Node, Mongoose - Referencing Document not working as expected - node.js

Question

In my app, users can create posts. I want to assign a user to each post by referencing the users ObjectId.
Post model and schema:
const postSchema = new mongoose.Schema({
postImage: {
type: String,
required: true
},
caption: {
type: String
},
user: {
type: mongoose.Schema.Types.ObjectId,
ref: 'User'
},
date: {
type: Date,
default: Date.now()
}
})
const Post = mongoose.model('Post', postSchema)
User model and Schema:
const userSchema = new mongoose.Schema({
username: {
type: String,
required: true,
unique: true,
minlength: 4,
maxlength: 50
},
password: {
type: String,
required: true,
minlength: 6,
maxlength: 75
}
})
userSchema.methods.generateAuthToken = () => {
const token = jwt.sign({ _id: this._id }, config.get('jwtPrivateKey'))
return token
}
const User = mongoose.model('User', userSchema)
In my post route, I apply the following middleware (auth middleware):
module.exports = (req, res, next) => {
const token = req.header('x-auth-token')
if (!token) return res.status(401).send('No token provided')
try {
const decoded = jwt.verify(token, config.get('jwtPrivateKey'))
req.user = decoded
next()
}
catch (ex) {
res.status(400).send('Invalid token')
}
}
Post route:
router.post('/', [auth, upload.single('postImage')], async (req, res) => {
const post = new Post({
postImage: `http://localhost:3000/${req.file.path}`,
caption: req.body.caption,
user: req.user._id
})
await post.save()
res.send(post)
})
When I make a request to the route, I set x-auth-token, and I get a response of the post object created. However in the database the user property on the post object has not been set . Only the following properties have been set on the post object created. _id, date, postImage, caption, __v. Does anybody know why the user is not being set as a property? Thanks.
UPDATE:
This is how I generate the jwt token, with _id attached:
userSchema.methods.generateAuthToken = () => {
const token = jwt.sign({ _id: this._id }, config.get('jwtPrivateKey'))
return token
}