Accessing subdocument properties in mongoose - node.js

Question

I am using a MEAN stack to build this application.
Here is my subject.js schema:
var mongoose = require('mongoose');
var schema = mongoose.Schema;
var topics = require('./topic');
var subjectSchema = new schema({
_category : {
type: String,
default: ""
},
topics: [topics.schema]
});
module.exports = mongoose.model('Subject', subjectSchema);
and my topics.js schema:
var mongoose = require('mongoose');
var schema = mongoose.Schema;
var otherstuff = require('./otherstuff');
var otherstuff2 = require('./otherstuff2');
var topicSchema = new schema ({
title: String,
otherstuff: [mongoose.model('otherstuff').schema],
otherstuff2: [mongoose.model('otherstuff2').schema]
});
module.exports = mongoose.model('Topic', topicSchema);
What I am having difficulty with is how to access my topicSchema to populate it with forms from my front end.
I can save information to the subjectSchema, but not the sub documents.
I have tried using this as outlined in another article:
var Subject = mongoose.model('Subject', subjectSchema);
Subject.find({}).populate('subjects[0].topics[0].title').exec(function(err, subjects) {
console.log(subjects[0].topics[0].title);
});
But I continue to get TypeError: Cannot read property 'title' of undefined. How do I access the title property?

Answer 1

populate in mongoose is used to populate referenced documents, that are marked with ref attribute (see more info in the docs). Sub-documents on the other hand are available when do a simple query because they are actually an array of custom objects, so if you remove the populate method your query will work as expected:
Subject.find({}).exec(function(err, subjects) {
console.log(subjects[0].topics[0].title);
});