Mongoose Schema hasn't been registered for model

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();

If someone coudn't fix it with the approach of the correct answer (like me), try to look at the creation of the schema. I wrote the 'ref' as 'User', but the correct was 'user'.

Wrong:

createdBy: {
    type: Schema.Types.ObjectId,
    ref: 'User'
}

Correct:

createdBy: {
    type: Schema.Types.ObjectId,
    ref: 'user'
}

IF YOU USE MULTIPLE mongoDB CONNECTIONS

---

beware that when using .populate() you MUST provide the model as mongoose will only "find" models on the same connection. ie where:

var db1 = mongoose.createConnection('mongodb://localhost:27017/gh3639');
var db2 = mongoose.createConnection('mongodb://localhost:27017/gh3639_2');
var userSchema = mongoose.Schema({
  "name": String,
  "email": String
});

var customerSchema = mongoose.Schema({
  "name" : { type: String },
  "email" : [ String ],
  "created_by" : { type: mongoose.Schema.Types.ObjectId, ref: 'users' },
});

var User = db1.model('users', userSchema);
var Customer = db2.model('customers', customerSchema);

Correct:

Customer.findOne({}).populate('created_by', 'name email', User)

or

Customer.findOne({}).populate({ path: 'created_by', model: User })

Incorrect (produces "schema hasn't been registered for model" error):

Customer.findOne({}).populate('created_by');