I want to increment many documents in mongodb - node.js

Question

router.put('/stockAccepted/:id', (req, res, next) =>{
stockSchema.findOneAndUpdate({_id: req.params.id}, {
$set:{
stockAccepted: req.body.stockAccepted
}
},(err, result) => {
if(err) res.json(err);
else res.json(result);
});
let stockItem = req.body.stockItem;
stockItem.forEach((element) => {
console.log("counter")
productSchema.update({_id: element.productId}, {
$inc:{
productQuantity: element.productQuantity
}
}
,(err, result) => {
if(err) res.json(err);
else res.json(result);
});
});
});
what is the best approach to achieve it?
This code throws an error that "headers are already sent".
How can I implement this correctly?

Answer 1

The problem is you are trying to send a JSON response in each db update callback. You can only send a response once for a request. You can either use Promise.all to wait for the stock and product(s) to be updated, or use Mongoose's bulkWrite feature.
The bulkWrite is more efficient as it only involves one request to the DB rather than multiple.
Here is an (untested) example of how bulkWrite could work with your code:
router.put('/stockAccepted/:id', (req, res, next) => {
stockSchema.findOneAndUpdate({
_id: req.params.id
}, {
$set: {
stockAccepted: req.body.stockAccepted
}
}).then((result) => {
let stockItem = req.body.stockItem;
let updates = []
stockItem.forEach((element) => {
updates.push({
updateOne: {
filter: {
_id: element.productId
},
update: {
$inc: {
productQuantity: element.productQuantity
}
}
}
})
})
return productSchema.bulkWrite(updates)
}).then((result) => {
res.json(result);
}).catch((err) => {
res.json(err);
})
});
I'm using the promise returned from a DB call in mongoose rather than the callbacks, as it reduces nesting.
Unlike your version, this code will wait for stock to be updated in DB. Then it will create a bulk operation and send that to the DB. Once this is completed, it will finally return the response from the bulk operation as JSON.
Note that the final catch handler will catch errors in both the stock update, or the bulk product update.

Answer 2

You are iterating over stockItem with a forEach loop, nothing wrong about that.
What you probably miss here is that after handling correctly the first element you use directly the res params which is why the next iteration when you process the same logic it fails with "headers are already sent" because you indeed, already fire an http response with res.
What I suggest is to aggregate your result during the loop and only fire the res with your aggregate results as json as below :
router.put('/stockAccepted/:id', (req, res, next) =>{
stockSchema.findOneAndUpdate({_id: req.params.id}, {
$set:{
stockAccepted: req.body.stockAccepted
}
},(err, result) => {
if(err) res.json(err);
else res.json(result);
});
let stockItem = req.body.stockItem;
const results = [];
const errors = [];
stockItem.forEach((element) => {
console.log("counter")
productSchema.update({_id: element.productId}, {
$inc:{
productQuantity: element.productQuantity
}
}
,(err, result) => {
if(err) errors.push(err);
else result.push(result);
});
});
res.json({
results,
errors
});
});