Trying to populate parent object's array but having trouble doing so - node.js

Question

So I have some users/posts/comments and I want to make sure that when I post a comment on a post, that user's comment array is updated to contain the comment they just made. I tried searching around and it seemed like the best way of doing this was to use mongoose's populate, but it doesn't seem to be working. I'm still a beginner with Mongoose so any help or direction would be appreciated. Thanks!
I tried something like this:
comment.save((err) => { //save the comment
if (err) return next(err);
res.status(200).json(comment);
});
User.find({username: username})
.exec((err, user) => {
user.comments.push(comment); // says comments is undefined, but should be []
user.comment_count++; // also, is there a way to set comment_count equal to the length of the comments array? Should I use .pre()?
user.save(() => {
if (err) return next(err);
});
});
This gives me a error like cannot push into users.comments (undefined).
Here are my Schemas:
const PostSchema = new Schema({
postedBy: {
type: String,
required: true
},
title: {
type: String,
required: true
},
content: {
type: String,
required: true
},
createdAt: {type: Date, default: Date.now},
comments: [{ type: Schema.Types.ObjectId, ref: 'CommentSchema' }],
likedBy: [User]
});
const CommentSchema = new Schema({
parentID: { type: String,
required: true,
},
postedBy: {
type: User,
required: true
},
content: {
type: String,
required: true
},
createdAt: {type: Date, default: Date.now},
editedAt: {type: Date, default: Date.now},
likedBy: [User],
});
const UserSchema = new mongoose.Schema({
username: {
type: String,
required: true,
},
email: {
comments: {
type: [{ type: Schema.Types.ObjectId, ref: 'CommentSchema' }],
default: [],
},
comment_count: { // how do I set this equal to the length of the comments array?
type: Number,
default: 0,
},
posts: {
type: [{ type: Schema.Types.ObjectId, ref: 'PostSchema' }],
default: [],
},
post_count: { // how do I set this equal to the length of the posts array?
type: Number,
default: 0,
},
});
I also tried population:
const comment = new Comment({
parentID,
postedBy,
content,
});
comment.save((err) => { //save the comment
if (err) return next(err);
res.status(200).json(comment);
});
User.find({ username: username })
.populate('comments')
.exec((err, user) => {
if(err) next(err);
console.log(user); // returns undefined for some reason
// also not sure what to do in this function...
});
This gives me an error like "cannot set headers after they are sent to the client."

Answer 1

first read more about how promises and call back worked.
quick fix would be.
comment.save((err) => { //save the comment
if (err) return next(err);
User.find({username: username})
.exec((err1, user) => {
if (err1) return next(err1);
res.status(200).json(user);
});
});
"cannot set headers after they are sent to the client." error occurred as you already send response.
res.status(200).json(comment);