Make this yet dramatically faster when a lot of the models don't have attachments

lib/paperclip.rb

@@ -138,8 +138,8 @@ module Paperclip
     def each_instance_with_attachment(klass, name)
       unscope_method = class_for(klass).respond_to?(:unscoped) ? :unscoped : :with_exclusive_scope
       class_for(klass).send(unscope_method) do
-        class_for(klass).find_each do |instance|
-          yield(instance) if instance.send(:"#{name}?")
+        class_for(klass).find_each(:conditions => "#{name}_file_name is not null") do |instance|
+          yield(instance)
         end
       end
     end