Handle interpolation of string IDs (supports mongo) as well as integers

lib/paperclip/interpolations.rb

@@ -143,7 +143,11 @@ module Paperclip
     # Returns the id of the instance in a split path form. e.g. returns
     # 000/001/234 for an id of 1234.
     def id_partition attachment, style_name
-      ("%09d" % attachment.instance.id).scan(/\d{3}/).join("/")
+      if (id = attachment.instance.id).is_a?(Integer)
+        ("%09d" % id).scan(/\d{3}/).join("/")
+      else
+        id.scan(/.{3}/).first(3).join("/")
+      end
     end
 
     # Returns the pluralized form of the attachment name. e.g.

test/interpolations_test.rb

@@ -88,13 +88,20 @@ class InterpolationsTest < Test::Unit::TestCase
     assert_equal 23, Paperclip::Interpolations.id(attachment, :style)
   end
 
-  should "return the partitioned id of the attachment" do
+  should "return the partitioned id of the attachment when the id is an integer" do
     attachment = mock
     attachment.expects(:id).returns(23)
     attachment.expects(:instance).returns(attachment)
     assert_equal "000/000/023", Paperclip::Interpolations.id_partition(attachment, :style)
   end
 
+  should "return the partitioned id of the attachment when the id is a string" do
+    attachment = mock
+    attachment.expects(:id).returns("32fnj23oio2f")
+    attachment.expects(:instance).returns(attachment)
+    assert_equal "32f/nj2/3oi", Paperclip::Interpolations.id_partition(attachment, :style)
+  end
+
   should "return the name of the attachment" do
     attachment = mock
     attachment.expects(:name).returns("file")