How to obtain the installable object from a polymorphic relationship

install_steps_controller.rb

class InstallStepsController < ApplicationController
  before_filter lambda {
    if params[:release_id]
      @installable = Release.includes(:product).find(params[:release_id])
    elsif params[:virtual_machine_id]
      @installable = VirtualMachine.find(params[:virtual_machine_id])
    else
      redirect_to root_url
    end
  }