yannisf yannisf - 4 months ago 139
Java Question

About the use of @ForceDiscriminator/@DiscriminatorOptions(force=true)

Why is

@ForceDiscriminator
or its equivalent
@DiscriminatorOptions(force=true)
necessary in some cases of inheritance and polymorphic associations? It seems to be the only way to get the job done. Are there any reasons not to use it?

Answer

As I'm running over this again and again, I think it might help to clarify: First, it is true that Hibernate does not require discrimination when using JOINED_TABLE mapping. However, it does require it when using SINGLE_TABLE. Even more importantly, other JPA providers mostly do require it.

What Hibernate actually does when performing a polymorphic JOINED_TABLE query is to create a discriminator named clazz on the fly, using a case-switch that checks for the presence of fields unique for concrete subclasses after outer-joining all tables involved in the inheritance-tree. You can clearly see this when including the "hibernate.show_sql" property in your persistence.xml. In my view this is probably the perfect solution for JOINED_TABLE queries, so the Hibernate folks are right to brag about it.

The matter is somewhat different when performing updates and deletes; here hibernate first queries your root-table for any keys that match the statement's where clause, and creates a virtual pkTable from the result. Then it performs a "DELETE FROM / UPDATE table WHERE pk IN pkTable" for any concrete class withing your inheritance tree; the IN operator causes an O(log(N)) subquery per table entry scanned, but it is likely in-memory, so it's not too bad from a performance perspective.

To answer your specific question, Hibernate simply doesn't see a problem here, and from a certain perspective they are correct. It would be incredibly easy for them to simply honour the @DiscriminatorValue annotations by injecting the discriminator values during entityManager.persist(), even if they do not actually use them. However, not honoring the discriminator column in JOINED_TABLE has the advantage (for Hibernate) to create a mild case of vendor lockin, and it is even defensible by pointing to superior technology.

@ForceDiscriminator or @DiscriminatorOptions(force=true) sure help to mitigate the pain a little, but you have to use them before the first entities are created, or be forced to manually add the missing discriminator values using SQL statements. If you dare to move away from Hibernate it at least costs you some code change to remove these Hibernate specific annotations, creating resistance against the migration. And that is obviously all that Hibernate cares about in this case.

In my experience, vendor lockin is the paradise every market leader's wildest dreams are about, because it is the machiavellian magic wand that protects market share without effort; it is therefore done whenever customers do not fight back and force a price upon the vendor that is higher than the benefits reaped. Who said that an Open Source world would be any different?

p.s, just to avoid any confusion: I am in no way affiliated to any JPA implementor.

p.p.s: What I usually do is ignore the problem until migration time; you can then formulate an SQL UPDATE ... FROM statement using the same case-switch-with-outer-joins trick Hibernate uses to fill in the missing discriminator values. It's actually quite easy once you have understood the basic principle.

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