Skip to main content
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [List Home]
Re: [eclipselink-users] How to get the "mappedBy" attribute name of a relation Attribute in a OneToOneMapping
Hello Mauro,

I misread your email and I guess my comment applies to the bug, not your email.  As stated, I don't believe the bug you filed is a bug.

If you are attempting to avoid the out join for this one use case that does not require it, I don't think anything in the JPA interfaces will directly help, and this is a provider specific optimization anyway that might not exist in other providers anyway.  You should not be accessing the mapping native api to build your query, but to decide which way to build it.  If the OneToOneMapping underneath has isForeignKeyRelationship set to true, then it owns the relationship and you can do the straight .  Otherwise it is what is referred to in native EclipseLink as a targetForeignKey relationship, where the target descriptor has/controls the foreign key, and you need to use a left outer join.

So something like:

DatabaseMapping mapping = ((SingularAttributeImpl) attribute).getMapping();
if (mapping.isOneToOneRelationships() && !((OneToOneMapping)mapping).isForeignKeyRelationship) {
  cq.where(cb.isNull(root.get(First_second)));
} else {
  cq.where(cb.isNull(root.join(First_second, JoinType.LEFT)));
}

Hope this helps.

Best Regards,
Chris





On 08/01/2016 12:10 PM, Mauro Molinari wrote:
Hi Chris,
thank you very much for your feedback.
Regarding the referenced bug, I understand your point of view (the specification dictates a general attitude on the matter), but the current behaviour is very counter-intuitive and brings to wrong results as soon as I simply change the owner of the one-to-one relationship. That is, a change in the mapping (which should reasonably be transparent) breaks a query in the object model, in which the intention to get "all the First instances that do not have a corresponding Second instances" is clear, rather than "give me always an empty result".

Anyway, the purpose of this message was not to bring that bug (or enhancement or what you like) to the devs attention (although that's good), but rather to request help to work around it, i.e. to implement the outer join you also suggest.

When processing the metamodel in a completely generic way, if I encounter the problematic case I need to get the foreign-key attribute of the owning side of the relationship starting from the non-owning side attribute (which is "mappedBy"). But I still couldn't succeed on it, as I wrote in my previous e-mail :-(

I'm now investigating on RelationshipAccessor class, but I've yet to figure out how to get an instance of it...

Thanks again,
Mauro

Il 08/01/2016 17:44, christopher delahunt ha scritto:
Hello Mauro,

Frameworks don't automatically treat the join as a left out join because traversing a path in JPA's criteria queries or JPQL is supposed to be treated as an inner join.  Section 4.4.4 of the JPA 2.1 specification requires that
"Path _expression_ navigability is composed using “inner join” semantics. That is, if the value of a non-terminal field in the path _expression_ is null, the path is considered to have no value, and does not participate in the determination of the result."
The fact that this works when the relationship is reversed is a result of EclipseLink optimizations that take advantage of the fact a join isn't required and not something intentionally done for null handling.  I believe I saw this behavior difference documented when the feature was done, I just don't have a link handy. 

If you want consistent, spec compliant behavior in your queries, you are required to specify the relationship use a left join in the query.  Ie:
  "Select first from First first left join first.second second where second is null"
or its equivalent in criteria expressions.

Best Regards,
Chris


One of the problems with considering this issue is that  This is why
On 08/01/2016 10:48 AM, Mauro Molinari wrote:
Hello all,
I'm trying to implement some sort of "findByExample" feature in my JPA application (using EclipseLink 2.6.1).
To do this, I'm inspecting the metamodel of my classes and dynamically creating predicates to be added to a query built with the Criteria API.

I encountered bug https://bugs.eclipse.org/bugs/show_bug.cgi?id=485414 which is causing me a lot of headache.
To work around it, I'm trying to implement the following logic: "if the attribute is mapping a bidirectional relationship which is not owned by this entity, then do an outer join and apply a condition on the corresponding relationship attribute of the target entity".

The simple case would be:
Predicate predicate = cb.equal(from.get(attribute), exampleValue);
which substantially is: "WHERE MyEntity.attribute = <exampleValue>"
But if exampleValue is NULL and attribute is a @OneToOne(mappedBy="inverseAttributeName"), so it's mapped to a non-owned relationship, I have to do a different thing to work around the aforementioned bug.
So, I have:
  • attribute, which is a SingularAttribute
  • attributeType, which is the attribute type (attribute.getType())
  • if attributeType.getPersistenceType() == PersistenceType.ENTITY (=> a relation attribute) then I get:
    DatabaseMapping mapping = ((SingularAttributeImpl) attribute).getMapping();
The DatabaseMapping object is promising, because it has the method getRelationshipPartnerAttributeName() (or even getRelationshipPartner()) which, if I understand it correctly, should give me the attribute name of the related entity, if this is a bidirectional mapping.
However what I see while debugging is that both methods always return null (also the protected field "mapping.mappedBy" is null!!), even if attribute is a SingularAttributeImpl of an attribute which is mapped like this:
@OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "inverseAttributeName", optional = true, orphanRemoval = true)
and the inverse mapping is present (so the relationship actually *is* bidirectional):
@Id @OneToOne(fetch = FetchType.EAGER, optional = false)

Something similar seems to be the fields mapping.targetToSourceKeyFields and mapping.sourceToTargetKeyFields, but those are maps containing DatabaseFields, not attributes, so I doubt I can use them to build my query (unless, of course, I am lucky and the attribute name matches the database column name).
On the other hand, I can't find anything else useful in SingularAttributeImpl (not to say the plain JPA interface...) to get the desired result.

I'm almost at it, but I already spent a lot of time on this final detail without success :-(
Any help would be *really* appreciated.

Thanks in advance,
Mauro


_______________________________________________
eclipselink-users mailing list
eclipselink-users@xxxxxxxxxxx
To change your delivery options, retrieve your password, or unsubscribe from this list, visit
https://dev.eclipse.org/mailman/listinfo/eclipselink-users



_______________________________________________
eclipselink-users mailing list
eclipselink-users@xxxxxxxxxxx
To change your delivery options, retrieve your password, or unsubscribe from this list, visit
https://dev.eclipse.org/mailman/listinfo/eclipselink-users



_______________________________________________
eclipselink-users mailing list
eclipselink-users@xxxxxxxxxxx
To change your delivery options, retrieve your password, or unsubscribe from this list, visit
https://dev.eclipse.org/mailman/listinfo/eclipselink-users

Back to the top