Mongodb - duplicate fields in $set and $setOnInsert

Question

In this post, the accepted answer explains that you cannot have the same fields under $set and $setOnInsert in an upsert operation.

Can someone explain why this is? It seems like the $setOnInsert shouldn't conflict with $set, since the former is used when a document is inserted, and the latter is used when the document is updated.

Answer 1

$set operator is used on upsert too. So it's nonsense to refer same fields both on $set and $setOnInsert.

Just try this on an empty collection:

db.items.remove();
db.items.update({},{$set:{a:1},$setOnInsert:{b:2}},{upsert:1})
db.items.find({});