Mongoose nested query on Model by field of its referenced model

Question

It seems like there is a lot of Q/A's on this topic on stackoverflow, but I can't seem to find an exact answer anywhere.

What I have:

I have Company and Person models:

var mongoose = require('mongoose');
var PersonSchema = new mongoose.Schema{
                        name: String, 
                        lastname: String};

// company has a reference to Person
var CompanySchema = new mongoose.Schema{
                        name: String, 
                        founder: {type:Schema.ObjectId, ref:Person}};

What I need:

Find all companies that people with lastname "Robertson" have founded

What I tried:

Company.find({'founder.id': 'Robertson'}, function(err, companies){
    console.log(companies); // getting an empty array
});

Then I figured that Person is not embedded but referenced, so I used populate to populate founder-Person and then tried to use find with 'Robertson' lastname

// 1. retrieve all companies
// 2. populate their founders
// 3. find 'Robertson' lastname in populated Companies
Company.find({}).populate('founder')
       .find({'founder.lastname': 'Robertson'})
       .exec(function(err, companies) {
        console.log(companies); // getting an empty array again
    });

I still can query companies with Person's id as a String. But it's not exactly what I want as you can understand

Company.find({'founder': '525cf76f919dc8010f00000d'}, function(err, companies){
    console.log(companies); // this works
});

Answer 1

You can't do this in a single query because MongoDB doesn't support joins. Instead, you have to break it into a couple steps:

// Get the _ids of people with the last name of Robertson.
Person.find({lastname: 'Robertson'}, {_id: 1}, function(err, docs) {

    // Map the docs into an array of just the _ids
    var ids = docs.map(function(doc) { return doc._id; });

    // Get the companies whose founders are in that set.
    Company.find({founder: {$in: ids}}, function(err, docs) {
        // docs contains your answer
    });
});