mongodb's $inc, incrementing a field that does not exists (upsert case)

Question

What happens when you increment a field that does not exist (upsert case) ?...

db.collection.findAndModify({query:{id:"id_in_param"},
                             update:{$inc:{score:1}},
                             upsert:true
                           })

Answer 1

The document said:

If the field does not exist, $inc creates the field and sets the field to the specified value.

In your case, new field score: 1 is added