Mongoose: popluate level 2 child object - mongodb

Question

I have the following code in a file "test.js", in which I am trying to populate story.fans[0].stories[0]
, but it doesn't work. The rest of the code runs fine, but when it is trying to populate fan[0]'s child object stories, it doesn't seem to work.
Can mongoose popluate 2 level child objects?
const mongoose = require('mongoose');
main().catch(err => console.log(err));
async function main() {
await mongoose.connect('mongodb://localhost:27017/testMongoose');
const Schema = mongoose.Schema;
const personSchema = Schema({
_id: Schema.Types.ObjectId,
name: String,
age: Number,
stories: [{ type: Schema.Types.ObjectId, ref: 'Story' }]
});
const storySchema = Schema({
author: { type: Schema.Types.ObjectId, ref: 'Person' },
title: String,
fans: [{ type: Schema.Types.ObjectId, ref: 'Person' }]
});
const Story = mongoose.model('Story', storySchema);
const Person = mongoose.model('Person', personSchema);
Story.
findOne({ title: 'Casino Royale' }).populate('fans').
exec(function (err, story) {
if (err) return handleError(err);
console.log('the story is',story.title);
console.log('The fans[0] is %s', story.fans[0].name);
story.fans[0].populate('stories');
console.log('the story written by fan is',story.fans[0].stories[0].title);
//option2
story.fans[0].populate('stories').exec(function(err,fan){
console.log('the story written by fan is',fan.stories[0].title);
});
});
}
Here is the error message:
----------------
This is my stories collection:
/* 1 */
{
"_id" : ObjectId("61cfd221256ef6d903523700"),
"author" : ObjectId("61cfd221256ef6d9035236fe"),
"title" : "Casino Royale",
"fans" : [
ObjectId("61cfee8b5059fb3fe37b3c5f")
],
"__v" : 1
}
/* 2 */
{
"_id" : ObjectId("61d09887abeb41f82a7e1678"),
"author" : ObjectId("61cfee8b5059fb3fe37b3c5f"),
"title" : "Story 001",
"fans" : [],
"__v" : 0
}
This is my people collection
/* 1 */
{
"_id" : ObjectId("61cfd221256ef6d9035236fe"),
"name" : "Ian Fleming",
"age" : 50,
"stories" : [],
"__v" : 0
}
/* 2 */
{
"_id" : ObjectId("61cfee8b5059fb3fe37b3c5f"),
"name" : "Fan 001",
"age" : 38,
"stories" : [
ObjectId("61d09fbfbd8f3fa20beaa616")
],
"__v" : 14
}

Answer 1

Considering the data provided by you, there is nothing in layer 2 to be populated, since the referenced id in Fan 001 is not matching with any of the stories.
Fans stories:
61d09fbfbd8f3fa20beaa616
==> is not found in
Available stories:
61d09887abeb41f82a7e1678
61cfd221256ef6d903523700
But if you fix your references what you want to be using is called deep Population
You basically populate a field in the object you just populated.
Your code then looks like this:
Story.findOne({ title: 'Casino Royale' })
.populate({ path: 'fans', model: 'Person', populate: { path: 'stories', model: 'Story' } }).
exec(function(err, story) {
if (err) return handleError(err);
console.log(story)
console.log('the story is', story.title);
console.log('The fans[0] is %s', story.fans[0].name);
console.log('the story written by fan is', story.fans[0].stories[0].title);
});
I rebuild the example but fixed the references and the output now looks like expected: