Elf Sternberg Elf Sternberg - 3 months ago 12
Node.js Question

Mongoose: ObjectId Comparisons fail inconsistently

I have a straightforward tool for building collections of documents and then automatically formatting them for EPUB or LaTeX rendering, written on top of ExpressJS. I'm using Coffeescript, if that matters (I doubt it).

Using Mongoose, I have the following:

DocumentSchema = new Schema
title: String

Offrefs = new Schema
ref: { type: ObjectId }
isa: String

BinderSchema = new Schema
title: String
contains: [Offrefs]

Offrefs doesn't specify what it refers to because because I want to be able to contain some binders in other binders, to create logical collections: "These are for the printer," "These are for epub," "These are web only," etc. (I've stripped out all the miscellaneous stuff out.)

Unfortunately, I have run into queries where, for retrieved objects

(story._id == offref.ref) -> True

And the two do indeed look the same. But:

(binder._id == offref.ref) -> False
(String(binder._id) == String(offref.ref)) -> True

And a visual comparison of the two references in the last two, they are the same ID number, but the
objects don't compare correctly.

I don't want to have to do string conversions constantly, which is a strong possiblity when I'm converting these complex objects into trees of data. Tree relationships are a bear in any DB; they shouldn't be difficult in MongoDB.

How do you do ObjectId comparisons in MongoDB?


A straight == (or ===) comparison will compare the two objects by reference, not value. So that will only evaluate to true if they both reference the very same instance.

Instead, you should be using the equals method of ObjectID to compare their values:


As @bendytree notes in the comments, if either value could be null (and you want nulls to compare as equal), then you can use the following instead:

String(story._id) === String(offref.ref)