Userei l'aggregate
metodo con $project
operatore.
Dai documenti web di mongodb.
db.article.aggregate(
{ $project : {
title : 1 ,
page_views : "$pageViews" ,
bar : "$other.foo"
}} );`
es.
db.mycol.aggregate({ $project : { name:1, value:"$age" }});
vedere http://docs.mongodb.org/manual/reference/aggregation/# _S_progetto