Properly referencing another document?

hi:

I have a MongoDB object schema defined as follows:

module.exports = mongoose.model('Thingy', {
    user_id    : String,
    thing_stuff: String
});

During a request, I'd like to grab the user and associate this thing with him:

var user = req.user;
var thing = new Thingy({
    user_id : user._id,
    thing_stuff: "whatever"     
});

Is this the proper way to store a reference to another mongodb document?