Can not populate array of objects - node.js

Question

EDIT added mongoose.model.
I'm pretty new to mongodb and mongoose. I'm not able to populate books to authors in mongoose.
Can anyone help me with that? Here is my code. I removed unnecessary fields
const authorSchema = mongoose.Schema({
_id:{
type: String,
required: true,
unique: true,
trim: true
},
name: {
type: String,
required: true,
trim: true
},
books: [{
type: String,
ref: "Book"
}]
}, {_id:false})
const Author = mongoose.model('Author', authorSchema)
module.exports = {Author}
And my books schema looks as following
const bookSchema = mongoose.Schema({
_id:{
type:String,
required:true,
unique:true,
trim: true
},
name: {
type: String,
required: true,
trim: true,
unique: true
},
description:{
type: String,
trim: true,
default: 'No description specified'
},
datePublished: {
type: Date,
trim: true,
default: '01/01/2001'
},
author:{
type: String,
ref: 'Author',
required: true
}
}, {_id:false})
const Book = mongoose.model('Book', bookSchema)
module.exports = {Book}
Here is the route to populate them together
AuthorRouter.get('/authors/:id', async(req, res)=>{
const authorID = req.params.id
try {
const author = await Author.findById(authorID)
try {
const populated = await author.populate({path: 'books.book', select: 'name description -_id'})
console.log(populated);
res.status(201).send(populated)
} catch (e) {
res.status(401).send(`${e}\n Unable to populate categories`)
}
} catch (error) {
res.status(401).send('Unable to find author')
}
})
Output is following with empty books array:
{
"_id": "123",
"name": "testuser",
"books": [],
"__v": 0
}

Answer 1

Populating an arrays of refs works the same way as a single ref. Just call the populate method on the query and an array of documents will be returned in place of the original _id's.
In your case that would be:
const populated = await author.populate({path: 'books', select: 'name description -_id'});

Answer 2

Hey i have modify your code , try to use this one
This is your author schema file
const Schema = mongoose.Schema;
const authorSchema = mongoose.Schema({
_id:{
type: String,
required: true,
unique: true,
trim: true
},
name: {
type: String,
required: true,
trim: true
},
books: [{
type: Schema.Types.ObjectId,
ref: "bookSchema"
}]
}, {_id:false})
const Author = mongoose.model('Author', authorSchema)
module.exports = {Author}
And this is your route
AuthorRouter.get('/authors/:id', async(req, res)=>{
const authorID = req.params.id
try {
const author = await Author.findById(authorID)
try {
const populated = await author.find().populate('books');
console.log(populated);
res.status(201).send(populated)
} catch (e) {
res.status(401).send(`${e}\n Unable to populate categories`)
}
} catch (error) {
res.status(401).send('Unable to find author')
}
})