我如何在我的MongoDB数组中选择一个特殊的字段并将其求和

我有一个MongoDB模式，有时我有一个数组，有时我有20个以上的数组值，每个数组都有一个字段值，我想将这些值求和并将其总和插入MongoDB的另一个字段中。这是我想说的，这是我的架构，我如何才能将插入到架构中的每个包装阵列的权重值加在一起，并使其成为我的总重量架构]

{
  "status": "In warehouse",
  "paymentStatus": "incomplete",
  "_id": "5d8b56476a3e4ae2d01f2953",
  "unitNo": "0002",
  "warehouseNo": "0001",
  "trackingNo": "FPZ848505936",
  "packages": [
    {
      "date": "2019-09-26T06:30:39.561Z",
      "_id": "5d8c5b0f756838f78d5205d7",
      "category": "chil",
      "quantity": "177",
      "description": "a valueablegoods",
      "width": 15,
      "itemweight": 123,
      "length": 17,
      "height": 21,
      "dimension": 31.25903614457831,
      "weight": 32.25903614457831 
    },
    {
      "date": "2019-09-26T06:30:39.561Z",
      "_id": "5d8c5b0f756838f78d5202dd,
      "category": "chil",
      "quantity": "177",
      "description": "a valueablegoods",
      "width": 15,
      "itemweight": 123,
      "length": 17,
      "height": 21,
      "dimension": 35.25903614457831,
      "weight": 30
    },
    {
      "date": "2019-09-26T06:30:39.561Z",
      "_id": "5d8c5b0f756838f78d51aeq",
      "category": "chil",
      "quantity": "177",
      "description": "a valueablegoods",
      "width": 15,
      "itemweight": 123,
      "length": 17,
      "height": 21,
      "dimension": 32.25903614457831,
      "weight": 44
    }
  ],
  "totalWeigth": "This should add all weight value in my packages array together and if it is only 1 it should brings only the one"
  "date": "2019-09-25T11:57:59.359Z",
  "__v": 0
}

这是将包添加到包数组字段的api路由，我希望在添加或更新新包时随时更新totalWeight

// @route   POST api/admins/addshipment/:unitNo
// @desc    Add shipment for each customer
// @access  Private
router.post(
  '/addshipment/:unitNo',
  passport.authenticate('jwt', { session: false }),
  (req, res) => {

     Shipments.findOne({unitNo: req.params.unitNo}, {paymentStatus: "incomplete"})
      .then(shipments => {
        if(shipments === null || shipments.paymentStatus === "complete"){
          const errwarehouse = "This user doesn't have an existing warehouse";
          return res.status(404).json(errwarehouse);
        }else{
          if (shipments.paymentStatus === "incomplete") {
         function getPrice(){
          if (initial > dimension){
            return initial
            }else if(initial === dimension) {
            return initial
          }else{
          return dimension
          }
        }
          const newPackages = {
          category: req.body.category,
          quantity: req.body.quantity,
          description: req.body.description,
          width: req.body.width,
          itemweight: req.body.itemweight,
          length: req.body.length,
          height: req.body.height,
          dimension,
          weight: getPrice(),
        };
          Shipments.findOneAndUpdate({unitNo: req.params.unitNo ,paymentStatus: "incomplete"}, 
            {"$push": {"packages": newPackages}}, {totalWeight: {"$sum" : {"packages.weight"}}}) //Here is were i add the package to the package array and here is where i tried sumup packages.weight for every time i add new package
            .then(shipments=> res.json(shipments))


          }
        }
        });
    });

谢谢

   var users =db.users.aggregate([
    {$unwind:"$packages"},
    {
           $group:
            {
               _id: "$_id",
              totalWeigth: { $sum: "$packages.weight" }
            }
        }
    ]).toArray()



 users.forEach((ele)=>{
  db.users.update({_id:ele._id},{$set:{totalWeigth:ele.totalWeigth}})  
 })