Mongodb authentication using MongoCredential - mongodb

Question

I have a grails application in which Im using db.authenticate for a login page but I understand this method has been deprecated and therefore I would like to upgrade my application to using the MongoCredential object for authentication. However, unlike the db.authenticate which nicely returns a boolean to get authentication done, the MongoCredential doesn't return a boolean so how can I go about accomplishing the code replacement with minimal headache. Ideally, I'd like to derive some kind of a boolean to tell me if authentication was achieved. Thanks for your patience. Im a newbie with Mongodb.
This is part of the code I need to replace which currently makes use of the deprecated method "authenticate":
MongoClient mongoClient = new MongoClient("localhost", 27017)
DB db = mongoClient.getDB("twcdb");
def userName = params.username
def passWord = params.password
//deprecated method being used in the line below.
boolean auth = db.authenticate(userName, passWord.toCharArray())
if (auth) {
userloggedin = params.username
render(contentType: 'text/json') {
[success: true, url: createLink(controller: 'admin', action: 'loggedin')]
}
}
else {
render(contentType: 'text/json') {
["success": false, "message": 'Login or Password is incorrect.']
}
Edit: I know that the answer must lie in testing a property of the MongoClient object somehow to see if it contains a valid authenticated connection but I am still stuck on how to do this. Given I knowingly feed the MongoClient constructor with a bogus MongoCredential, it still creates an object that isn't null. I was betting on the null test but no joy. So, how do I replace the deprecated db.authenticate?