How to fetch the "Describes Parts" of a Document in ThingWorx Mashup?

I have configured my ThingWorx to connect to Windchill using ptc-demo-windchill-thing. Now, I want to search for a document number which should fetch all the document attributes along with all "Describes Parts" from Windchill. I am able to fetch all attributes but not the "Describes Parts". Can some one help?