Symfony2 Doctrine merge

$em->merge() is used to take an Entity which has been taken out of the context of the entity manager and 'reattach it'.

• If the Entity was never managed, merge is equivalent to persist.
• If the Entity was detached, or serialized (put in a cache perhaps) then merge more or less looks up the id of the entity in the data store and then starts tracking any changes to the entity from that point on.

Cascading a merge extends this behavior to associated entities of the one you are merging. This means that changes are cascaded to the associations and not just the entity being merged.

I know this is an old question, but I think it is worth mentioning that $em->merge() is deprecated and will be removed soon. Check here

Merge operation is deprecated and will be removed in Persistence 2.0. Merging should be part of the business domain of an application rather than a generic operation of ObjectManager.

Also please read this doc v3 how they expect entities to be stored

https://www.doctrine-project.org/projects/doctrine-orm/en/latest/cookbook/entities-in-session.html#entities-in-the-session

It is a good idea to avoid storing entities in serialized formats such as $_SESSION: instead, store the entity identifiers or raw data.