Template
single collection
multiple collections
mgodatagen
update
indexe
explain
Stage
Database
bson
mgodatagen
[{a:5,b:3},{a:2,b:1},]
Query
db.collection.aggregate([{"$project":{"test":{"$let":{"vars":{"sum":{$add:["$a","$b"]},"d":3},"in":{"c":"$$sum","d":{"$multiply":["$$sum","$b"]}}}}}}])
Result