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Read/write DiscriminatorValue from/to DiscriminatorColumn when combined with InheritanceType#JOINED

Description

The JPA spec states that DiscriminatorColumn can be used in conjunction with JOINED inheritence. The only section that seems to deal with this combination is 11.1.10 DiscriminatorColumn Annotation where it says

For the SINGLE_TABLE mapping strategy, and typically also for the JOINED strategy, the persistence
provider will use a type discriminator column. The DiscriminatorColumn annotation is used to
define the discriminator column for the SINGLE_TABLE and JOINED inheritance mapping strategies.

Apparently this is meant to imply that even if providers do not need/use the discriminator for concrete class resolution (reading) they are still expected to write it out to the database. Dunno, thats just what we have been told 🙂

Anyway, like I said, Hibernate itself does not need the discriminator column/value to interpret the concrete classes. However, in the interest of portability we certainly could write it out.

Longer term we could possibly even use the DiscriminatorColumn/DiscriminatorValue (if present) instead of the SQL CASE statement thing we do now.

See https://hibernate.atlassian.net/browse/HHH-4358#icft=HHH-4358 for the origins of this request...

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RyanRJanuary 28, 2014 at 8:15 PM

Thanks for clearing this up. I removed the @DiscriminatorColumn annotation as suggested and now it does indeed work without the property set. I did some more googling and it seems JOINED inheritance is not well defined in JPA and varies based on vendor (http://en.wikibooks.org/wiki/Java_Persistence/Inheritance#Joined.2C_Multiple_Table_Inheritance). Some implementations require a discriminator column with JOINED inheritance.

Steve EbersoleJanuary 28, 2014 at 7:48 PM

Well you have combined joined inheritance with discriminator-based inheritance. Hibernate, unlike other JPA providers, does not need the discriminator to make joined inheritance work. This is all laid out in the discussion above.,

If you want to use joined inheritance and do not want to force Hibernate to use its legacy behavior by setting "hibernate.discriminator.ignore_explicit_for_joined" then simply remove the discriminator.

RyanRJanuary 28, 2014 at 7:41 PM

While upgrading from hibernate 3.6.5 to 4.3.1 I ran into this issue. Setting the property "hibernate.discriminator.ignore_explicit_for_joined" to "true" seems to make things work again. I am curious what code changes I need to make to get this to work without the property set to "legacy" behavior. The error I get without the property set is SQL Error 17003, SQLState 99999; Invalid column Index. The error occurs when saving a new hierarchical entity and Hibernate tries to insert a new record into the parent table. I'm on Oracle 10g Dialect. I've got the discriminator columns annotated in my entity as @Enumerated,(EnumType.STRING) and I have the entity constructors coded to set the discriminator column with the appropriate values. If I remove the set discriminator column code from the constructors then I get a constraint violation exception because the discriminator column isn't set (Beans Validation). If I remove the @NotNull then I'll end up with a "ORA-01400 cannot insert null into column" error. How is this supposed to work now?

Brett MeyerDecember 16, 2013 at 5:47 PM

pushed to 4.2

Steve EbersoleDecember 13, 2013 at 4:39 PM

Log staments added. All of them are debug (just to be able to track the flow through the decision point), except for the "no longer ignore explicit DiscriminatorColumn" case, which logs a WARN:

10:30:44,200 WARN AnnotationBinder:910 - HHH000457: Joined inheritance hierarchy [org.hibernate.test.joinedsubclass.JoinedSubclassWithExplicitDiscriminatorTest$Animal] defined explicit @DiscriminatorColumn. Legacy Hibernate behavior was to ignore the @DiscriminatorColumn. However, as part of issue HHH-6911 we now apply the explicit @DiscriminatorColumn. If you would prefer the legacy behavior, enable the `hibernate.discriminator.ignore_explicit_for_joined` setting (hibernate.discriminator.ignore_explicit_for_joined=true)

Bit wordy perhaps, but better to let upgraders know...

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Created December 21, 2011 at 4:00 AM
Updated July 14, 2016 at 7:47 PM
Resolved December 16, 2013 at 5:47 PM

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