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

@@ -14,25 +14,42 @@ def obtain_attachments
   end
 end
 
+def for_all_attachments
+  klass     = obtain_class
+  names     = obtain_attachments
+  instances = klass.find(:all)
+
+  instances.each do |instance|
+    names.each do |name|
+      result = if instance.send("#{ name }?")
+                 yield(instance, name)
+               else
+                 true
+               end
+      print result ? "." : "x"; $stdout.flush
+    end
+  end
+  puts " Done."
+end
+
 namespace :paperclip do
   desc "Regenerates thumbnails for a given CLASS (and optional ATTACHMENT)"
   task :refresh => :environment do
-    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
-        else
-          true
-        end
-        print result ? "." : "x"; $stdout.flush
+    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
-    puts " Done."
   end
 end