Mongoose MongoDB updating a nested array - node.js

Question

I have the following schema and I'm trying to update the heart_rates and glucose_levels arrays by adding a new entry:
"days": [
{
"day": "16-12-2020",
"data": {
"average_score": 80,
"metabolic_score": 80,
"cardio_score": 80,
"heart_rates": [{
"start_date": "timestamp",
"end_date": "timestamp",
"value": 100
}],
"glucose_levels": [{
"start_date": "timestamp",
"end_date": "timestamp",
"value": 100
}],
"weekly_health_recap": {
"id": "",
"start_date": "timestamp",
"end_date": "timestamp",
"rating": "Excellent"
},
"summaries": [{
"type": "meal",
"actions": [
"sweets", "fruits"
],
"title": "",
"start_date": "",
"end_date": ""
}],
"articles": [{
"id": "",
"name": "",
"text": "",
"image_url": ""
}]
}
}
]
I tried various solutions, however they didn't give the expected results.
Currently, I'm using the following code to find if there is an existing entry for the day, if there is not, I'm creating it using findByIdAndUpdate and upsert, if there is already a day entry then I'm using update and push to add a new entry in the heart_rates array. The problem is that the update inserts the new entry in some other day entry and not the one that I'm querying for.
exports.updateDayHeartrate = async (req, res) => {
User.find({ "_id": req.body._id, "days.day": req.body.day}, function (err, docs) {
if (err){
return done(err);
}
if (docs.length > 0) {
User.update({"_id": req.body._id, "days.day": req.body.day},
{$push: {"days.0.data.heart_rates": req.body.data}},
(err, user) => {
if (err) {
res.status(500).send({ message: err });
return;
}
else if (user) {
res.status(200).send("Updated!")
}
}
)
}
else{
User.findByIdAndUpdate(req.body._id,
{$push: {"days": {"day": req.body.day, "data": {"heart_rates": req.body.data}}}},
{upsert:true, new:true, setDefaultsOnInsert: true},
(err, user) => {
if (err) {
res.status(500).send({ message: err });
return;
}
else if (user) {
res.status(200).send("Updated!")
}
}
);
}
});
};