Template
single collection
multiple collections
mgodatagen
update
indexe
explain
Stage
Database
bson
mgodatagen
[{"_id":ObjectId("5a934e000102030405000000"),"key":1},{"_id":ObjectId("5a934e000102030405000001"),"key":2}]
Query
db.collection.aggregate([{$set:{createdAt:{$toDate:"$_id"}}}])
Result