It’s not an issue with model export. I had the same issue.
The real issue is that require statements for the models
var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/news');
require('./models/Posts');
require('./models/Comments');
were below the routes dependencies. Simply move the mongoDB dependencies above the routes dependencies. This is what it should look like:
// MongoDB
var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/news');
require('./models/Posts');
require('./models/Comments');
var routes = require('./routes/index');
var users = require('./routes/users');
var app = express();