db={"bookPurchases":[{"userId":"801879404207931443","userName":"Philip","bookId":"111101"},{"userId":"801892568375361586","userName":"Josh","bookId":"211104"}],"userReferrals":[{"userId":"801879404207931443","userName":"Philip","referredUserId":"692597720661229598"},{"userId":"1119157325891129495","userName":"Ryan","referredUserId":"1052088956281421824"}]}

db.bookPurchases.aggregate([{$project:{_id:0,userId:1,userName:1,point:{$toInt:1}}},{$unionWith:{coll:"userReferrals",pipeline:[{$project:{_id:0,userId:1,userName:2,point:{$toInt:2}}}]}},{$group:{_id:"$userId",userName:{$first:"$userName"},points:{$sum:"$point"}}},{$sort:{points:-1}},{$project:{_id:0,userId:"$_id",userName:1,points:1}}])