NodeJS and MongoDB - use aggregate and $lookup together with findById - node.js

Question

I want to make a relation between two collections - a book and author collections.
If i use only get and display all of my books and integrate the data about author by id it works.
Author schema:
const AuthorSchema = new mongoose.Schema({
name: { type: String, required: true },
surname: { type: String, required: true },
dateOfBirth: { type: String, required: true },
countryOfBirth: { type: String, required: true },
});
Book schema:
const BookSchema = new mongoose.Schema({
owner: { type: String, required: true },
pagesNo: { type: String, required: true },
releaseDate: { type: String, required: true },
country: { type: String, required: true },
authorID: { type: Schema.Types.ObjectId, ref: "Author", required: true }, <-- HERE I NEED DATA ABOUT AUTHOR
});
My express function that works for fetching all data:
router.get("/", async (req, res) => {
try {
let books = await Book.aggregate([
{
$lookup: {
from: "authors",
localField: "authorID",
foreignField: "_id",
as: "author",
},
},
]);
res.status(200).json(books);
} catch (err) {
res.status(404).json({ success: false, msg: "Book is not found" });
}
});
But now I want to display that joined data also when i search for a single book by ID (findById()).
I got an error status if I use a function like this:
router.get("/:bookId", async (req, res) => {
try {
let book= await Book.aggregate([
{
$lookup: {
from: "authors",
localField: "authorID",
foreignField: "_id",
as: "author",
},
},
]);
book= book.findById({ _id: req.params.bookId});
res.status(200).json(book);
} catch (err) {
res.status(404).json({ success: false, msg: "Book is not found" });
}
});
Thank you for your help

Answer 1

use the $match to find only one book for the same query
const mongoose = require('mongoose');
const ObjectId = mongoose.Types.ObjectId();
router.get("/:bookId", async (req, res) => {
try {
let book= await Book.aggregate([
{
$match: { _id : ObjectId("book _id") }
},
{
$lookup: {
from: "authors",
localField: "authorID",
foreignField: "_id",
as: "author",
},
},
]);
book= book.findById({ _id: req.params.bookId});
res.status(200).json(book);
} catch (err) {
res.status(404).json({ success: false, msg: "Book is not found" });
}
});