Create a presenter object on the fly in Rails

gistfile1.rb

# Inspired by Ryan Bates pro RailsCast "Presenters from Scratch"
#  This is a proof of concept for 'automagically' creating presenter objects
#
# This assumes you have presenter objects that follow the convention
#   Object => ObjectPresenter
#
# It intercepts the 'render' method in the controller, and does some
#  introspection to see if there is a presenter defined for the class
#  of the instance variable being looked at. If so, it makes a presenter
#  and adds it as an instance variable
#

class ApplicationController < ActionController::Base
  protected
  def render(options = nil, deprecated_status = nil, &block)
    self.instance_variables.each do |iv_sym|
      next if iv_sym[0] == "@" && iv_sym[1] == "_"

      instance_variable = self.instance_variable_get iv_sym  # value of @foo
      iv_class = instance_variable.class.to_s # Person

      presenter_class = "#{iv_class}Presenter"
      if Object.const_defined? presenter_class
        presenter = presenter_class.constantize.send(:new, instance_variable)
        # following defines the presenter variable name as class_presenter
        # you may want to use the instance variable name directly, so you can have
        # current_user_presenter and user_presenter, ie.
        self.instance_variable_set "@#{iv_class.tableize.singularize}_presenter".to_sym, presenter
      end
    end
    # call the ActionController::Base render to show the page
    super
  end
end