[{_id:"id_1",item:"journal",instock:[{warehouse:"A",qty:5},{warehouse:"C",qty:15},{warehouse:"C",qty:20},{warehouse:"C",qty:25}]}]

db.collection.aggregate([{"$match":{"_id":"id_1"}},{"$project":{"item":1,"instock":{"$filter":{"input":"$instock","cond":{"$gt":["$$this.qty",20]}}}}}])