MissingSchemaError: Schema hasn't been registered for model

How to reference another schema properly?

Error:

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

Model file:

// module dependencies
var mongoose = require('mongoose')
  , Schema = mongoose.Schema
  , CategoryMain = mongoose.model('CategoryMain')
  , CategorySub = mongoose.model('CategorySub');


// set up the schema
var CategoryProductSchema = new Schema({
  name: { type: String },
  _category_main : [CategoryMainSchema],
  _category_sub : [CategorySubSchema]

},
{
  collection: 'categories_product'
}
)

// before save function equivalent
CategoryProductSchema.pre('save', function(next){
  var now = new Date();
  this.updated_at = now;
  if ( !this.created_at ) {
    this.created_at = now;
  }
  next();
})

CategoryProductSchema.set('toObject', { getters: true });

mongoose.model('CategoryProduct', CategoryProductSchema);

---

EDIT

This is a small project I took over, and I'm new to MongoDB/Mongoose. I found this in app.js from the previous owner:

//load models
var models_path = __dirname + '/models/'
fs.readdirSync(models_path).forEach(function (file) {
  if(~file.indexOf('.js')){
    require(models_path + '/' + file);
  }
})

It simply goes through the folder and registers each schema one by one. However, in the folder my child schema is before my parent schema, so it's getting registered first.

I added a models.js files that looks like this:

var models = ['token.js',
'user.js', 
'category_main.js',
'category_sub.js',
'category_product.js',
'product.js'
];
exports.initialize = function() {
    var l = models.length;
    for (var i = 0; i < l; i++) {
        require(models[i]);
    }
};

And then replaced the initial code in app.js to call require this new models file like so:

require('./models/models.js').initialize();

I got this idea from one of the answers in this popular question:

mongoose schema creation

However, now I'm getting a ReferenceError: CategoryMainSchema is not defined coming from my category_sub.js model file.

It's not a MissingSchemaError, however.