_id not preserved when saving to different collection

This statement did not preserve the _id's of the original (new ones were created):

// move posts to the attic
var attic = connect("localhost:27017/nomic_attic");
db.Post.find(

).sort({_id:-1}).forEach( function(vouchPost) {
attic.VouchPost.save(vouchPost);
});

I had to do this workaround to get the _id to remain unchanged in the new collection:

// move posts to the attic
var attic = connect("localhost:27017/nomic_attic");
db.Post.find(

).sort({_id:-1}).forEach( function(vouchPost) {
var copy = {};
for (var key in vouchPost) {
if (vouchPost.hasOwnProperty(key))

}
attic.VouchPost.save(copy);
});