Comparing mongoose _id and strings

246
votes

I have a node.js application that pulls some data and sticks it into an object, like this:

var results = new Object();

User.findOne(query, function(err, u) {
    results.userId = u._id;
}

When I do an if/then based on that stored ID, the comparison is never true:

if (results.userId == AnotherMongoDocument._id) {
    console.log('This is never true');
}

When I do a console.log of the two id's, they match exactly:

User id: 4fc67871349bb7bf6a000002 AnotherMongoDocument id: 4fc67871349bb7bf6a000002

I am assuming this is some kind of datatype problem, but I'm not sure how to convert results.userId to a datatype that will result in the above comparison being true and my outsourced brain (aka Google) has been unable to help.

7
node.jsmongodbmongoose

7 Answers

427
votes

Mongoose uses the mongodb-native driver, which uses the custom ObjectID type. You can compare ObjectIDs with the .equals() method. With your example, results.userId.equals(AnotherMongoDocument._id). The ObjectID type also has a toString() method, if you wish to store a stringified version of the ObjectID in JSON format, or a cookie.

If you use ObjectID = require("mongodb").ObjectID (requires the mongodb-native library) you can check if results.userId is a valid identifier with results.userId instanceof ObjectID.

Etc.

68
votes

ObjectIDs are objects so if you just compare them with == you're comparing their references. If you want to compare their values you need to use the ObjectID.equals method:

if (results.userId.equals(AnotherMongoDocument._id)) {
    ...
}
27
votes

converting object id to string(using toString() method) will do the job.

9
votes

According to the above,i found three ways to solve the problem.

  1. AnotherMongoDocument._id.toString()
  2. JSON.stringify(AnotherMongoDocument._id)
  3. results.userId.equals(AnotherMongoDocument._id)
8
votes

The accepted answers really limit what you can do with your code. For example, you would not be able to search an array of Object Ids by using the equals method. Instead, it would make more sense to always cast to string and compare the keys.

Here's an example answer in case if you need to use indexOf() to check within an array of references for a specific id. assume query is a query you are executing, assume someModel is a mongo model for the id you are looking for, and finally assume results.idList is the field you are looking for your object id in.

query.exec(function(err,results){
   var array = results.idList.map(function(v){ return v.toString(); });
   var exists = array.indexOf(someModel._id.toString()) >= 0;
   console.log(exists);
});
2
votes

The three possible solutions suggested here have different use cases.

  1. Use .equals when comparing ObjectId on two mongoDocuments like this
results.userId.equals(AnotherMongoDocument._id)
  1. Use .toString() when comparing a string representation of ObjectId to an ObjectId of a mongoDocument. like this
results.userId === AnotherMongoDocument._id.toString()
1
votes

I faced exactly the same problem and i simply resolved the issue with the help of JSON.stringify() as follow:- 

if (JSON.stringify(results.userId) === JSON.stringify(AnotherMongoDocument._id)) {
        console.log('This is never true');
}