Mongoose: ObjectId Comparisons fail inconsistently

问题:

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 ObjectId 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?

回答1:

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:

story._id.equals(offref.ref)

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)

回答2:

This goes somewhat beyond the original asked question, but I have found that the .equals method of ObjectID's will return false in some cases where a string comparison will return true even when the values are not null. Example:

var compare1 = invitationOwningUser.toString() === linkedOwningUser.toString();
var compare2 = invitationOwningUser.equals(linkedOwningUser);
var compare3 = String(invitationOwningUser) === String(linkedOwningUser);
logger.debug("compare1: " + compare1 + "; " + "compare2: " + compare2 + "; " + "compare3: " + compare3);

Output:

compare1: true; compare2: false; compare3: true 

This occurred when invitationOwningUser (an ObjectID) came from a Collection created using a Mongoose schema, and linkedOwningUser (also an ObjectID) came from a Collection not created using Mongoose (just regular MongoDB methods).

Here is the document containing invitationOwningUser (the owningUser field):

{
    "_id" : ObjectId("5782faec1f3b568d58d09518"),
    "owningUser" : ObjectId("5781a5685a06e69b158763ea"),
    "capabilities" : [
        "Read",
        "Update"
    ],
    "redeemed" : true,
    "expiry" : ISODate("2016-07-12T01:45:18.017Z"),
    "__v" : 0
}

Here is the document containing linkedOwningUser (the owningUser field):

{
    "_id" : ObjectId("05fb8257c95d538d58be7a89"),
    "linked" : [
        {
            "owningUser" : ObjectId("5781a5685a06e69b158763ea"),
            "capabilities" : [
                "Read",
                "Update"
            ]
        }
    ]
}

So, as a bottom line for me, I'll be using the string comparison technique to compare ObjectID's, not the .equals method.