If an update operation with upsert: true results in an insert of a
document, then $setOnInsert assigns the specified values to the fields in
the document. If the update operation does not result in an insert,
$setOnInsert does nothing.
또한 보기:
Expr::setOnInsert()
또한 보기:
https://docs.mongodb.org/manual/reference/operator/update/setOnInsert/
