enable thumbnailing on S3

- added url to optimized image model
- refactored s3_store & local_store

db/migrate/20130728172550_add_url_to_optimized_images.rb

@@ -0,0 +1,22 @@
+class AddUrlToOptimizedImages < ActiveRecord::Migration
+  def up
+    # add a nullable url column
+    add_column :optimized_images, :url, :string
+    # compute the url for existing images
+    execute "UPDATE optimized_images
+             SET url = substring(u.url from '^\/uploads\/[^/]+\/')
+                    || '_optimized/'
+                    || substring(oi.sha1 for 3) || '/'
+                    || substring(oi.sha1 from 4 for 3) || '/'
+                    || substring(oi.sha1 from 7 for 11) || oi.extension
+            FROM optimized_images oi
+            JOIN uploads u ON u.id = oi.upload_id
+            WHERE optimized_images.id = oi.id;"
+    # change the column to be non nullable
+    change_column :optimized_images, :url, :string, null: false
+  end
+
+  def down
+    remove_column :optimized_images, :url
+  end
+end