ExpressJS require() load order and Mongoose missing ModelSchema - node.js

Question

I'm learning the MEAN stack and have found myself with a load order issue that doesn't seem to make sense.
The below code shows my server.js loading the routes file, which in turn pulls in the controller for a model, which in turn requires the model itself.
If I don't include a reference to the model from routes.js I get a MissingSchemeError when I startup the server. Why? Am I missing something regarding the loading of resources?
My understanding was that the exports for a file would be completely imported by the require() prior to attempting to run any code.
server.js
// modules =================================================
var express = require('express');
var bodyParser = require('body-parser');
var methodOverride = require('method-override');
var mongoose = require('mongoose');
var morgan = require('morgan');
var app = express();
// configuration ===========================================
// config files
var db = require('./config/db');
// set our port
var port = process.env.PORT || 8080;
// connect to our mongoDB database
mongoose.connect(db.url);
// get all data/stuff of the body (POST) parameters
// parse application/json
app.use(bodyParser.json());
// parse application/vnd.api+json as json
app.use(bodyParser.json({ type: 'application/vnd.api+json' }));
// parse application/x-www-form-urlencoded
app.use(bodyParser.urlencoded({ extended: true }));
// override with the X-HTTP-Method-Override header in the request. simulate DELETE/PUT
app.use(methodOverride('X-HTTP-Method-Override'));
// set the static files location /public/img will be /img for users
app.use(express.static(__dirname + '/public'));
// set morgan to log requests
app.use(morgan('dev'));
// routes ==================================================
require('./app/routes')(app);
routes.js
//this line is the problem. why is this needed?
var Customer = require('./models/customer'); // <--
var customers = require('./controllers/customer-server-controller');
module.exports = function(app) {
app.route('/api/customers')
.get(customers.list);
//.post(customer.create);
}
customer-server-controller.js
var mongoose = require('mongoose');
var Customer = mongoose.model('Customer');
/**
* List of Customers
*/
exports.list = function(req, res) {
Customer.find().sort('-created').exec(function(err, customers) {
if (err) {
return res.status(400).send({
message: "ERROR: " + err
});
} else {
res.jsonp(customers);
}
});
};
Got a good portion of the biolerplate from this tutorial on Scotch.io

Answer 1

I take it that you're missing the model from the tutorial
You should have this somewhere.
Create a models folder and add todo.js and add the following:
// define model =================
var Todo = mongoose.model('Todo', {
text : String
});

Answer 2

Your customer-server-controller.js doesn't require() the model file, it just tries to reference the model by asking Mongoose for it (Mongoose doesn't load the model file for you!):
var Customer = mongoose.model('Customer');
You need to require() the module file from your controller, otherwise the model isn't registered with Mongoose and you get the error that you got.