mongoose find one and update function - node.js

Question

Suppose that we have a shopping cart in the site, we have specified number of a good in database. Whenever the user wants to validate his/her cart and finalize it, I want to subtract the number of goods that the user purchased from total count of the product. How can I do that?
router.post('/done', (req, res) => {
// substract from the count
for (var i = 0; i < req.body.cart.length; i++){
Product.findOne({_id: req.body.cart[i]._id}).exec((err, result) => {
result.count = result.count - req.body.cart[i].count;
// update the value of result.count and save it on database
})
}
})

Answer 1

You just need to save, and also check for errors.
router.post('/done', (req, res) => {
// substract from the count
for (var i = 0; i < req.body.cart.length; i++){
Product.findOne({_id: req.body.cart[i]._id}).exec((err, result) => {
if(result)
{
result.count = result.count - req.body.cart[i].count;
result.save((err) => {
if (err) // do something
});
}
//If you don't find the product for some reason, do something
})
}
})

Answer 2

You can use mongoose's findOneAndUpdate() and $inc reference operator with a negative value to decrement the count.
Docs
The following update() operation uses the $inc operator to decrease the quantity field by 2 (i.e. increase by -2) and increase the "metrics.orders" field by 1:
db.products.update(
{ sku: "abc123" },
{ $inc: { quantity: -2, "metrics.orders": 1 } }
)
Making those changes
'use strict';
router.post('/done', (req, res) => {
let {cart} = req.body;
if (!Array.isArray(cart) || cart.length === 0) {
return res.status(400).send({
msg: 'Empty cart'
});
}
let updates = cart.map(item => {
return Product.findOneAndUpdate({
_id: item._id
}, {
$inc: {
count: -1 * item.count
}
}, {
upsert: false
});
});
Promise.all(updates).then(() => {
return res.status(200).send({
msg: 'inventory updated'
});
}).catch(err => {
console.log('err', err.stack);
return res.status(500).send({
msg: 'inventory update failed!'
});
});
});