[{animals:{cats:[{weight:3},{weight:5}],cows:[{weight:15},{weight:21},{weight:14}]}},{animals:{dogs:[{weight:6},{weight:7}],chickens:[{weight:2},{weight:3},{weight:2.5}],mouses:[{weight:0.5}]}}]

db.collection.aggregate([{$set:{animals:{"$objectToArray":"$animals"}}},{$match:{"animals.v.weight":{$lt:1}}}])