update document using a unique filed with findOneAndUpdate is thorwing exception - node.js

Question

I am new to node.js and mean.
I have nearly 20 fields to update in a document.
var UserSchema = mongoose.model('UserSchema');
I am passing these 20 fields in the request body and updating the UserSchema model object.
var newUser =new UserSchema(req.body);
Actually i am not passing _id from my request body but when i saw my newUser._id it is filled up with some id, so it resulting in an error called
exception: After applying the update to the document {_id: ObjectId('560e506ad4369d2a02a6ad6d') , ...}, the (immutable) field '_id' was found to have been altered to _id: ObjectId('560e65f395857e210313d3a7')
UserSchema.findOneAndUpdate({ userName: newUser.userName }, newUser, { new: true }, function (err, newuser) {
if (err) {
console.log(err);
return res.status(400).json({ message: err });
}
return res.status(200).json({ result: newUser });
});

Answer 1

Because you are creating new record set by following statement,
var newUser =new UserSchema(req.body);
Above statement will match all provided fields by req.body with registered schema for UserSchema Model and will generate new ObjectId for it. So newUser variable have _id field and when you try update existing record with newUser, it causes error as you are trying to change immutable _id field.
Instead you can re-write whole as following,
var UserSchema = mongoose.model('UserSchema');
var newUser = req.body;
UserSchema.findOneAndUpdate({ userName: newUser.userName }, newUser, { new: true }, function (err, newuser) {
if (err) {
console.log(err);
return res.status(400).json({ message: err });
}
return res.status(200).json({ result: newUser });
});
And if provided record does not exists then findOneAndUpdate will insert new as you provided new: true flag.