merge in doctrine 2 not working giving error - doctrine-orm

Question

I am trying to update a row in a table. I am using doctrine 2 ORM. I am trying to update a row using merge(), which is said can be used to update a row. But it gives a error saying
Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry
I am new to doctrine 2. please suggest what can i do?

Answer 1

When to merge
First off: You only need $em->merge() when you have an entity that isn't managed by the EntityManager, but you want it to be. A common use-case is when you have a serialized entity, and want the EntityManager to start managing it.
So merging entities is not directly related to updating entities in the database.
If you simply find (using $repo->find*()) an entity and make changes, calling $em-flush() is sufficient. The entity is already managed by the EntityManager and there's no need to merge it.
How to merge
A common mistake when using $em->merge() is that the passed entity itself becomes managed. This isn't true, $em->merge() returns a new object that represents the managed entity.
$managedEntity = $em->merge($detachedEntity);
After this line of code, $detachedEntity is still detached (meaning it still isn't managed by the EntityManager). It's $managedEntity which you can start using to make changes.
Your code
Given the code you've put in the comments, you probably want to do something like this:
$user = $entityManager->getRepository('User')->find($_REQUEST['id']);
$user->setName($_REQUEST['name']);
$user->setPassword($_REQUEST['pass']);
$entityManager->flush();
PS: It looks like you're saving the the plain-text password in the database. That's never a good idea.