[hibernate-dev] Joins over type variable defined assocations NPE problems

[Steve Ebersole]

Well, to be clear I said to have PropertyMapping for the super to treat
that path as illegal...

On Wed, Mar 14, 2018 at 1:21 AM Christian Beikov <christian.beikov at gmail.com>
wrote:

> Thanks for the feedback. I tried removing the attribute from the
> PropertyMapping at first, but it didn't work out in the end. Not
> remembering the full details, but it had something to do with load plans
> trying to resolve the properties contained in the subclass propery closure
> against the super type. I didn't see a nice way to make this work, so I
> decided to register "null" and empty join columns for such unresolvable
> types. Now, in the load plan, I simply skip association attributes that
> have empty join columns.
>
> The fix passes the testsuite and the original reporter confirmed that the
> fix works for him.
>
> Mit freundlichen Grüßen,
> ------------------------------
> *Christian Beikov*
> Am 13.03.2018 um 20:06 schrieb Steve Ebersole:
>
> I think it is ok to disallow implicit downcasting in cases where the
> downcast is non-deterministic.  From you example domain, I think it is ok
> to disallow the implicit downcast in : `from BaseEntity e where
> e.relation.id = ...' with either a specific error ("cannot implicitly
> downcast BaseEntity to (EntityA, EntityB) due to named attributes being
> unrelated") or a general "un-resolved attribute" error.  On option to
> address that would be to change your registering a null, to instead
> recognizing that the passed property fits this description in its
> PropertyMapping impls and throw such an exception.
>
> And I think as long as you remove it from that entity's/component's
> various attribute-info arrays it should handle the SQL generation too.
>
> If you like that approach better...
>
> On Mon, Mar 12, 2018 at 12:02 PM andrea boriero <andrea at hibernate.org>
> wrote:
>
>> Hi Christian,
>> I have looked at your PR and at the moment I cannot find another easy
>> clean
>> solution for this issue not requiring as you said quite a lot of work.
>>
>>
>>
>> On 9 March 2018 at 20:20, Christian Beikov <christian.beikov at gmail.com>
>> wrote:
>>
>> > Hey,
>> >
>> > so the problems related to the type variable handling I did aren't fully
>> > solved. There is one other case that was reported today:
>> > https://hibernate.atlassian.net/browse/HHH-12375
>> >
>> > I'm not sure how to handle this. Let me explain you the model and the
>> > problem.
>> >
>> > @Entity class BaseEntity { @Id Integer id; }
>> > @Entity class EntityA extends BaseEntity { @ManyToOne EntityC relation;
>> }
>> > @Entity class EntityB extends BaseEntity { @ManyToOne EntityD relation;
>> }
>> > @Entity class EntityC {...}
>> > @Entity class EntityD {...}
>> >
>> > The essence is, that EntityA and EntityB define properties with the same
>> > names but have unrelated types. It's actually the same problem for
>> > collections.
>> >
>> > Since EntityA and EntityB extend BaseEntity and because BaseEntity is an
>> > @Entity, the Hibernate code tries to add "EntityA.relation" and
>> > "EntityB.relation" to the PropertyMapping of BaseEntity. Before my fix,
>> > the property which is added first won and subsequent adds are ignored.
>> > This is presumably done to support implicit downcasting. My fix was
>> > about making use of a common super type if possible and disallowing
>> > ambiguous access.
>> >
>> > The problem the user now reported is, when using FetchType.EAGER there
>> > is an exception which states that property "relation" could not be found
>> > on type "BaseEntity". This is because I register "null" as type to
>> > signal that it's not really possible to use that property with that
>> > persister(BaseEntity in this case). During booting, the load plan for
>> > BaseEntity is built, for which the SQL is pre-generated. At this point,
>> > the exception happens, because "null" is registered as type for the
>> > property "relation". This will cause the property to be considered as
>> > non-existant wrt getPropertyType. The code before my fix was essentially
>> > broken, because it doesn't handle eager fetching properly due to the
>> > "first property wins" strategy. A property might only be loaded through
>> > a secondary query due to lazy loading rather than by the defined fetch
>> > style. An implicit downcasted join using the property might lead to
>> > non-deterministic results.
>> >
>> > One of the reasons for doing the fix, was to prevent the possibility of
>> > being hit by the non-determinism. I had a mapping that was using the
>> > type variable approach and I had to find out the non-determinism the
>> > hard way. I'd like to prevent access to such conflicting attributes from
>> > the top level type through implicit downcasting, so IMO the only real
>> > solution is to exclude the conflicting attributes in the load plan.
>> >
>> > The "best" solution would be to introduce a synthetic type that
>> > represents the union of the unrelated property types, but to me this
>> > seems to be quite a lot of work.
>> >
>> > I have implemented the proposed solution here:
>> > https://github.com/hibernate/hibernate-orm/pull/2190
>> >
>> > We need to release this fix soon as the current state is broken for the
>> > EAGER configuration. What do you think about the solution or the matter
>> > in general?
>> >
>> > --
>> >
>> > Mit freundlichen Grüßen,
>> > ------------------------------------------------------------------------
>> > *Christian Beikov*
>> > _______________________________________________
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>> > hibernate-dev at lists.jboss.org
>> > https://lists.jboss.org/mailman/listinfo/hibernate-dev
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