Question about many-to-one relation and polymorphism

gewuerzgurke · **Joined:** Tue Sep 09, 2008 7:53 am **Posts:** 3

Hi everyone,

I'm using the latest hibernate version (=3.3.0 SP1), PostgreSQL (=8.2.4) and Spring (=2.5.5).
The problem that stresses me is about a many-to-one relation that points to a entity that has
a inheritance mapping:

Code:

<class name="sample.Parent" table="parent">

        <meta attribute="scope-class">
           public abstract
        </meta>
        <meta attribute="implements">
           FooInterface
        </meta>  

        <cache usage="read-write"/>

        <id name="id" type="java.lang.Integer" column="parent_id">
            <meta attribute="scope-set">protected</meta>
            <generator class="increment"/>
        </id>
        
        <property name="someParentProperty"/>

   <joined-subclass name="sample.Child1" table="child_one">
      <key column="child_one_id"/>
      <property name="someChildProperty"/>
   </join-subclass>

   <joined-subclass name="sample.Child2" table="child_two">
      <key column="child_two_id"/>
      <property name="someOtherChildProperty"/>
   </join-subclass>
</class>

The mapping that refers to the inheritance mapping:

Code:

<class name="sample.Shop" table="shop">
   <cache usage="read-write" />

        <id name="id" type="java.lang.Integer" column="shop_id">
            <meta attribute="scope-set">protected</meta>
            <generator class="increment"/>
   </id>

   <many-to-one name="parent" class="sample.Parent" column="parent_id" not-null="false" />
</class>

Now when I get an Instance (managed by hibernate) from sample.Shop and try to cast the result of
getParent() to sample.Child1 (because I know that the instance must be of the type) the following
error is thrown:

java.lang.ClassCastException: sample.Parent_$$_javassist_28 cannot be cast to sample.Child1
at com.infinconsystems.tests.spring.HibernateTest.testRiskAnswers(HibernateTest.java:22)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at junit.framework.TestCase.runTest(TestCase.java:168)
at junit.framework.TestCase.runBare(TestCase.java:134)
at junit.framework.TestResult$1.protect(TestResult.java:110)
at junit.framework.TestResult.runProtected(TestResult.java:128)
at junit.framework.TestResult.run(TestResult.java:113)
at junit.framework.TestCase.run(TestCase.java:124)
at junit.framework.TestSuite.runTest(TestSuite.java:232)
at junit.framework.TestSuite.run(TestSuite.java:227)
at org.junit.internal.runners.JUnit38ClassRunner.run(JUnit38ClassRunner.java:81)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:38)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:460)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:673)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:386)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:196)

I've already found some posts facing this problem.

So two questions:
a) Is the one and only solution to use proxy="" at the joined-subclass and the superclass?
b) Is hibernate's codegen able to produce an interface and an implementing class that I could use at a) ?
c) When there is another way, how it's implemented :) ?

Best regards,
Alexander Müller

Froestel · **Posted:** Tue Sep 29, 2009 3:56 pm

Hi gewuerzgurke,

I also had problems with this issue. I searched quite a while for a proper solution but found only the interface-way.

I managed to handle it by implementing an interface for each class in the inheritance tree and used them for mapping the proxy-attribute. The entity class and the generated proxy class therefore inherit from the same parent interface to which you can then cast the elements of your result sets. Works out fine so far.

CU
Froestel

gewuerzgurke · **Joined:** Tue Sep 09, 2008 7:53 am **Posts:** 3

Hi froestel,

I've fixed this Problem by enabling the bytecode instrumentation ;) let me know if you want to See the solution.

Best regards,
Alex

Froestel · **Posted:** Tue Sep 29, 2009 5:41 pm

Hi gewuerzgurke,

I tried that, too, but for some reason it didn't work. But I didn't bother with it.

What are your performance experiences with bytecode instrumentation? Is it "expensive"?

CU
Froestel

gewuerzgurke · **Joined:** Tue Sep 09, 2008 7:53 am **Posts:** 3

No I haven't seen any differences in runtime-performance. The only con is that you've to implement this bytecode instrumentation into the build process, that was tricky :)

bgranvea · **Joined:** Wed Sep 09, 2009 8:49 am **Posts:** 3

Hi,

Can you please explain this bytecode instrumentation trick to avoid defining interfaces?

Thanks.