Eu usaria o
aggregate
método com $project
operador. Dos documentos da web do mongodb.
db.article.aggregate(
{ $project : {
title : 1 ,
page_views : "$pageViews" ,
bar : "$other.foo"
}} );`
por exemplo.
db.mycol.aggregate({ $project : { name:1, value:"$age" }});
consulte http://docs.mongodb.org/manual/reference/aggregation/# _S_projeto