db={Order:[{"_id":{"$oid":"6276ba9dd1dfd6f5bf4b501f"},"Id":"1","OrderDate":"2012-07-04 00:00:00.000","OrderNumber":"542378","CustomerId":"85","TotalAmount":"440.00"},{"_id":{"$oid":"6276ba9dd1dfd6f5bf4b5020"},"Id":"2","OrderDate":"2012-07-05 00:00:00.000","OrderNumber":"542379","CustomerId":"79","TotalAmount":"1863.40"}],Customer:[{"_id":{"$oid":"6276ba2dd1dfd6f5bf4b4f53"},"Id":"85","FirstName":"Maria","LastName":"Anders","City":"Berlin","Country":"Germany","Phone":"030-0074321"},{"_id":{"$oid":"6276ba2dd1dfd6f5bf4b4f54"},"Id":"79","FirstName":"Ana","LastName":"Trujillo","City":"México D.F.","Country":"Mexico","Phone":"(5) 555-4729"}]}

db.Order.aggregate([{$lookup:{from:"Customer",localField:"CustomerId",foreignField:"Id",as:"CustomerInfo"}},{$project:{CustomerId:1,CustomerInfo:{$first:"$CustomerInfo"}}},{$group:{_id:"$CustomerInfo.Id",CustomerOrderNumber:{$sum:1},CustomerInfo:{$first:"$CustomerInfo"}}},{$project:{_id:0,CustomerId:"$_id",CustomerOrderNumber:1,CustomerName:{$concat:["$CustomerInfo.FirstName"," ","$CustomerInfo.LastName"]}}},{$setWindowFields:{sortBy:{CustomerOrderNumber:-1},output:{denseRankHighestOrder:{$denseRank:{}}}}},{$match:{denseRankHighestOrder:1}}])