SucksRocks.rb

class RockScore
  NoScore = Class.new

  def self.for_term(term)
    positive = SearchEngine.count_results(%{"#{term} rocks"}).to_f
    negative = SearchEngine.count_results(%{"#{term} sucks"}).to_f
    score = 10 * positive / (positive + negative)
    score.nan? ? NoScore : score
  end
end

class ScoreCache
  def self.for_term(term)
    begin
      CachedScore.for_term(term)
    rescue CachedScore::NoScore
      score = RockScore.for_term(term)
      CachedScore.save_score(term, score)
      score
    end
  end
end


class CachedScore < ActiveRecord::Base
  attr_accessible :term, :score
  class NoScore < RuntimeError; end

  def self.for_term(term)
    cached_score = find_by_term(term) or raise NoScore
    result = cached_score.score
    result || RockScore::NoScore
  end

  def self.save_score(term, score)
    score = nil if score == RockScore::NoScore
    create!(:term => term, :score => score)
  end
end

The RockScore::NoScore sentinel object is inert and returned instead of nil. The same if checks are needed for it as a nil but you get feedback as to where our no score came from.

The controller calls ScoreCache which is really the public interface here. Sorry if that's not super clear. This example is stolen from DestroyAllSoftware but I've been using the heck out of it at work.