Tuesday, September 14, 2010

Entity inheritance-single table per class hierarchy strategy with JPA + Hibernate

I happened to have the chance to implement entity inheritance using single table per class hierarchy strategy. At first I though I would sail smooth and arrive at the promised land easily :) but as usual with programming, it is a rare thing a code would sail smooth at the first time.
Persisting entity works like charm, retrieving it makes me speechless. Here's an example:

The base class (ItemReference)

Subclass#1 (ItemReferenceAge)

Subclass#2 (ItemReferenceSex)


One class that I don't include here is the class which has a collection of ItemReferenceSex and a collection of ItemReferenceAge, let's call this class Wrapper.

The scenario is I add 2 instances of ItemReferenceAge to wrapper collection and persisted it. What happened when I try to load the same wrapper instance is both collections are having elements in them. Remember, I only add ItemReferenceAge instance which means the collection of ItemReferenceSex should be empty.
Looking at the query generated shows that the discriminator column is not used in the where clause of the select query. What I can think of the design reasoning is that it is assumed that there will not be two or more type of instances loaded in the same parent object which in my case it is.
Luckily for me, there's a solution for this and unfortunately it's hibernate specific annotation (is it maybe because of JPA doesn't state how to handle my case or is it hibernate implementation?). All I need to do is to add ForceDiscriminator annotation in my base class and the problem is solved!

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