Based on the original post (http://robots.thoughtbot.com/code-show-and-tell-polymorphic-finder), I think this would have been a simpler refactor.

application_controller.rb

class ApplicationController < ActionController::Base
  private
  def requested_purchaseable
    RequestPurchase.new(params).find
  end
end

request_purchase.rb

RequestPurchase = Struct.new(:params) do
  def find
    product || individual_plan || team_plan || section || raise_unknown
  end

  private
  def product
    if id = params.slice(:product_id, :screencast_id, :book_id, :show_id).compact.first
      Product.find(id)
    end
  end

  def individual_plan
    if id = params[:individual_plan_id]
      IndividualPlan.where(sku: id).first!
    end
  end

  def team_plan
    if id = params[:team_plan_id]
      TeamPlan.where(sku: id).first!
    end
  end

  def section
    if id = params[:section_id]
      Section.find(id)
    end
  end

  def raise_unknown
    raise "Could not find a purchaseable object from given params: #{params}"
  end
end

In what directory would you normally store request_purchase.rb?