Results in a pretty significant performance improvement.
Many thanks to Matt Taylor for noticing the unusual glut of calls
to ObjectId#to_s. See here: http://unhalting.com/?p=18
This allows exact definition of the fields you want included in the query results. For example :fields => {:name => 1, :people => 0} will include the name key but exclude the people key.
Kyle Banker
Houdini
Cyril Mougel