[{"username":"Stack Overflow User1","votes":5,"answer":true,"question":false},{"username":"Stack Overflow User2","votes":10,"answer":true,"question":false},{"username":"Stack Overflow User1","votes":5,"answer":false,"question":true},{"username":"Stack Overflow User1","votes":8,"answer":true,"question":false},{"username":"Stack Overflow User2","votes":7,"answer":true,"question":false}]

db.collection.aggregate([{$group:{_id:"$username",answer_count:{$sum:{$cond:["$answer",1,0]}},total_votes:{$sum:{$cond:["$answer","$votes",0]}},max_votes:{$max:{$cond:["$answer","$votes",null]}}}}])