Added new task for cleaning out invalid attachments.

lib/paperclip/attachment.rb

@@ -203,6 +203,7 @@ module Paperclip
       @styles.each do |name, args|
         begin
           dimensions, format = args
+          dimensions = dimensions.call(instance) if dimensions.respond_to? :call
           @queued_for_write[name] = Thumbnail.make(@queued_for_write[:original], 
                                                    dimensions,
                                                    format, 

tasks/paperclip_tasks.rake

@@ -14,19 +14,15 @@ def obtain_attachments
   end
 end
 
-namespace :paperclip do
-  desc "Regenerates thumbnails for a given CLASS (and optional ATTACHMENT)"
-  task :refresh => :environment do
+def for_all_attachments
   klass     = obtain_class
   names     = obtain_attachments
   instances = klass.find(:all)
 
-    puts "Regenerating thumbnails for #{instances.length} instances of #{klass.name}:"
   instances.each do |instance|
     names.each do |name|
       result = if instance.send("#{ name }?")
-          instance.send(name).reprocess!
-          instance.send(name).save
+                 yield(instance, name)
                else
                  true
                end
@@ -34,5 +30,26 @@ namespace :paperclip do
     end
   end
   puts " Done."
+end
+
+namespace :paperclip do
+  desc "Regenerates thumbnails for a given CLASS (and optional ATTACHMENT)"
+  task :refresh => :environment do
+    for_all_attachments do |instance, name|
+      instance.send(name).reprocess!
+      instance.send(name).save
+    end
+  end
+
+  desc "Cleans out invalid attachments. Useful after you've added new validations."
+  task :clean => :environment do
+    for_all_attachments do |instance, name|
+      if instance.valid?
+        true
+      else
+        instance.send("#{name}=", nil)
+        instance.save
+      end
+    end
   end
 end