title: why is mongodb returning success and it not posting value into mongodb array
body: <p>I am working on sending an array value to an existing table in mongodb using the <code>$push</code> and the res.json returns this</p>
  
  <pre><code>// @route   POST api/addshipment/:unitNo
  // @desc    Add shipment for each customer
  // @access  Private
  router.post(
    '/addshipment/:user_id',
    passport.authenticate('jwt', { session: false }),
    (req, res) =&gt; {
  
      Shipments.findOne({_id: req.params.user_id}, {paymentStatus: "incomplete"})
        .then(shipments =&gt; {
          if(!shipments){
            const errshipments = 'This user doesnot have an existing warehouse';
             return res.status(404).json(errshipments);
          }else{
            const newPackages = {
            category: req.body.category,
            quantity: req.body.quantity,
            description: req.body.description,
            trackingno: 1214112,
            length: req.body.length,
            height: req.body.height,
            width: req.body.width,
            weight :  12  
          };
            Shipments.updateOne({_id: req.params.user_id}, {paymentStatus: "incomplete"}, { "$push": { "packages": newPackages}})
            .then(shipments=&gt; res.json(shipments));
          }
        });
      });
  </code></pre>
  
  <p>here is the res.json that i got </p>
  
  <pre><code>{
    "n": 1,
    "nModified": 0,
    "opTime": {
      "ts": "6739809611714396360",
      "t": 3
    },
    "electionId": "7fffffff0000000000000003",
    "ok": 1,
    "operationTime": "6739809611714396360",
    "$clusterTime": {
      "clusterTime": "6739809611714396360",
      "signature": {
        "hash": "Olz9Fehw4JRj/a4rrLOhCxZIO7E=",
        "keyId": "6719390010044317697"
      }
    }
  }
  </code></pre>
  
  <p>instead of something like this</p>
  
  <pre><code>{
    "paymentStatus": "incomplete",
    "_id": "5d85a4fc8e7efe74e4dee830",
    "warehouseNo": "0001",
    "packages": [{
       "quantity": 12,
       "category" : "Hazardous",
              }],
    "date": "2019-09-23T10:46:23.065Z",
    "__v": 0
  }
  </code></pre>