Find mongoose objects with pointer

Question

I have a model like this...

var studentSchema = new Schema({
    classroomId: Schema.Types.ObjectId,
    // etc
});

var Student = mongoose.model('Student', studentSchema);

var classroomSchema = new Schema({
    // doesn't matter for this question
});

And I have a method on classroom who's job it is to return students. It seems to work with two different syntaxes...

classroomSchema.methods.students = function() {
    // this works
    return Student.find({ classroomId:this._id });

    // and this also seems to work?
    return Student.find({ classroomId:this });
}

Questions:

• Why do this and this._id both appear to generate the same results? Is it just syntax sugar?
• Can I rely on this in general? Like, can I assign an object rather than an object ID to a pointer attribute?
• Who is providing me this nice feature (if that's what it is), is it mongo or mongoose?

Answer 1

Okay so I did some research on this. Created classrooms and students collections as with some documents as shown below :

So it seems, the Native MongoDB driver doesn't return anything if we do something like this :

db.collection("classrooms").findOne({_id: 1}, function(err, classroom){
    console.log("Got classroom as : " + JSON.stringify(classroom));
    db.collection("students").find({classroomId: classroom}).toArray(function(err, students){
        if(err) console.log(err);
        else console.log(students);

        //Close connection
        db.close();
    });
});

It returns an empty array.

On the other hand if I do something like this :

db.collection("classrooms").findOne({_id: 1}, function(err, classroom){
    console.log("Got classroom as : " + JSON.stringify(classroom));
    db.collection("students").find({classroomId: classroom._id}).toArray(function(err, students){
        if(err) console.log(err);
        else console.log(students);

        //Close connection
        db.close();
    });
});

Then I get an array containing 3 students as are in my students collection with classroomId: 1

So I suppose this is the magic done by mongoose and NOT mongodb.

Also not quite sure if you can rely on this in general.

Hope this helps.