Hi Michael,
Regarding seeing OPENJPA-1365 on 'vanilla' OpenJPA 1.2.1, I'll have to take
a look at that. It probably happens if the query does not fetch records from
the database. If that's what you're seeing then I just need to update the
JIRA with that info.
I'm not terribly familiar with JDO. I was under the impression that
OpenJPA's FetchPlans were identical (or nearly identical) to those in JDO.
To use my example (Employee and Manager) you would do something like this :
OpenJPAEntityManager em = getEntityManager();
OpenJPAQuery q = em.createQuery("SELECT m from Manager m ");
q.getFetchPlan().addField(Manager.class, "employees");
List<Manager> managers = q.getResultList();
There's a lot more in the manual about it, I'd start looking at [1].
[1]
http://openjpa.apache.org/builds/1.2.1/apache-openjpa-1.2.1/docs/manual/manual.html#ref_guide_fetch
Hope this helps,
-mike
On Tue, Nov 17, 2009 at 11:02 AM, Michael Simons <[email protected]
> wrote:
> Hi Mike,
>
> Do you mean fetchplan like described in the manual in chapter 1.7.4?
> Or is there a possibility to use JDO-FetchPlan with OpenJPA? If so, is
> there a description, how
> to do that?
>
> kind regards,
> Michael
>
> Michael Dick schrieb:
> > Hi all,
> >
> > Daryl's correct, the duplicates returned by a JOIN FETCH clause are
> correct
> > behavior. If you don't want the duplicates to be returned you should use
> the
> > DISTINCT keyword and OpenJPA will remove duplicates from the list.
> >
> > There are several outstanding issues though (which I'm working on):
> >
> > 1.) OPENJPA-894: When results are returned from the database OpenJPA
> > automatically removes duplicates from the list. If the results are
> fetched
> > from memory the duplicates reappear.
> >
> > 2.) OPENJPA-1365: After you apply the fix for OPENJPA-894 the distinct
> > keyword doesn't work. This is because OpenJPA merely prepends the
> DISTINCT
> > keyword to the SQL generated which doesn't work if you're selecting
> across
> > multiple tables. Instead we need to filter the result list after
> retrieving
> > from the database.
> >
> > There are two proposed fixes for OPENJPA-894 each of which have some
> > drawbacks.
> >
> > 3a.) Mike's fix : supports pagination but does not support multiple JOIN
> > FETCH statements (ie SELECT m FROM Manager m JOIN FETCH m.employees JOIN
> > FETCH m.projects returns the wrong number of results).
> >
> > 3b.) Fay's fix : supported multiple JOIN FETCH statements, but does not
> > support pagination (ie query.setMaxResults(), query.setFirstResult()
> doesn't
> > page forward as expected).
> >
> > So there is work being done, but it's turned out to be a very ticklish
> issue
> > to solve.
> >
> > At the risk of muddying the waters a bit if you're migrating a JDO
> > application have you considered using OpenJPA's FetchPlan implementation
> to
> > eagerly load some fields? Over medium - large datasets I've found this to
> be
> > significantly faster than using a JOIN FETCH, but YMMV.
> >
> > Hope this helps,
> >
> > -mike
> >
> > On Tue, Nov 17, 2009 at 8:48 AM, Daryl Stultz <[email protected]>
> wrote:
> >
> >> On Tue, Nov 17, 2009 at 9:27 AM, Michael Simons
> >> <[email protected]>wrote:
> >>
> >>
> >>> You state, that you're query with distinct and join fetch does work
> >>> properly. But this would
> >>> mean OpenJPA-1365 doesn't occur, does it?
> >>>
> >> The JIRA states:
> >> This issue occurs if the proposed fix for OPENJPA-894 is in place.
> >>
> >> So 1365 does not occur unless you've patched your code such that 894 is
> >> fixed. What version of OpenJPA are you using and do you have any patches
> in
> >> place?
> >>
> >>
> >>> When we call "select a from A a join fetch B" we get n instances of A,
> >> with
> >>> n = numbers of
> >>> A-B-associations.
> >>>
> >> This is the correct behavior. I have found with OpenJPA 1.2.1, I get
> >> distinct rows of A which sounds like what you (and I) want but is
> improper.
> >> 894 shows that a second run of the query in the same EntityManager
> yields
> >> duplicates A's (with LEFT JOIN FETCH). What happens when you do this:
> >>
> >> select distinct a from A a join fetch a.bs
> >> ?
> >>
> >> How about these two:
> >> select a from A a left join fetch a.bs
> >> select distinct a from A a left join fetch a.bs
> >>
> >> I don't want to give the impression that I'm an expert on the matter,
> just
> >> that I've dealt with this issue and I want to be sure my understanding
> of
> >> things is accurate.
> >>
> >> --
> >> Daryl Stultz
> >> _____________________________________
> >> 6 Degrees Software and Consulting, Inc.
> >> http://www.6degrees.com
> >> mailto:[email protected]
> >>
> >
>
>
>
