Question

Is there a way to atomically pop (remove and retrieve) a random element with MongoDB – like Redis’s SPOP?

I’ve read the RandomAttribute tutorial but now I need to make sure that the element is also removed when fetched, and this must be done atomically.

I guess as an alternative I could push the data into an array field pre-sorted, but I’d really prefer to have it fetch a random record.

Looking at $pop‘s documentation, it seems it can’t take arguments, so it either removes the first or the last element of an array.

Answer 1

Updates within collections in Mongo are atomic, so combining findAndModify with the RandomAttribute approach will do the trick:

var rand = Math.rand();
db.collection.findAndModify({query: {random: {$gte: rand}}, remove: true});

Just make sure to also query for random: {$lt: rand} when the above returns null.

Protip: some drivers have findAndRemove, which is the same as findAndModify with remove: true.