Database
[{"_id":111,"recs":[{"date":"01-01-2021","sales":[{"quantity":{"P":14,"K":0,"U":0}}]},{"date":"02-01-2021","sales":[{"quantity":{"P":23,"K":0,"U":0}}]}]},{"_id":21,"recs":[{"date":"01-01-2021","sales":[{"quantity":{"P":11,"K":0,"U":0}}]},{"date":"02-01-2021","sales":[{"quantity":{"P":31,"K":0,"U":0}}]}]}]
Query
db.collection.aggregate([{"$project":{_id:1,recs:{"$map":{"input":"$recs","in":{"$mergeObjects":["$$this",{sales:{$first:"$$this.sales.quantity"}}]}}}}}])