Mam dwa pliki - jeden dla kategorii, a drugi dla usera.

Kategoria:
var users = require('./users');

var category = mongoose.model('categories', new mongoose.Schema({
    _id:          String,
    name:         String,
    id_owner:     [
      {type: String, ref: 'users'}
    ]
}));

user:
var categories = require('./dashboard');

var user = mongoose.model('users', new mongoose.Schema({
    _id:          String,
    login:        String,
    password:     String,
    email:        String,
    categories: [{
      id_category: String,
      ref: 'categories'
    }]
}));