block8.js

db.getCollection("embeddedOrders").aggregate([
  { $unwind:  "$orders" },
  { $unwind:  "$orders.lineItems" },
  { $project: { "lineitems": "$orders.lineItems"   }},
  { $group:{  _id:{ "lineitems_prodId":"$lineitems.prodId" ,"lineitems_productName":"$lineitems.productName"  },
             "lineitems_itemCount-sum":{$sum:"$lineitems.itemCount"}  }
  },
  { $sort:{  "lineitems_itemCount-sum":-1 }},
  { $limit:  10 },
],{allowDiskUse: true});