Mongoose - pushing refs - cannot read property "push" of undefined - mongodb

Question

I would like to add a category and then if successed, push it's ref to user' collection. That's how I'm doing this:
That's mine "dashboard.js" file which contains categories schema.
var users = require('./users');
var category = mongoose.model('categories', new mongoose.Schema({
_id: String,
name: String,
ownerId: { type: String, ref: 'users' }
}));
router.post('/settings/addCategory', function(req, res, next) {
console.log(req.body);
var category_toAdd = new category();
category_toAdd._id = mongoose.Types.ObjectId();
category_toAdd.name = req.body.categoryName;
category_toAdd.ownerId = req.body.ownerId;
category.findOne({
name: req.body.categoryName,
ownerId: req.body.ownerId
}, function(error, result) {
if(error) console.log(error);
else {
if(result === null) {
category_toAdd.save(function(error) {
if(error) console.log(error);
else {
console.log("Added category: " + category_toAdd);
<<<<<<<<<<<<<<<<<<<THE CONSOLE LOG WORKS GOOD
users.categories.push(category_toAdd);
}
});
}
}
});
Here is my "users.js" file which contains "users" schema.
var categories = require('./dashboard');
var user = mongoose.model('users', new mongoose.Schema({
_id: String,
login: String,
password: String,
email: String,
categories: [{ type: String, ref: 'categories' }]
}));
So, the category add proccess works well and I can find the category in database. The problem is when I'm trying to push the category to user.
This line:
users.categories.push(category_toAdd);
I get this error:
Cannot read property "push" of undefined.
I need to admit once more that before that pushing there is console.log where the category is printed properly.
Thanks for your time.

Answer 1

The users object is a Mongoose model and not an instance of it. You need the correct instance of the users model to add the category to.
dashboard.js
...
category_toAdd = {
_id: mongoose.Types.ObjectId(),
name: req.body.categoryName,
ownerId: req.body.ownerId
};
// Create the category here. `category` is the saved category.
category.create(category_toAdd, function (err, category) {
if (err) console.log(err);
// Find the `user` that owns the category.
users.findOne(category.ownerId, function (err, user) {
if (err) console.log(err);
// Add the category to the user's `categories` array.
user.categories.push(category);
});
});