jta - Best way to handle JPA merge? -

i'm new whole jpa thing have multiple questions best way handle jpa merge , persist.

1. i have user object should updated (some values date , name). have merge passed object first or safer find new object?

   currently code updating user looks this:

   public void updateusername(user user, string name) {      // maybe first merge it?      user.setname(name);      user.setchangedate(new date());      em.merge(user);     } 

   how can sure user has not been manipulated before update method called? safer this:

    public void updateusername(int userid, string name) {      user user = em.getreference(user.class, userid);      user.setname(name);      user.setchangedate(new date());      em.merge(user);     } 

   maybe other solutions? watched multiple videos , saw many examples different , nobody explained best practice is.

2. what best approach add children relationships? example user object has connection multiple groups. should call jpa handler users , add new group user group list or should create group in group handler persist , add manually user object?

hope here has clue ;)

1. it depends on want achieve , how information have origin of object you're trying merge.

   firstly, if invoke em.merge(user) in first line of method or in end doesn't matter. if use jta , cmt, entity updated when method invocation finishes. difference if invoke em.merge(user) before changing user should use returned instance instead of parameter, either is:

   public void updateusername(user user, string name) {    user manageduser = em.merge(user);    manageduser.setchangedate(new date());    // no need of additional em.merge(-) here.    // jta automatically commits transaction business method. } 

   or

   public void updateusername(user user, string name) {    user.setchangedate(new date());    em.merge(user);    // jta automatically commits transaction business method. } 

   now updating entity.
   if want update well-defined fields in entity - use second approach it's safer. can't sure if client of method hasn't modified other fields of entity. therefore, em.merge(-) update them might not wanted achieve.

   on other hand - if want accept changes made user , override / add properties changedate in example, first approach fine (merge whole entity passed business method.) depends on use-case.

2. i guess depends on cascading settings. if want automatically persist / merge groups whenever user entity changed - it's safe add user's collection (something user#addgroup(group g) { groups.add(g)}. if don't want cascading, can create own methods propagate other side of relationship. might like: user#addgroup(group g) automatically invokes g.adduser(this);.