to_xml and to_json behaviour with nil values

Hi,

i was wondering if there is any way to just skip attributes having nil values when to_xml or to_json on ActiveRecord. The default behavior is to include a nil value: xml: <identity-url nil="true"/> json: "identity_url": null

Is there an option way to make this value just not appear ? Or should it be a hack in the serialization itself ? Thank you,

Elise