http://github.com/durran/mongoid/issues/#issue/203

gistfile1.txt

--- a/lib/mongoid/document.rb
+++ b/lib/mongoid/document.rb
@@ -11,6 +11,15 @@ module Mongoid #:nodoc:
       attr_reader :new_record
 
       delegate :primary_key, :to => "self.class"
+
+      unless self.instance_of?(Class) and self.name == ""
+        (@@descendants ||= {})[self] = :seen
+      end
+    end
+
+    # Returns the classes that have included Mongoid::Document
+    def self.descendants
+      (@@descendants ||= {}).keys
     end
 
     module ClassMethods #:nodoc:

merged. Thanks dude!