GAE with JPA: Update entity

Question

I have problems updating entities in Googles App Engine.

EntityManager em = ... // constructed like in the doc

MyEntity myE = new MyEntity();
myE.setType("1");      // String
em.persist(myE);em.refresh(myE);

myE.setType("2");
em.merge(myE);em.refresh(myE);

I expect a entity with type="2", but there is only one entity with type="1" :-(

Answer 1

That's the correct behaviour, let me explain (I assume that all your code runs in the same persistence context / transaction).

# This line sets the value in the in-memory object without changing the database
myE.setType("2");

# this line doesn't do anything here, as the entity is already managed in the current 
# persistence context. The important thing to note is that merge() doesn't save the 
# entity to the DB.
em.merge(myE);

# This reloads the entity from the DB discarding all the in-memory changes.
em.refresh(myE);

Answer 2

I was facing similar issue too. My issue is solved after my put the Reresh() after Commit().

It would be something like:

em.getTransaction().begin();
//Code to update the entity
em.persist(myE);
em.getTransaction().commit();
em.refresh(myE)

This will ensure the updated entity in JPA Cache gets refreshed with the updated data. Hope this helps.

Answer 3

It's because merge creates a new instance of your entity, copies the state from the supplied entity, and makes the new copy managed. You can find more info on merge vs. persist here and a full discussion about it here