Re: [eclipselink-users] multible fetch joins

[Michael Simons <michael.simons@xxxxxxxxxxx>]

Hi Tom,

so now I'm using hints on queries like
query.setHint("eclipselink.join-fetch", "e.address");

Unfortunately this doesn't work like expected, at least not always.

(Model: Article(1:N)LocalizedArticle

The following code
		query = em.createQuery ("SELECT a from Article a");
		query.setHint ("eclipselink.left-join-fetch", "a.locals");
		query.getResultList ();

leads to the following SQL:

[EL Fine]: 2009-12-01
18:42:20.748--ServerSession(8279577)--Connection(7438069)--Thread(Thread[main,5,main])--SELECT
t1.article_id, t1.needs_addtank, t1.ext_id, t1.jdo_version, t1.alt_ex_id, t1.product_id,
t1.caution_id, t0.localized_article_id, t0.nme, t0.jdo_version, t0.article_id, t0.lnguage_id
FROM article t1 LEFT OUTER JOIN localized_article t0 ON (t0.article_id = t1.article_id)

[EL Fine]: 2009-12-01
18:42:20.763--ServerSession(8279577)--Connection(7438069)--Thread(Thread[main,5,main])--SELECT
localized_article_id, nme, jdo_version, article_id, lnguage_id FROM localized_article WHERE
(article_id = ?)

The second statement is repeated for each Article.

I don't understand why the second statement is generated at all. All columns are already part of
the first statement.

Might this be a bug? I guess it might because this happens not always.
I assured that the association is FetchType=LAZY in both directions.
Any help would be appreciated.

-Michael


Tom Ware schrieb:
> Hi Michael,
> 
>   The limitation on JPQL fetch joins comes from the JPA specification.
> 
>   Have a look at the following link to see how you can get multiple
> fetch joins in all versions of EclipseLink.
> 
> http://wiki.eclipse.org/Using_EclipseLink_JPA_Extensions_%28ELUG%29#Join_Fetch
> 
> 
>   We are currently considering what the feature set will be for
> EclipseLink 2.1 and the following enhancement request covers expanding
> EclipseLink's JPQL capabilities beyond the JPA spec.  Please feel free
> to vote for it and add any comments you have.
> 
> https://bugs.eclipse.org/bugs/show_bug.cgi?id=219814
> 
> -Tom
> 
> 
> 
> Michael Simons wrote:
>> Hello [EclipseLink 1.2.0, database=MySQL 5],
>>
>> I'm trying to port a client/server application first to JPA to enable
>> the 3-tier migration.
>> At the moment I try to use EclipseLink as JPA implementation.
>>
>> EclipseLink 1.2.0 does not support multible fetch joins like
>> select a from A a fetch join A.b b ...
>> because the alias 'b' is already denied.
>>
>> Will this be supported in the 2.x of EclipseLink?
>>
>> Kind Regards,
>> Michael
>>
>> _______________________________________________
>> eclipselink-users mailing list
>> eclipselink-users@xxxxxxxxxxx
>> https://dev.eclipse.org/mailman/listinfo/eclipselink-users
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