mongodb update field a to be value of field b - mongodb

Question

I am trying to run a mongo query to update the value of one field with the value of another field. I have the following documents:
{ "_id" : ObjectId("56e0a3a2d59feaa43fba49d5"), "old" : 16, "new" : 17 }
{ "_id" : ObjectId("56e0a3a2d59feaa43fba49d3"), "old" : 11, "new" : 12 }
I would like to make it look like this after update:
{ "_id" : ObjectId("56e0a3a2d59feaa43fba49d5"), "old" : 16, "new" : 16 }
{ "_id" : ObjectId("56e0a3a2d59feaa43fba49d3"), "old" : 11, "new" : 11 }
I've tried the following with no luck
db.runCommand(
{
findAndModify: "testData",
query: { $where: "this.new != this.old" },
update: { old : this.new },
upsert: true
}
)
and
db.testData.update( { $where: "this.new != this.old" }, { $set: { old: this.new } } );
Is this even possible with mongoDb?
I would like to do it in a single query and not iterate through each document.
Any ideas would be greatly appreciated.
Thank you

Answer 1

You can try something like this and probably change _id when you want to update other document:
db.testData.find().forEach(function(elem) {
db.testData.update({
"_id": "56e0a3a2d59feaa43fba49d5"
}, {
$set: {
new: elem.old
}
});
});

Answer 2

You can't do that in MongoDB yet (note to visitors from the future: I'm referring to V3.2).
You have to iterate on the documents.
NB there's a trick in case you don't mind deleting the old field: use rename to rename old to new.
NB2 for some SQLike-fu actions (not this one) the aggregation framework can be useful.