Hi,
Here's a problem that I have not been able to find a workaround to.
I'd be thankful for any help/insights.
I have two cmp-ejbs A and B, that have a many-to-many CMR-relation.
Starting from an instance of A i want to pre-load the related B-objects.
I try to optimize this by adding <left-join> tags to jbosscmp-jdbc.xml.
This works fine if there ARE some B-objects related to A , but if not
i get an exception "Internal error setting instance field IdB".
The reason seems to be that JBoss tries to initialize a B-object from a NULL-valued key-field.
Here is the sql that JBoss produces (table-names edited by me for clarity):
SELECT A.IdA, A.Description, B.IdB, B.Attribute FROM
A LEFT OUTER JOIN AtoB ON A.IdA=AtoB.IdA
LEFT OUTER JOIN Attribute B ON B.IdB=AtoB.IdB WHERE A.IdA=?
Here, the fields AtoB.IdB and B.IdB and B.Attribute will be null when
there is no match.
So, JBoss tries to construct a B-object from the read-ahead resultset
even if there is no such object!
I have not tested if this happens also for empty one-to-many relations
of if this is just a problem with many-to-many relations.
I've tried to change the type of IdB to be Integer instead of int and hence
allow null's. This works in a way, but requires that i also make all other fields in B
(e.g the field B.Attribute) into something that can be null.
I'e a B-object with all fields set to null will be created. I'm not sure i want that..
***
*** So, my questions are:
***
*** Is there a workaround?
*** Am I doing something wrong?
*** Is this fixed in later versions. (I'm using JBoss 3.2.5)
***
cheers,
andreas
