart = guest_user.shopping_cart
      shopping_cart.user_id = current_user.id
      shopping_cart.save!
    end
    
    if guest_user.favourites.any?
      guest_user.favourites.each do |favourite|
        favourite.user_id = current_user.id
        favourite.save!              
      end
    end
  end

  def create_guest_user
    u = User.new(:email => "guest_#{Time.now.to_i}#{rand(99)}@example.com")
    u.skip_confirmation!
    u.save!(:validate => false)
    session[:guest_user_id] = u.id
    u
  end
end