How to create an embedded document that follows a model with Mongoose?

Question

I have two models, Post and Comment:

My Post model (models/post.js):

var mongoose = require('mongoose');
var Comment = require('../models/comment');
var Schema = mongoose.Schema;

module.exports = mongoose.model('Post', new Schema({
    text: {type: String, trim: true},
    postedBy: String,
    comments: [Comment]
}));

My Comment model (models/comment.js):

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

module.exports = mongoose.model('Comment', new Schema({
    user: String,
    comment: {type: String, trim: true},
    created: {type: Date, default: Date.now(), select: false}
}));

When I attempt to create a new post without any comments, the post is created perfectly fine.

Although when I try to $push a comment to the post after creation, nothing happens.

Post.findOneAndUpdate(
    {"_id": req.params.id}, 
    {$push: {comments: {
        comment: "Hello World",
        user: "933ujrfn393r"
    }}
}).exec(function(err, post) {
    console.log(post);
    res.json({success: true});
});

Why is this failing to push the comment to the post? My console.log(post) line simply logs undefined, so not too sure what is happening here. I tried a simple test of Post.findOne({"_id": req.params.id}) and it returned the post successfully, so there is no problem with the find query.

Instead of using two different collections for this, use single collection and in that collection you can separately define schema for the field comments. — Jitendra Khatri, May 31 '16 at 06:44

Matt · Answer 1 · 2017-01-02T02:43:29.953

Embedded sub documents

Your usage implies an embedded sub document inside the model which only requires a schema definition for the sub document. This will store both schema's in a single document in a single collection in MongoDB

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var CommentSchema = new Schema({
    user: String,
    comment: {type: String, trim: true},
    created: {type: Date, default: Date.now(), select: false}
});

var PostSchema = new Schema({
    text: {type: String, trim: true},
    postedBy: String,
    comments: [CommentSchema]
});

module.exports = mongoose.model('Post', PostSchema);

Then create comments as you were.

Post.findOneAndUpdate(
    {"_id": req.params.id}, 
    {$push: {comments: {
        comment: "Hello World",
        user: "933ujrfn393r"
    }}
}).then(function (post) {
    console.log(post);
    res.json({success: true});
});

Document references

If you want to keep the two models then you would need to use a reference in your Post schema instead. This will create seperate documents in seperate collections in MongoDB and use the _id to look up the second document.

var PostSchema = new Schema({
    text: {type: String, trim: true},
    postedBy: String,
    comments: {
      type: mongoose.Schema.Types.ObjectId, 
      ref: 'Comment'
    }
});

Then comments need to be created before you can reference them in the Post model.

c = new Comment({ comment: 'x' })
c.save().then(function (result) {
  return Post.findOneAndUpdate(
    { _id: req.params.id },
    { $push: { comments: result._id } }
  );
}).then(function (result) {
  console.log('updated post');
});

Population can be used to easily retrieve the "foreign" documents.

If using the first method you mention (different models for each, although same document), how would I save the comment to the post document? I have tried creating the new comment like `c = new Comment({ comment: 'x' })` and then `$push`ing to the `Post`, although it doesn't seem to push anything... — Fizzix, May 31 '16 at 11:47
`new Comment` is only for the second method where `Comment` is a model. When comments are just a Schema and embedded documents, the query should work as you have in your question. Note that schema's and models are separate things in Mongoose. — Matt, May 31 '16 at 12:53

score 0 · Answer 2 · edited Jun 20 '20 at 09:12

0

Based on this question, I believe your problem is that you're embedding the Comment Model instead of the Comment Schema.

Try changing post.js from:

var Comment = require('../models/comment');

to:

var Comment = require('../models/comment').schema;

This also makes sense after looking at the example on the Mongoose docs regarding sub-docs.

P.S. What helped me investigate this was outputting the err object of the exec callback to see what was actually going on...

edited Jun 20 '20 at 09:12

Community

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answered May 31 '16 at 07:03

OzW

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That would work but creating a model also creates a separate collection in the database. If you are not using the model, then there's no point instantiating the model from the schema in the first place. – Matt May 31 '16 at 07:12

How to create an embedded document that follows a model with Mongoose?

2 Answers2

Embedded sub documents

Document references