[{item:"journal",instock:[{warehouse:"A",qty:null},{warehouse:"C",qty:15}]},{item:"notebook",instock:[{warehouse:"C",qty:5}]},{item:"paper",instock:[{warehouse:"A",qty:60},{warehouse:"B",qty:15}]},{item:"planner",instock:[{warehouse:"A",qty:40},{warehouse:"B",qty:5}]},{item:"postcard",instock:[{warehouse:"B",qty:15},{warehouse:"C",qty:35}]}]

db.collection.find({"instock.0.qty":{$type:"null"}})