Why to_xml on ActiveResource does not work well?

I have several ActiveResource model in my project. There was so
strange to me when I called to_xml on my ActiveResource. The options
that I passed to to_xml like :only and :except doesn't work at all. On
ActiveRecord, it works really well. Anyone knows?

class Node < ActiveResource::Base
   self.site = NODE_SERVER

# node has uuid, name, type attributes
node = Node.find("3333")
node.to_xml(:only => [:uuid])

# after here, i still get all attributes
<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<node>\n <uuid type=
\"integer\">0</uuid>\n <name>AAAA</name>\n <type></type>\n</node>