Push ObjectId to nested array in Mongoose - mongodb

Question

(Basic library CRUD application)
I am trying to create a document containing some global data about a given book, and then within a User document, add the ObjectId of the newly-created book to an array containing all books belonging to that user.
I have three data models in my application:
var userSchema = new mongoose.Schema({
name: String,
password: String,
email: String,
books: [BookInstanceSchema],
shelves: [String]
});
var bookSchema = new mongoose.Schema({
title: {
type: String,
required: true
},
author: {
type: String,
required: true
},
description: String,
pageCount: Number,
ISBN: String,
googleID: String,
thumbnail: String,
publisher: String,
published: String,
});
var BookInstanceSchema = new mongoose.Schema({
bookId: {
type: mongoose.Schema.Types.ObjectId,
ref: 'Book'
},
userReview: String,
userRating: {
type: Number,
get: v => Math.round(v),
set: v => Math.round(v),
min: 0,
max: 4,
default: 0
},
shelf: String
});
The User model contains a nested array of BookInstances, which contain user-specific data such as ratings or reviews for a given book. A bookInstance in turn contains a reference to the global data for a book, to avoid duplicating data that isn't specific to any user.
What I'm trying to do is first save the global data for a book (thus generating an _id), and when done, save a bookInstance containing that _id in a given user's array of books:
router.post('/save/:id', function(req, res) {
var url = encodeurl('https://www.googleapis.com/books/v1/volumes/' + req.params.id);
request(url, function(err, response, data) {
parsedData = JSON.parse(data);
var newBook = {
title: parsedData.volumeInfo.title,
author: parsedData.volumeInfo.authors[0],
description: parsedData.volumeInfo.description,
pageCount: parsedData.volumeInfo.pageCount,
ISBN: parsedData.volumeInfo.description,
googleID: parsedData.id,
publisher: parsedData.volumeInfo.publisher,
published: parsedData.volumeInfo.publishedDate,
thumbnail: parsedData.volumeInfo.imageLinks.thumbnail
};
Book.create(newBook, function(err, newBook) {
if (err) {
console.log(err);
}
else {
console.log(newBook._id);
console.log(mongoose.Types.ObjectId.isValid(newbook._id));
User.findByIdAndUpdate(req.session.id, {
$push: {
"books": {
bookId: newBook._id,
userRating: 0,
userReview: ''
}
}
},
{
upsert: true
},
function(err, data){
if(err) {
console.log(err);
}
else {
res.redirect('/');
}
});
}
});
});
});
I'm getting the error:
message: 'Cast to ObjectId failed for value "hjhHy8TcIQ6lOjHRJZ12LPU1B0AySrS0" at path "_id" for model "User"',
name: 'CastError',
stringValue: '"hjhHy8TcIQ6lOjHRJZ12LPU1B0AySrS0"',
kind: 'ObjectId',
value: 'hjhHy8TcIQ6lOjHRJZ12LPU1B0AySrS0',
path: '_id',
reason: undefined,
Every time, the value in the error (in this case, jhHy8T...) is different than the newBook._id I'm attempting to push into the array:
console.log(newBook._id); // 5a120272d4201d4399e465f5
console.log(mongoose.Types.ObjectId.isValid(newBook._id)); // true
It seems to me something is wrong with my User update statement:
User.findByIdAndUpdate(req.session.id, {
$push: {
"books": {
bookId: newBook._id,
userRating: 0,
userReview: ''
}
}...
Any help or suggestions on how to better organize my data are appreciated. Thanks!