MissingSchemaError: Schema hasn’t been registered for model “User”

In my models/user.js file:

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var userSchema = new Schema({
    (define schema)
});
...
(save user)
...
(check password)
...
mongoose.model('User', userSchema);

And in my router/index.js, I have:

var mongoose = require('mongoose');
var User = mongoose.model('User');

which throws the error:

MissingSchemaError: Schema hasn't been registered for model "User".

If however, in user.js, I do (in the last line)

module.exports = mongoose.model('User', userSchema);

and in index.js I do var User = require('../models/User');, then everything works.

But it should not, because in config/pass.js I am doing var User = mongoose.model('User'); and it’s working flawlessly.

The require('../models/User'); syntax isn’t working on Ubuntu, but is on my Mac.

What should I do? How do I fix it? I have looked at tons of sample apps, including MEAN but nothing was really helpful.