block9.js

db.getCollection("lineitems").aggregate([
  { $group:{ _id:{ "prodId":"$prodId"  },
             "itemCount-sum":{$sum:"$itemCount"} }
  },
  { $sort:{  "itemCount-sum":-1 }},
  { $limit:  10 },
  { $lookup:
     { from:         "products",
       localField:   "_id.prodId",
       foreignField: "_id",
       as:           "product"
     }
  },
  { $project: {
          "ProductName": "$product.productName"  ,  
          "itemCount-sum": 1  ,  
          "_id": 1
         }
  },
],{allowDiskUse: true});