Mongoose nested query on Model by field by its referenced model.

mongoose-ref-model-service.js

/*
Find all companies that people with lastname "Robertson" have founded
*/
var mongoose = require('mongoose');

var PersonSchema = new mongoose.Schema {
                    firstname: String, 
                    lastname: String
                  };
var Person = mongoose.model('Person', PersonSchema);

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

// 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
    });
});