mongodb get first element after find query

Question

I can't find the answer. I have a query that can return many answer and I want to get the last one. How to do it?

db.users.find({username: "bob", sort: {createdAt: -1}}).first()

I would like something like this.

Answer 1

do this to get last N records :

function last(N) {
    return db.collection.find().skip(db.collection.count() - N);
}