Modifying the last element of an array in MongoDB

Question

I have an object structure like this:

 {
    name: "...",
    pockets: [
        {
            cdate: "....",
            items: [...]
        }
        ...
    ]
 }

In an update operation, I want to add some records into the items field of the last pocket item. Using dot notation is the only way that I know to access a sub document, but I can't get what I want. So, I'm looking for something like these:

• pockets.-1.items
• pockets.$last.items

Is it possible to modify the last element? If yes, how?

Answer 1

I don't know of a way to do this using a single-line query. But you could select the record, update and then save it.

var query = <insert query here>;
var mydocs = db.mycollection.find(query);
for (var i=0 ; i<mydocs.length ; i++) {
    mydocs[i].pockets[pockets.length-1].items.push('new item');
    db.mycollection.save(mydoc);
}

Answer 2

I don't believe it is possible to do it atomically. There is a request for this functionality to be added to MongoDB.

If you can assure thread-safety in your application code, you could probably use a sequence of $pop from pockets array (that removes the last element from pockets) to variable p and then $addToSet to p.items, now you can $push p back into pockets. But if your application doesn't have a way to assure only one process may be doing this at one time, then another process could modify the array in the middle of those steps and you may end up losing that update.

You might also look into "Update if current" semantics here to see another way you can work around possible race by multiple threads issue.