Class hierarchy: how to HQL test class type in where clause?

davout_uk · **Joined:** Tue Apr 10, 2007 10:02 am **Posts:** 56

I have a hierarchy of classes mapped to a single table, with an integer discriminator column. In this situation is it possible to include a test of the class type in a HQL where clause?

For example, take a class mapping that has a common field called 'target' and the following class hierarchy....

Code:

     BaseClass
         |---SubClass1
         |---SubClass2

.... how would I create a HQL query that equates to ....

Code:

from BaseClass as m where (m.target = 1 and m.class = subClass1) or (m.target = 3 and m.class = subClass2)

Any suggestions?

Froestel · **Posted:** Fri Feb 05, 2010 12:30 pm

Hi davout_uk,

hm, I guess this is only possible if you'd be using content-based discrimination. Then you would have access to the discriminator in queries. Otherwise, I'm afraid the discriminator cannot be used from outside.

CU
Froestel

davout_uk · **Joined:** Tue Apr 10, 2007 10:02 am **Posts:** 56

I found a way of mapping to the discriminator, by adding a new read only property, as shown below...

Code:

<discriminator column="ActionDiscriminator" type="java.lang.Integer" />
<property name="typeID" insert="false" update="false" type="java.lang.Integer">
      <column name="ActionDiscriminator" />
</property>

Froestel · **Posted:** Thu Feb 11, 2010 12:41 pm

Hi davout_uk,

congrats on the solution. If it works - fine. This is very close to content-based discrimination as I suggested. I personally think, content-based discrimination is more elegant.

CU
Froestel

Francky · **Joined:** Thu Jun 30, 2005 1:04 pm **Posts:** 26

You can use query like this:

Code:

from Cat cat where cat.class = DomesticCat

Found it in the documentation
Then you request wille be :

Code:

from BaseClass as m where (m.target = 1 and m.class = SubClass1) or (m.target = 3 and m.class = SubClass2)

davout_uk · **Joined:** Tue Apr 10, 2007 10:02 am **Posts:** 56

What do you mean by "content-based discrimination"? I can't see anything listed on this topic in the link you provided.

Froestel wrote:

Hi davout_uk,

congrats on the solution. If it works - fine. This is very close to content-based discrimination as I suggested. I personally think, content-based discrimination is more elegant.

CU
Froestel

Froestel · **Posted:** Thu Feb 11, 2010 1:52 pm

Hi

Hibernate online documentation, see Section 23.4.4 Content based discrimination.

CU
Froestel