[{username:"bob",data:[{sport:"football",events:[{event:1,_id:"h4jh234ht34j"},{event:2,_id:"4h326ol2j3ld"},{event:2,_id:"fdsfasdfa3ws"}/** ... more events*/]},{sport:"cricket",events:[{event:3,_id:"hd2g234ht3gj"},{event:2,_id:"33326oldwqld"},{event:6,_id:"461fa24gsa3s"}/** ... more events*/]},/** ... more sports*/]}]

db.collection.aggregate([{$match:{"data.events.event":2}},{"$unwind":"$data"},{"$unwind":"$data.events"},{$match:{"data.events.event":2}},{"$replaceRoot":{"newRoot":"$data.events"}}])