Question

Is there a way to update (or delete) many documents matching a certain criteria and get the list of IDs of actually updated/deleted documents (or some other fields of those documents)? I cannot simply query the documents matching my criteria beforehand because I need kinda atomicity for this operation. And I can’t use findAndModify because it can only process one document at a time which is too slow because of round-trips. Suggestions?

Answer 1

MongoDB only supports atomic operations on a single document.
http://www.mongodb.org/display/DOCS/Atomic+Operations

The only way to do this is to do what you said you didn’t one to:

First query the collection to find id’s for our query:

db.things.find({"name":"john"}, {_id:1});

Then, use the same query to remove:

db.things.remove({"name":"john"}, {_id:1});

Not ideal, and not atomic, but it’s as good as you’re going to get in this scenario.