How can we achieve Joins in MongoDB? - mongodb

Question

I have two collections in db,
1. products (product_id, name, status, price, unit_price)
2. cart_details (cart_id, product_id, quantity)
Problem I am facing is, I want to show descriptive record on cart details page from these both collections. Where unit_price and product_name will come from products collection while quantity will be used from cart_details collection. I referred Mongo DB's own documentation it do returns me collection with nested objects but problem with that is if i want to update quantity in cart_details collection it throw an exception of "type mismatch".
Don't know if there is any other way to join tables and on update of one collection it won't throw any error.
here is the update cart code.
exports.update = function(req, res) {
var cart = req.cart;
cart = _.extend(cart, req.body);
cart.save(function(err) {
if (err) {
return res.status(400).send({
message: errorHandler.getErrorMessage(err)
});
} else {
res.json(cart);
}
});
};
and here is the cart service:
angular.module('core').factory('Cart', ['$resource',
function($resource) {
return $resource('cart/:cartId', { cartId: '#_id'
}, {
update: {
method: 'PUT'
}
});
}
]);