MongoDB: atomic call on 2 collections (find + insert)

Question

How can I atomically get the latest "rounds" record ObjectId and use that when inserting to the "deposits" collection?

This post answer says it can't be done: Is there any way to atomically update two collections in MongoDB? Is this still true?

In process A, I want to atomically FIND the latest round id (rid) and INSERT that into deposits. The race condition is that after A finds rid, another process B might insert into rounds, so now A has an rid that isn't the latest, but is 1 behind. How can A finds rid in rounds + insert this rid into deposits (act on these 2 collections) atomically?

    // GET ROUND ID (RID) OF LATEST

    var rid;
    db.rounds.find().limit(1).sort({$natural:-1}, function(err, latestInsertedRound){
        rid = latestInsertedRound[0]._id;
        print(rid, 'rid'); // if another process B inserts now, this rid is 1 behind

        // INSERT INTO DEPOSITS

        db.deposits.insert({uid:uid, iid:iid, rid:rid}, function(err, insertedDeposit){
             print(insertedDeposit, 'insertedDeposit');
        });
    });

Answer 1

Inserting a document in Mongodb has a callback function that can be used. This callback function has a second parameter which returns the document inserted.

I tried printing the second parameter using console.log. It looks like :

{ result: { ok: 1, n: 1 },
ops:
    [ { username: 'user1',
        password: 'password1',
        _id: 562099bae1872f58b3a22aed } ],
    insertedCount: 1,
    insertedIds: [ 562099bae1872f58b3a22aed ]
}

insertedIds is the array that holds the _ids of the inserted document or documents.

So you can insert your object in the second collection in the function callback of the insertion of first collection. A little confusing.

In simple terms : Insert the document in first collection. In it's callback, insert the document in the second collection.

MongoClient.connect(MONGOLAB_URI, function (err, db) {
    if (err)
        console.log("We have some error : " + err);

    else {            
        db.createCollection('rounds', function (err, rounds) {
            if (err) {
                console.log('Error while creating rounds collection');
                throw err;
            }

            else {
                rounds.insert({ 'username' : 'user1', 'password' : 'password1' }, function(err,docsInserted){
                    console.log('Last document inserted id :', docsInserted.insertedIds[0]);

                    //inserting the document in the function callback
                    db.createCollection('deposits', function (err, deposits) {
                        if (err) {
                            console.log('Error while creating deposits collection');
                            throw err;
                        }

                        else {
                            //change array index according to your need
                            //you may be inserting multiple objects simultaneously
                            deposits.insert({'last_inserted_object' : docsInserted.insertedIds[0]);

                            console.log('inserted into deposits collection');
                        }
                    });
                });
            }
        });
    }
});

Answer 2

It seems it's not possible to operate atomically on 2 collections in MongoDB, as explained in this answer:

Is there any way to atomically update two collections in MongoDB?

I leave the question up because it has a slightly different focus (not 2 updates, but find+insert).