Simple callback-esque way to calculate in the background, and have the result later on by storing a reference to it in the session

calculate_the_thing.rb

class CalculateTheThing
  include Celluloid

  def calculate
    sleep 5
    result = rand
    yield(result) if block_given?
    result
  end
end

calculation.rb

# id, result
class Calculation < ActiveRecord::Base
  attr_accessible :result
end

pages_controller.rb

class PagesController < ApplicationController

  def dashboard
    if session[:calculation_id].blank?
      Calculation.create.tap do |calc|
        CalculateTheThing.new.async.calculate do |result|
          Calculation.find(calc.id).update_attributes(results: result)
        end
        session[:calculation_id] = calc.id
      end
    end
    calculation = Calculation.find(session[:calculation_id])
    @counter = calculation.result
  end

end