Template
single collection
multiple collections
mgodatagen
update
indexe
explain
Stage
Database
bson
mgodatagen
[{_id:1,items:[[1,2,3],[4,5,6]]}]
Query
db.collection.aggregate([{"$project":{"items":{"$reduce":{"input":"$items",initialValue:[],"in":{$concatArrays:["$$this","$$value"]}}}}}])
Result