[{"traces":[{"act":{"account":"555","name":"Bob"}},{"act":{"account":"666","name":"Mary"}}],"other":[{"act":{"account":"555","volume":"PPPPP"}}]},{"traces":[{"act":{"account":"666","name":"Mary"}}],"other":[{"act":{"account":"555","volume":"PPPPP"}}]}]

db.collection.find({"traces.act.account":{$in:["555"]}})