Template
single collection
multiple collections
mgodatagen
update
indexe
explain
Stage
Database
bson
mgodatagen
[{"cityId":NumberInt(1),"stateId":NumberInt(2)},{"cityId":NumberInt(9),"stateId":NumberInt(8)}]
Query
db.collection.update({},[{"$set":{"id":"$_id"}}],{"multi":true})
Result