pres_model.rb

class Car < ActiveRecord::Base
  attr_accessible :make, :car_model, :colour
  
  def lots_of_model_code
    # etc, etc  
  end
end

pres_presenter.rb

require 'delegate'

class CarPresenter < SimpleDelegator
  
  def description
    # we can take advantage of memoization to speed it up if repeated calls to methods :)
    @description ||= "This is a #{model.make} #{model.car_model} of #{colour_or_empty} colour"  
  end
  
  def colour_or_empty
    @colour ||= model.colour.blank? ? "unknown" : model.colour
  end
  
  # Returns ref to the object we're decorating
  #  a method to help readability.
  # If using this n>1 times then consider moving upto superclass
  def model
    __getobj__
  end
end

pres_tests.rb

require 'test_helper'

class CarPresenterTest < ActiveSupport::TestCase

    def setup
        @car = cars(:car_one) # a car with Ford, Focus and Red attributes

        @car_presenter = CarPresenter.new(@car)
    end
    
    # a non-exhaustive few tests for class
    
    test "should give description with correct attributes" do
        assert_equal "This is a Ford Focus of Red colour", @car_presenter.description
    end
    
    test "should give description when colour is empty" do
      @car.colour = nil # empty the colour attribute

      car_presenter = CarPresenter.new(@car)
      
      assert_equal "This is a Ford Focus of unknown colour", car_presenter.description
    end
end