Mongodb upsert only update selected fields, but insert all

Question

I am trying to use upsert in MongoDB to update a single field in a document if found OR insert a whole new document with lots of fields. The problem is that it appears to me

Answer 1

MongoDB 2.4 has $setOnInsert

db.somecollection.update(
    {name: "some name"},
    {
        $set: {
            "lastseen": "2012-12-28"
        },
        $setOnInsert: {
            "firstseen":   # set on insert, not on update
        }
    },
    {upsert:true}
)