Mongoose many to many population with uni-directional references - node.js

Question

I have the following schemas which are used to represent a many-to-many relationship :
var CategorySchema = new Schema({
title: {type: String},
});
mongoose.model('Category', CategorySchema);
var ProductSchema = new Schema({
title: {type: String},
categories: [
{
type: Schema.ObjectId,
ref: 'Category'
}
]
});
mongoose.model('Product', ProductSchema );
When I query the Categories or the Products I want to be able to get in the result all the linked documents.
Populating the categories when querying the Product is straightforward:
Product.find().populate('categories').exec(...)
But how to do this from the Category side? I know I can add an array of ObjectId ref to the Product documents in the CategorySchema. But I'd like to avoid bi-directional referencing (I don't want to maintain it, and have a risk of inconsistency).
EDIT: here is the solution I implemented
/**
* List all Categories
*/
exports.all = function (req, res) {
//Function needed in order to send the http response only once all
//the categories' product has been retrieved and added to the returned JSON document.
function sendResponse(categories) {
res.json(categories);
}
AppCategory.list(function (err, categories) {
if (err) {
errors.serverError();
} else {
_.forEach(categories, function (category, index) {
category.products = [];
Product.byCategory(category._id, function (err, products) {
category.products= category.products.concat(products);
if (index === categories.length - 1) {
sendResponse(categories);
}
});
});
}
});
};
ProductSchema.statics = {
byCategory: function (categoryId, callback) {
this.find({'categories': categoryId})
.sort('-title')
.exec(callback);
}
};

Answer 1

You probably don't want to do that. :-) I would guess a product can be in some reasonably-small number of categories, but a category might have many thousands of products. In that case, trying to do Category.populate('products') is not going to work from an efficiency standpoint. You'll use lots of memory, not be able to do pagination in a straightforward way, load duplicate product data into memory when a product belongs to several categories, etc. Better to load the products in a category by querying directly against the products collection. You can filter by category easily enough a la Product.find({'categories._id': $in: arrayOfCategoryIds}}).