Mongoose: ObjectId Comparisons fail inconsistently

Question

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 us

Answer 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)

Answer 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.