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
     

lib/couchrest/support/rails.rb

@@ -28,6 +28,7 @@ CouchRest::CastedModel.class_eval do
   # The to_param method is needed for rails to generate resourceful routes.
   # In your controller, remember that it's actually the id of the document.
   def id
+    return nil if base_doc.nil?
     base_doc.id
   end
   alias_method :to_param, :id

spec/couchrest/more/casted_model_spec.rb

@@ -326,6 +326,11 @@ describe CouchRest::CastedModel do
       @toy1.base_doc.title = 'Tom Foolery'
       @course.title.should == 'Tom Foolery'
     end
+    
+    it "should return nil if not yet casted" do
+      person = Person.new
+      person.base_doc.should == nil
+    end
   end
   
   describe "calling base_doc.save from a nested casted model" do