How to get devise to redirect to a specific page after user sign up

registrations_controller.rb

class Users::RegistrationsController < Devise::RegistrationsController

  # Use this if you are sending a confirmation email before completing sign up
  def after_inactive_sign_up_path_for(resource)
    # resource is an instance of User
    "http://www.example.com?user_id=#{resource.id}"
  end

  # Use this one once the account is active
  def after_sign_up_path_for(resource)
    # resource is an instance of User
    "http://www.example.com?user_id=#{resource.id}"
  end
end

routes.rb

# We need to have our own custom controller for registrations
devise_for :users, :controllers => { :registrations => "users/registrations" }