Mongoose saving for populate - node.js

Question

I'm new to Mongoose and Nodejs developement in general and I've got a bit of confusion around how to properly set up saving my records. Here are my two schemas:
Download
var mongoose = require("mongoose");
var Schema = mongoose.Schema;
var downloadSchema = Schema({
title : String,
description : String,
_project : { type: Schema.Types.ObjectId, ref: 'Project' }
});
module.exports = mongoose.model('Download', downloadSchema);
Project
...
var projectSchema = Schema({
name : String,
url : String,
pwd : String,
_downloads : [{type: Schema.Types.ObjectId, ref: 'Download' }]
});
module.exports = mongoose.model('Project', projectSchema);
This appears to be working correctly. The documentation explains my use-case of saving a download and linking a project, but I'm not sure how to properly populate the Project._downloads. Here's what I've done:
Express route handler:
function createDownload(req, res) {
// the Project Id is passed in the req.body as ._project
var dldata = req.body;
Project.findOne({ _id : dldata._project }, function(err, project) {
var dload = new Download(dldata);
dload.save( function (err, download) {
project._downloads.push(download._id);
project.save( function(err){
var msg = {};
if(err) {
msg.status = 'error';
msg.text = err;
}else {
msg.status = 'success';
msg.text = 'Download created successfully!';
}
res.json(msg);
});
});
});
}
This seems overcomplicated to me. Am I supposed to be manually pushing to the ._downloads array, or is that something Mongoose is supposed to handle internally based on the schema? Is there a better way to achieve it so that I can do:
Download.find().populate('_project').exec( ...
as well as:
Project.findOne({_id : _projectId}).populate('_downloads').exec( ...

Answer 1

According to the mongoose docs there are 2 ways to add subdocs to the parent object:
1) by using the push() method
2) by using the create() method
So I think that your code can be a bit simplified by eliminating the operation of saving a new Download item:
function createDownload(req, res) {
var dldata = req.body;
Project.findOne({ _id : dldata._project }, function(err, project) {
// handle error
project._downloads.push(dldata);
project.save(function(err) {
// handle the result
});
});
}
or
function createDownload(req, res) {
var dldata = req.body;
Project.findOne({ _id : dldata._project }, function(err, project) {
// handle error
project._downloads.create(dldata);
project.save(function(err) {
// handle the result
});
});
}