[{"_id":ObjectId("5fa506a9fe8d8429b0f7a85c"),"userid":"5c35f1045643180d9488112f","pt":2,"date":ISODate("2021-02-28T08:17:45.489Z"),"wtg":0.92,"rank":1},{"_id":ObjectId("5fa506a9fe8d8429b0f7a85d"),"userid":"5c35f1045643180d9488112g","pt":2,"date":ISODate("2021-02-28T08:17:45.489Z"),"wtg":0.82,"rank":2},{"_id":ObjectId("5fa506a9fe8d8429b0f7a85e"),"userid":"5c35f1045643180d9488112f","pt":1,"date":ISODate("2021-02-28T08:17:45.489Z"),"wtg":0.92,"rank":1},{"_id":ObjectId("5fa506a9fe8d8429b0f7a85f"),"userid":"5c35f1045643180d9488112f","pt":3,"date":ISODate("2021-02-28T08:17:45.489Z"),"wtg":21.92,"rank":1},]

db.collection.aggregate([{$group:{_id:{user:"$userid",exam:"$pt"},rank1:{$sum:{$cond:[{$eq:["$rank",1]},1,0]},},rank2:{$sum:{$cond:[{$eq:["$rank",2]},1,0]},},rank3:{$sum:{$cond:[{$eq:["$rank",3]},1,0]},}}},{$group:{_id:"$_id.exam",users:{$push:{uud:"$_id.user",rank1:"$rank1",rank2:"$rank2",rank3:"$rank3"}}}}])