Problems getting populate to work correctly - mongodb

Question

I am trying to build a forum and have come across some problems when trying to add answers with information of the user to a specific post. I am able to list the answers for a single post in the frontend, however I am unable to populate the user information, as for now I only get an id. Here are my models and endpoints to post and get the answers. What am I missing?
const User = mongoose.model('User', {
username: {
type: String,
required: [true, 'Username is required!'],
unique: true,
lowercase: true,
trim: true
},
email: {
type: String,
required: [true, 'Email is required!'],
unique: true,
lowercase: true,
trim: true,
validate: {
validator: (value) => {
return /^\w+([\.-]?\w+)*#\w+([\.-]?\w+)*(\.\w{2,3})+$/.test(value)
},
message: "Please enter a valid email address"
}
},
password: {
type: String,
required: [true, 'Password is required!'],
minlength: [8, 'Password must be a minimum of 8 characters!']
},
accessToken: {
type: String,
default: () => crypto.randomBytes(128).toString('hex')
}
})
const Post = mongoose.model('Post', {
title: {
type: String,
required: [true, 'Field cannot be blank'],
minlength: [2, 'Your post must be a minimum of 2 characters!'],
maxlength: [50, 'Your post must be a maximum of 50 characters!']
},
post: {
type: String,
required: [true, 'Field cannot be blank'],
minlength: [2, 'Your post must be a minimum of 2 characters!'],
},
votes: {
type: Number,
default: 0
},
createdAt: {
type: Date,
default: Date.now()
},
answers: [{
type: mongoose.Schema.Types.ObjectId,
ref:'Answer'
}],
user: {
type: mongoose.Schema.Types.ObjectId,
ref: 'User'
}
})
const Answer = mongoose.model('Answer', {
answers: {
type: String,
},
number: {
type: Number,
default: 0
},
post: [{
type: mongoose.Schema.Types.ObjectId,
ref: 'Post'
}],
user: {
type: mongoose.Schema.Types.ObjectId,
ref: 'User'
},
createdAt: {
type: Date,
default: Date.now()
}
})
app.get('/posts/:id/answers', authenticateUser, async (req, res) => {
const { id } = req.params
const newAnswer = await Post.findById(id).populate('user', 'username').populate('answers').exec()
res.json({ success: true, newAnswer })
})
app.post('/posts/:id/answers', authenticateUser, async (req, res) => {
const { id } = req.params
const { _id } = req.user
const { answers, post } = req.body
try {
const user = await User.findById(_id)
// get answer and save post id
const newAnswer = new Answer({
answers: req.body.answers,
post: id,
user
})
// save answer
const result = await newAnswer.save()
// find post and push the comment into the array in the same moment
const answeredPost = await Post.findByIdAndUpdate(id, {
$push: {
answers: result
}
}).populate('user', 'username').populate('answers').exec()
res.json({ success: true, answeredPost})
} catch (error) {
res.status(400).json({ success: false, message: 'Invalid request', error })
}
})

Answer 1

Try this:
Post.findById(id).populate('user', 'username').populate({ path : 'answers', populate : { path : 'user'}}).exec()