cache all photos in class variable

app/models/photo.rb

@@ -1,6 +1,7 @@
 class Photo
   attr_accessor :id, :thumbnail_url, :large_url, :index
   require 'open-uri'
+  @@allphotos = nil
 
   def initialize(attributes = {})
     attributes.each do |name, value|
@@ -9,6 +10,7 @@ def initialize(attributes = {})
   end
 
   def self.all
+    return @@allphotos if @@allphotos
     doc = Nokogiri::XML(open("http://api.flickr.com/services/rest/?method=flickr.photosets.getPhotos&api_key=c794c3f00be2130085e3cdef2f06aeb6&photoset_id=72157625847846478&extras=url_sq%2Curl_m%2Curl_l%2Curl_o"))
     photos = Array.new
     doc.css("photo").to_a.each_index do |index|
@@ -20,7 +22,7 @@ def self.all
         :index => index
       )
     end
-    photos
+    @@allphotos = photos
   end
 
   def self.find(id)