MongoDB: Capped collection findAndModify
===

I'm trying to atomically insert an empty document if the capped collection is empty or return the last naturally sorted document if not empty.  Can I do this with findAndModify?

    db.collection.findAndModify({
        query: { _id: { $exists: true }},
        sort: { $natural: -1 },
        update: {},
        upsert: true,
        new: true
    });

I would have expected this to either return the latest document (if the collection is non empty) or insert a new document if none exist, however, it inserts a blank document (without an _id) every single time it's called.  Does findAndModify work with capped collections?

Thanks.

-Scott