Template
single collection
multiple collections
mgodatagen
update
indexe
explain
Stage
Database
bson
mgodatagen
[{"a":[{x:1},{x:2}],"b":[{x:3},{x:4}]}]
Query
db.collection.aggregate([{$project:{_id:0,data:{$concatArrays:["$a","$b"]}}}])
Result