base_doc should be nil for unassociated casted models

lib/couchrest/more/casted_model.rb

@@ -30,7 +30,7 @@ module CouchRest
     # Gets a reference to the top level extended
     # document that a model is saved inside of
     def base_doc
-      raise "Cannot call base_doc on a model that is not yet casted by a document" unless @casted_by
+      return nil unless @casted_by
       @casted_by.base_doc
     end