Pushing into Mongoose array is not updating the array - node.js

Question

I have a User schema with a collections property that holds collection objects. These collection objects have a property called items that is an array of objects.
//User Schema
const UserSchema = new mongoose.Schema({
username: {
type: String,
required : true,
},
password: {
type: String,
required: true
},
collections: [{type : mongoose.Schema.Types.ObjectId, ref: 'Collection'}]
});
//Collection Schema
const CollectionSchema = new mongoose.Schema({
items : [{type : mongoose.Schema.Types.ObjectId, ref: 'Item'}]
});
//Item Schema
const ItemSchema = new mongoose.Schema({
title: {
type: String,
required: true
});
Using the code below, I tried creating and pushing a new Item Schema into the first collection object. I pass the _id of the collection object I want to update using the findById function. After finding the collection with my id, I simply just push a Item Schema into the collection object's items array. I get the res.status(200) yet my items array is never updated. Does anyone else have this issue? Thank you for you help.
userRouter.post('/addItem', passport.authenticate('jwt', {session: false}), (req, res) => {
const item = new Item(req.body)
item.save(err => {
if(err)
res.status(500).json({message: {msgBody: "Error has occured", msgError: true }});
else {
Collection.findById(req.body.search, function(err, coll){
coll.items.push(item);
req.user.save(err => {
if(err)
res.status(500).json(err)
else{
res.status(200).json(coll)
}
})
})
}
})
});

Answer 1

You don´t perform any update operation on the Collection with the newly created Item document in the database.
Add the created Item document by using the $addToSet operator (https://docs.mongodb.com/manual/reference/operator/update/addToSet/) to the items property in Collection (I would suggest by using mongoose findByIdAndUpdate method or updateOne method).
It would be something like this using callbacks:
Collection.findByIdAndUpdate(req.body.search, { $addToSet: {items: item._id}}, callback);
If you have multiple Item documents which you need to insert into Collection, you can use $addToSet in combination with the $each operator (https://docs.mongodb.com/manual/reference/operator/update/each/).