[{"userId":1,"data":"aaaaa",version:1,mainVersion:1},{"userId":2,"data":"bbbbb",version:1,mainVersion:1},{"userId":2,"data":"bbbbbx",version:2,mainVersion:1},{"userId":2,"data":"bbbbbxx",version:3,mainVersion:1},{"userId":1,"data":"cancelled!!!",version:2,mainVersion:1},{"userId":2,"data":"bbbbbxxx",version:4,mainVersion:1},{"userId":1,"data":"aaaaa-",version:1,mainVersion:2},{"userId":2,"data":"bbbbbxxx-",version:4,mainVersion:2},{"userId":2,"data":"bbbbbxxx-x",version:5,mainVersion:2},{"userId":2,"data":"bbbbbxxx-",version:4.5,mainVersion:2},{"userId":1,"data":"aaaaa-x",version:2,mainVersion:2},{"userId":1,"data":"cancelled!!!",version:3,mainVersion:1}]

db.collection.aggregate({"$group":{_id:"$userId","latestVersion":{"$max":{"$mergeObjects":[{"primaryVersion":"$mainVersion"},{"vversion":"$version"},"$$ROOT"]}}}})