insert - MongoDB copy object while changing parameters -

in mongo, what's best way copy record while changing property on it?

for example have user object , want copy while changing properties. (logically):

db.users.find({_id:"fred"}).foreach( function(r) { db.users.insert(r.copy("_id"="barney","age":25)) } ); 

of course there no "copy" function, that's i'm looking for: way copy document, while changing properties (of course key _id).

you don't have copy anything, example work fine:

db.users.     find( { _id: "fred" } ).     foreach( function (r) {          r._id = "barney";          r.age = 25;         db.so.insert( r );     } ); 

i don't quite understand why want though. makes more sense insert new document original fields again new "barney" document.