This is because NP entities only exists in the memory of the server and the browser for the time they exists and not in the database. NP entities can refer to persitable entities but not the other way.
You can consider using a persistable entity without ever storing it.
Hi Marie ,
We can't able to create an association from persistable to non persistable.
You can but you have to make sure that the persistable entity does not refer to the non-persistable entity.
You can either have many to one (from non persistable to persistable) or
you can have many to many (where the non persistable is the owner of the association)