Description:

This is part of the many changes to $out in 4.2. If the "uniqueKey" includes the _id and the document is missing an _id at the time of the write in $out, we'll generate one for them automatically (the usual ObjectID).

Engineering Ticket Description:

If the uniqueKey contains _id, we should relax the enforcement that all fields of the unique key must be present in a given document since the update/insert should auto generate an _id for us.