db.getCollection('tmp').aggregate( [
   // First Stage
   {
     $unwind: { path: "$faceAmtMap", preserveNullAndEmptyArrays: true }
   },
   // Second Stage
   {
     $group:
       {
         _id: "$faceAmtMap.age",
         averagePrice: { $max: "$faceAmtMap.faceAmt" }
       }
   },
   // Third Stage
   {
     $sort: { "averagePrice": -1 }
   }
] )