feed.rb

class Feed < ActiveRecord::Base
  validates :uri, presence: true,
                  format: { with: /(^$)|([a-z0-9]+([\-\.]{1}[a-z0-9]+)*\.[a-z]{2,5}(([0-9]{1,5})?\/.*)?$)/ix }

  def load_feed
    @cache ||= Redis.new

    if @cache.exists("Feed:#{self.uri}")
      return Feedzirra::Feed.parse(@cache.get "Feed:#{self.uri}")
    else
      cache_feed
      return Feedzirra::Feed.parse(@cache.get "Feed:#{self.uri}")
    end

  end

  def cache_feed
    @cache.set "Feed:#{self.uri}", Feedzirra::Feed.fetch_raw(self.uri)
    @cache.expire "Feed:#{self.uri}", 1
    self.updated_at = Time.now
    self.save
  end
end

spec.rb

  it "should update if feed cache is older than 15 minutes" do
    Timecop.freeze
    feed = valid_feed.load_feed
    time_one = valid_feed.updated_at
    Timecop.travel(16.minutes)
    feed = valid_feed.load_feed
    time_two = valid_feed.updated_at
    time_one.should_not == time_two
  end