MongoDB Java - update a field in a record - mongodb

Question

I would like to update a field of a record which has been already been stored in the database. How can I achieve that? All of the examples I've come across so far are either in Python/Php or an older version of MongoDB (Java) and the api is not longer current.
Basically I would like to load an object with a particular userId in memory and change its username value. (I'd like to refer you to the code below)
Thank you so much
EDIT
So far I've got the following code:
public class UserDAO {
MongoOperations mongoDb;
public UserDAO(){
mongoDb = MongoDBInstanceFactory.getMongoDBinstance();
}
public User getUserByUsername(String username){
Query searchUserQuery = new Query(Criteria.where("username").is(username));
return mongoDb.findOne(searchUserQuery, User.class);
}
public User getUserById(String id){
Query searchUserQuery = new Query(Criteria.where("id").is(id));
return mongoDb.findOne(searchUserQuery, User.class);
}
public User addUser(String username){
mongoDb.save(new User(username));
return getUserByUsername(username);
}
}
#Document(collection = "users")
public class User {
public User(String username) {
this.username = username;
}
public User() {
}
#Id
private String id;
private String username;
//getter, setter, toString, Constructors
/**
* #return the id
*/
public String getId() {
return id;
}
/**
* #return the username
*/
public String getUsername() {
return username;
}
}
#RequestMapping(value = "/registerUser")
public #ResponseBody
String registerUser(#RequestParam(value = "username") String username, HttpServletResponse response) throws IOException {
sb = new StringBuilder();
user = userDao.getUserByUsername(username);
if (user == null) {
user = userDao.addUser(username);
userQueryPhaseDao.addUserQueryPhase(user.getId(), null, "0");
sb.append("1|").append(user.getId());
response.addCookie(new Cookie("userId", user.getId()));
} else {
sb.append("0|User with that handle already exists!");
}
return sb.toString();
}
Thank you for your time

Answer 1

To change an existing document, use the method DBCollection.update.
The method takes two parameters.
The first parameter tells MongoDB which document to update. It works exactly like find or findOne.
The second is the document that document will be replaced with. When you only want to update a single field and not replace the whole document, you need to use the $set operator.