add selective processing for thumbanils

Closes #499

lib/paperclip/attachment.rb

@@ -11,6 +11,7 @@ module Paperclip
         :url                   => "/system/:attachment/:id/:style/:filename",
         :path                  => ":rails_root/public:url",
         :styles                => {},
+        :only_process          => [],
         :processors            => [:thumbnail],
         :convert_options       => {},
         :default_url           => "/:attachment/:style/missing.png",
@@ -41,6 +42,7 @@ module Paperclip
       @path                  = options[:path]
       @path                  = @path.call(self) if @path.is_a?(Proc)
       @styles                = options[:styles]
+      @only_process          = options[:only_process]
       @normalized_styles     = nil
       @default_url           = options[:default_url]
       @default_style         = options[:default_style]
@@ -107,7 +109,7 @@ module Paperclip
 
       @dirty = true
 
-      post_process if @post_processing
+      post_process(*@only_process) if @post_processing
 
       # Reset the file size if the original file was reprocessed.
       instance_write(:file_size,   @queued_for_write[:original].size.to_i)

test/attachment_test.rb

@@ -250,6 +250,24 @@ class AttachmentTest < Test::Unit::TestCase
     end
   end
 
+  context "An attachment with :only_process" do
+    setup do
+      rebuild_model :styles => {
+                      :thumb => "100x100",
+                      :large => "400x400"
+                    },
+                    :only_process => [:thumb]
+      @file = StringIO.new("...")
+      @attachment = Dummy.new.avatar
+    end
+
+    should "only process the provided style" do
+      @attachment.expects(:post_process).with(:thumb)
+      @attachment.expects(:post_process).with(:large).never
+      @attachment.assign(@file)
+    end
+  end
+
   context "An attachment with :convert_options that is a proc" do
     setup do
       rebuild_model :styles => {