Mongoose populate empty collection with other collections - node.js

Question

I am trying to create a method for populating collection with a pre-defined Schema static method. I want it to grab all documents from 2 different collections and return them. It seems logical to me to create a separate Schema and return this.find().populate() callback as a result, but I can't get it to work...
Here are my Schemas:
dht.model.js
var mongoose = require('mongoose'),
Schema = mongoose.Schema;
// Dht Schema from MongoDB
var DhtSchema = mongoose.Schema({
location: {
type: String,
required: true
},
reading: {
type: String,
required: true
}
}, {
timestamps: {
createdAt: 'created_at',
updatedAt: 'updated_at'
},
});
co2.model.js
var mongoose = require('mongoose'),
Schema = mongoose.Schema;
// CO2 Schema from MongoDB
var CO2Schema = mongoose.Schema({
location: {
type: String,
required: true
},
reading: {
type: String,
required: true
}
}, {
timestamps: {
createdAt: 'created_at',
updatedAt: 'updated_at'
},
});
excel.model.js
This is the schema I want to populate dynamically, since new data is added to the database from MQTT middleware every 20 minutes and it has to return old and new results every time it has been invoked.
var mongoose = require('mongoose'),
Schema = mongoose.Schema;
// Dht Schema from MongoDB
var ExcelSchema = mongoose.Schema({
co2: [{
type: Schema.ObjectId,
ref: 'co2'
}],
dht: [{
type: Schema.ObjectId,
ref: 'dhts'
}]
});
ExcelSchema.statics.showAll = function (cb) {
return this.find().populate('co2 dht').exec(function (err, cb) {
if (err) {
return err;
}
return cb;
});
};
module.exports = mongoose.model('merged_sensors', ExcelSchema);
When method is invoked within the API, it freezes and returns 504. My only assumption is, that it can't access the collections for some reason, but I am, most likely, wrong.
Update:
Here is a controller, which makes an API call for that Collection:
excel.controller.js
var Xls = require('./../models/excel.model'),
express = require("express"),
router = express.Router();
router.get("/all", function (req, res) {
Xls.showAll(function (err, results) {
if (err) throw err;
res.json(results);
})
});
module.exports = router;