Question

I am currently playing around with node.js and MongoDB using the node-mongo-native driver.

I tested a bit around using the Mongo console storing and retrieving JS objects. I figured out, that if I store an object that contains functions/methods the methods and functions will also be stored in the collection. This is interesting since I thought that functions could not be stored in MongoDB (with the exception of the system.js collection, as suggested by the Mongo docs).
Also it will not only store the methods but actually each method and member of the object’s entire prototype chain. Besides that I dont like this behaviour and think it’s unintuitive I mustn’t have it.

I was going to manage users in a Mongo collection. To do this I have a User object containing all of the users methods functioning as a prototype for each instance of an user. The user object itself would only contain the users attributes.

If I store a user in the Mongo collection I only want to store the own properties of the user object. No prototype members and especially no prototype methods. Currently I do not see how to cleanly do this. The options that I figured might work are:

1. creating a shallow copy using foreach and hasOwnProperty and storing this copy in the collection.
2. Add a data attribute to each user that contains all the object’s attributes and can be stored in the collection.
3. This just came to my mind writing this: I could also set all the prototypes properties to not enumerable which should prevent them from being stored in the collection.

However, I do have the same issues the other way around: when loading a user from a collection. AFAIK there is no way to change an objects prototype in JavaScript after it was created. And there’s also no way to specify a prototype to use when Mongo instantiates objects it retrieved from a collection. So basically I always get objects that inherit from Object using Mongo. As far as I can tell I have 2 options to restore a usable user object from this point on:

1. Create a fresh object inheriting from User and copying each attribute on the result object to the newly created object. (Compatible to storing mechanisms 1 & 3)
2. Create a fresh object inheriting from User and storing the result object as a data attribute on the newly created object. (Compatible to storing mechanism 2)

Are my assumptions, especially about the possibility to specify a prototype for query results, correct? What’s the right way to do it, and why? I’m surely not the first person struggling to store and resurrect objects in/from MongoDB using node.js.

Currently I would go with the approach 2/2. I don’t really like it, but it is the most efficient and the only one that works cleanly with the API. However, I’d much rather hear that actually the API does nothing wrong, but I do for not knowing how to use it correctly. So please, enlighten me 🙂

Answer 1

I just recently realized, that it actually is possible to change an objects prototype in V8/node. While this is not in the standard it is possible in various browsers and especially in V8/node!

function User(username, email) {
    this.username = username;
    this.email = email;
}

User.prototype.sendMail = function (subject, text) {
    mailer.send(this.email, subject, text);
};

var o = {username: 'LoadeFromMongoDB', email: '[e-mail]'};
o.__proto__ = User.prototype;
o.sendMail('Hello, MongoDB User!', 'You where loaded from MongoDB, but inherit from User nevertheless! Congratulations!');

This is used all over various modules and plugins – even core modules make use of this technique, allthough it is not ECMAScript standard. So I guess it is safe to use within node.js.