DBRefs support

[daaru00]

In according with https://docs.mongodb.com/v3.2/reference/database-references/ I've try to do that:

db.collection("users").findOne({"username": "test"}, function(err, user){
    if(err) console.error(err);
    db.collection("test").save({
        "prop1": "val1",
        "prop2": "val2",
        "prop3": "val3",
        "user": { "$ref" : "users", "$id" : user._id}  //using mongodb syntax
    }, function(err, data){
        if(err) console.error(err);
        console.log(data);
        db.collection("test").find({}).toArray(function(err, results){
            if(err) console.error(err);
            console.log(results);
        })
    })
})

but I received an error: key $ref must not start with '$'.
If I try to using the native class DBRef() used in nodejs connector:

db.collection("users").findOne({"username": "test"}, function(err, user){
    if(err) console.error(err);
    db.collection("test").save({
        "prop1": "val1",
        "prop2": "val2",
        "prop3": "val3",
        "user": new DBRef("users", user._id) //using DBRef class
    }, function(err, data){
        if(err) console.error(err);
        console.log(data);
        db.collection("test").find({}).toArray(function(err, results){
            if(err) console.error(err);
            console.log(results);
        })
    })
})

I receiving an other error: ReferenceError: DBRef is not defined

I'm doing somethings wrong?