gistfile1.rb

module ModelPresentable
  class ModelPresentableError < StandardError; end

  def self.included(base)
    base.extend(ClassScope)
    base.class_eval { include InstanceScope }
  end

  module InstanceScope
    delegate :to_param, :to_key, :new_record?, :to => :@presentable

    def presents_model(presentable)
      @presentable = presentable
      self.class.presentable_class = @presentable.class
    end
  end

  module ClassScope
    delegate :model_name, :to => :presentable_class

    def presentable_class=(presentable_class)
      @presentable_class = presentable_class
    end

    def presentable_class
      if @presentable_class.nil?
        raise ModelPresentableError, "#{self} does not call presents_model."
      end
      @presentable_class
    end
  end
end

# Example usage:

class MyPresenter
  include ModelPresentable

  def initialize(model_instance)
    presents_model model_instance
  end
end