How to deal with Symfony and doctrine merge

symfony_doctrine_merge.md

You should use merge operation on entities which are in detached state and you want to put them to managed state.

Merging should be done like this $entity = $em->merge($detachedEntity). After that $entity refers to the fully managed copy returned by the merge operation. Therefore if your $form contains detached entities, you should adjust your code like this:

$doctrineManager = $this->getDoctrine()->getManager();
foreach ($form->getData()->getEntities() as $detachedEntity) {
    $entity = $doctrineManager->merge($detachedEntity);  
    $doctrineManager->remove($entity);
}
$doctrineManager->flush();

However, in case that the $form does not contain detached entities, you should remove the merge operation, like this:

$doctrineManager = $this->getDoctrine()->getManager();
foreach ($form->getData()->getEntities() as $entity) {
    $doctrineManager->remove($entity);
}
$doctrineManager->flush();

This image should help you to understand entity state transitions. It is taken from Java Persistence API, but in Doctrine2 it is about the same.

Source: StackOverflow Reference: https://openjpa.apache.org/builds/1.2.3/apache-openjpa/docs/jpa_overview_em_lifecycle.html