Add a $noop operations so you can effectively do an insert if some criteria doesn't already exist.

db.coll.update({_id:25, name:"John"}, {$noop:1}, true, false)

This question came up in IRC and is the reason for this request: "I want to insert some data into a collection if and only if it doesn't exist already?"

This can be done with a query + insert but would be closer to the RDBMS version (which is wrapped in a transaction normally) using the syntax above (which is atomic).

I could also see this working by using a null second param for update (this is not currently allowed), but the $noop is clearer.