Data is mutated but not updated in the database - node.js

Question

I have a sever connected to a mongodb database. When I add a first level data and then save that, it works.
For example :
// this works fine
router.post('/user/addsomedata', async (req,res)=>{
try {
const user = await User.findOne({email : req.body.email})
user.username = req.body.username
await user.save()
res.send()
} catch(e) {
res.status(404).send(e)
}
})
BUT if I try to save the object with deeper level data, it's not getting saved. I guess the update is not detected and hence the user didn't get replaced.
Example :
router.post('/user/addtask', auth ,async (req,res)=>{
const task = new Task({
name : req.body.name,
timing : new Date(),
state : false,
})
try {
const day = await req.user.days.find((day)=> day.day == req.body.day)
// day is found with no problem
req.user.days[req.user.days.indexOf(day)].tasks.push(task)
// console.log(req.user) returns exactly the expected results
await req.user.save(function(error,res){
console.log(res)
// console.log(res) returns exactly the expected results with the data filled
// and the tasks array is populated
// but on the database there is nothing
})
res.status(201).send(req.user)
} catch(e) {
res.status(400).send(e)
}
})
So I get the tasks array populated on the console even after the save callback but nothing on the db image showing empty tasks array

Answer 1

You're working on the user from the request, while you should first find the user from the DB like in your first example (User.findOne) and then update and save that model.

Answer 2

Use .lean() with your find queries whenever you are about to update the results returned by mongoose. Mongoose by default return instance objects which are immutable by nature. lean() method with find returns normal js objects which can be modified/updated.
eg. of using lean()
const user = await User.findOne({email : req.body.email}).lean();
You can read more about lean here
Hope this helps :)