I keep wishing I could have a computed field:

Basic idea:

db.people.find( { firstname : "John" }, { _id: 1, name : function() {return this.firstname + ' ' + this.lastname;} } )

Where it gets interesting:

db.people.find( { score: {'$gt': 10} }, { _id: 1, score: function() {return this.foo + this.bar + this.baz;} } ).sort(score: 5)

Obviously if you know in advance that you're going to need to run a query like this frequently it makes sense to try to precompute the value, but sometimes a query is so rare that precomputing is a waste, and in some cases you don't know enough information in advance.