'populate' and working with parent / child models in Mongoose / MongoDB - node.js

Question

I have a pretty simple setup where I'm trying to populate my Mongoose JSON responses with all Comments that belong to a Post
I thought that calling 'populate' on Post would return all comments related to the Post, but instead I'm getting an empty array. I just don't get what I'm doing wrong.
post.js
const mongoose = require('mongoose');
const db = require('./init');
const postSchema = new mongoose.Schema({
title: String,
url: String,
body: String,
votes: Number,
_comments: [{type: mongoose.Schema.Types.ObjectId, ref: "Comment"}]
});
const Post = mongoose.model('Post', postSchema);
module.exports = Post;
comment.js
const mongoose = require('mongoose');
const db = require('./init');
const commentSchema = new mongoose.Schema({
// post_id: post_id,
_post: { type: String, ref: 'Post'},
content: String,
posted: { type: Date, default: Date.now() }
});
const Comment = mongoose.model('Comment', commentSchema);
module.exports = Comment;
posts.js
router.get('/', function(req, res, next) {
// An empty find method will return all Posts
Post.find()
.populate('_comments')
.then(posts => {
res.json(posts)
})
.catch(err => {
res.json({ message: err.message })
})
});
and within the posts.js file I've set up a route to create a comment when a post request is sent to posts/post_id/comments
commentsRouter.post('/', function(req, res, next) {
console.log(req.params.id)
//res.json({response: 'hai'})
comment = new Comment;
comment.content = req.body.content;
comment._post = req.params.id
comment.save((err) => {
if (err)
res.send(err);
res.json({comment});
});
});
Comments are being created when I post to this route, and they are created with the correct _post value, however populate isn't picking them up.
For example, this post has been created, and it doesn't populate the associated comment below:
{
"post": {
"__v": 0,
"votes": 0,
"body": "Test Body",
"url": "Test URL",
"title": "Test Title",
"_id": "587f4b0a4e8c5b2879c63a8c",
"_comments": []
}
}
{
"comment": {
"__v": 0,
"_post": "587f4b0a4e8c5b2879c63a8c",
"content": "Test Comment Content",
"_id": "587f4b6a4e8c5b2879c63a8d",
"posted": "2017-01-18T10:37:55.935Z"
}
}

Answer 1

When you create a comment, you also have to save the comment instance _id to a post. So within the save() callback, you can do something like
commentsRouter.post('/', function(req, res, next) {
console.log(req.params.id)
//res.json({response: 'hai'})
comment = new Comment({
content: req.body.content;
_post: req.params.id
});
comment.save((err, doc) => {
if (err)
res.send(err);
Post.findByIdAndUpdate(req.params.id,
{ $push: { _comments: doc._id } },
{ new: true },
(err, post) => {
if (err)
res.send(err);
res.json({doc});
}
)
});
});