Template
single collection
multiple collections
mgodatagen
update
indexe
explain
Stage
Database
bson
mgodatagen
[{"name":"rajesh","age":38},{"name":"vijay","age":27}]
Query
db.collection.aggregate([{$addFields:{"temp":{$concat:["$name","_",{$toString:"$age"}]}}},{$project:{_id:0}}])
Result