Create a presenter object on the fly in Rails

application_controller.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
  alias_method_chain :render, :presenters
  def render_with_presenters(options = nil, deprecated_status = nil, &block)
    self.instance_variables.reject {|iv_sym| iv_sym[1] == "_" }.each do |iv_sym|
      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
        self.instance_variable_set("@#{iv_sym}_presenter".to_sym, presenter_class.constantize.send(:new, instance_variable))
      end
    end
    # call the ActionController::Base render to show the page
    render
  end
end

base.rb

class ActionPresenter::Base                                                                                                                                        
  def self.inherited(base)                                                                                                                                         
    @children << base                                                                                                                                              
  end                                                                                                                                                              

  def self.presents(*args)                                                                                                                               
  end                                                                                                                                                              
end                                                                                                                                                                

foo_presenter.rb

class FooPresenter < ActionPresenter::Base                                                                                                                         
  presents :singular_class_name # SingularClassName class
  # presents AClassObject
  # presents lambda {|object| true } # Returns true if can present object.
  # presents [:alist, :of, AnyOne, lambda {|object| true }]                                                                                                                             
end                                                                                                                                                                

ActionPresenter::Base.presenters_for(object)