How do I reference documents from other collection in express - node.js

Question

I have 2 collections here >>course & author
I need to prepare my course schema in such a way that it references the author and at the post request I only need to put the id.I am using #joi13 This is what I have done so far.
course schema
const mongoose = require('mongoose')
const Joi = require('joi')
Joi.objectId= require('joi-objectid')(Joi)
// schema
const courseSchema = new mongoose.Schema({
...
author: {
type: mongoose.Schema.Types.ObjectId,
ref: 'Author'
}
})
// the model
const Course = mongoose.model('Courses', courseSchema)
// validation
const courseValidation = (course) => {
const schema = {
...
authorId: Joi.objectId().required()
}
return Joi.validate(course, schema)
}
the course router
const {Course, validate} = require('../models/course')
const express = require('express')
const router = express.Router()
const {Author} = require('../models/author')
// post
router.post('/', async (req, res) => {
const {error} = validate(req.body)
if (error) return res.status(400).send(error.details[0].message)
const title = await Course.findOne({title : req.body.title})
if (title) return res.status(400).send('That user exists')
const author = await Author.find()
if (!author) return res.status(404).send('No such Author')
let course = new Course({
...
author: {
_id: author._id,
username: author.username
}
})
try {
course = await course.save()
res.send(course)
}
catch(er){
console.log(er)
}
})
The error

Answer 1

At line, const author = await Author.find() will return array of authors
while creating course at following lines of you are using author._id which will be undefined, So you have to find a specific Author using findOne (return a single author as object) or you have to use an indexed element of an author like author[0]._id
let course = new Course({
...
author: {
_id: author._id,
username: author.username
}
})