Database
[{_id:1,ip:"1.2.3.4",top_ports:{"12.168.16.253":{state:{state:"up"}}}}]
Query
db.collection.aggregate([{"$addFields":{"top_ports":{"$objectToArray":"$top_ports"}}},{$match:{"top_ports.k":"12.168.16.253"}},{"$addFields":{"top_ports":{"$arrayToObject":"$top_ports"}}}])