Hi,
> In rich domain entities it is often required to get a reference to an
EntityManager to be able to create and
> persist new entities or to perform queries.
JPA has deliberately took the route where a persistent domain model is
orthogonal to persistence context. A persistent object is *managed by* a
persistence context, but a persistent object *should not* operate on a
persistence context. This suggestion would violate that key decision.
The idea was not to save the developer by denying them a facility (that
does seem lucrative, but unhealthy in the long run, like fast food), but
to maintain a consistent semantics of the API itself (essentially we are
saving ourselves, not the developers :)
> I have the issue in every non-trivial project with a rich domain
object model.
The richness of a persistent domain object model stems from its ability
to warranty referential integrity (e.g. maintaining consistency of
bi-directional relational, or enforcing finite cardinality of a -to-many
relation etc) that relates to its persistent properties.
The example of Bike/Training cited shows a different aspect : computing
averages (essentially a set based computation that JPA does support via
query). The only issue is that computational logic should reside *outside*
the domain object scope in a place that has natural access to persistence
context and the persistent objects.
Regards --
Pinaki Poddar
Apache OpenJPA Chair
http://openjpa.apache.org/
JPA Expert Group Member
Application & Integration Middleware
From: Adam Bien <abien_at_adam-bien.com>
To: jsr338-experts_at_jpa-spec.java.net
Date: 06/18/11 03:53 AM
Subject: [jsr338-experts] EntityManager access in the entities
Hi All,
I would like to introduce a LifecycleListener and Lifecycle Call Back
methods like:
@PostAttach
public void postAttach(EntityManager manager){
}
@PreDetach
public void preDetach(){}
Reason:
In rich domain entities it is often required to get a reference to an
EntityManager to be able to create and persist new entities or to perform
queries.
Currently it is only possible with workarounds like using ThreadLocal or
ThreadSynchronizationRegistry. With an extension of the lifecycle listener
you would get
a reference to an EntityManager and will get notified in case it is no
more valid.
Injection of the EntityManager would be the best/easiest solution, however
JPA-entities are managed by the EntityManager and not by the container...
I have the issue in every non-trivial project with a rich domain object
model. There are, however, no issues with an anemic (
http://www.martinfowler.com/bliki/AnemicDomainModel.html) model, because
usually the EJBs / CDI managed beans do implement the business logic.
see also:
http://www.adam-bien.com/roller/abien/entry/why_sometimes_the_entitymanager_in
best regards,
adam