Database
[{"_id":ObjectId("614513461af3bf569fdc420e"),"item":"postcard","status":"A","size":{"h":10,"w":15.25,"uom":"cm"},"instock":[{"warehouse":"B","qty":15},{"warehouse":"C","qty":35}]}]
Query
db.collection.aggregate([{"$match":{"_id":ObjectId("614513461af3bf569fdc420e")}},{"$project":{"_id":0,"qty":{"$getField":{"field":"qty","input":{"$last":"$instock"}}}}}])