db={"users":[{_id:ObjectId("62a2a0422ec90fea68390aaa"),name:"Robert Yamashita",username:"robyama",email:"robert.yamashita@rocketmail.com",},{_id:ObjectId("62a2a0452ec90fea68390aad"),name:"Charles X",username:"cvx",email:"charles.xxx@rocketmail.com",}],"points":[{userId:ObjectId("62a2a0422ec90fea68390aaa"),action:"Liked a post",points:10,},{userId:ObjectId("62a2a0422ec90fea68390aaa"),action:"Liked a post",points:10,},{userId:ObjectId("62a2a0452ec90fea68390aad"),action:"Liked a comment",points:5,}]}

db.users.aggregate([{$lookup:{from:"points",localField:"_id",foreignField:"userId",as:"userPoints"}},{$unwind:"$userPoints"},{$group:{_id:{name:"$name",email:"$email",username:"$username",},count:{$sum:"$userPoints.points"}}},{"$sort":{count:-1}},{"$group":{"_id":null,"docs":{"$push":"$$ROOT",}}},{"$unwind":{path:"$docs",includeArrayIndex:"rownum"}},{"$match":{"docs._id.username":"robyama"}},{"$addFields":{"docs.rank":{"$add":["$rownum",1]}}},{"$replaceRoot":{"newRoot":"$docs"}}])