Hello,

I'd like to know if it is at all possible to do a collection mapping of a class to the same class.

Eg, I have a class named Category, and this class contains a list of other categories (which are in fact subcategories).

Would Hibernate manage some how to achieve such a mapping? If not, why? Is it conceptually stupid? Should I just subclass Category by another class (Subcategory), but that would mean two SQL tables.

I have read the documentation and spent quite some time googling, but I did not find anything, so I 'll be happy to take any link or help (I am new to Hibernate).

Jean-Noël

Absolutely no problem. This case is even a part of the CaveatEmptor. See the "book forum" to see subject about it.

Guess what, in CaveatEmptor, there's a class called Category that is self-referencing :-). The only difference with what you want to do is that Category only has a many-to-one reference to its parent. But your mapping still will be ok if considering a list of children from Category.

To see if a problem is manageable with Hibernate, just verify that it's storable in a db. If yes, you're done :). In your case, it's just a column in the Category table which references its parent.

HTH.

Good to know that it can be done. However, I cannot manage to accomplish it. Here is my mapping:

<hibernate-mapping>
<class name="com.example.client.Category" table="ShopCategories">
<id name="id" column="id">
<generator class="native"/>
</id>
<property name="name" />

<set access="field" name="subcategories_list">
<key column="parent_category_id" />
<one-to-many class="com.example.client.Category"/>
</set>

<!-- The non-inverse side of the one-to-many/many-to-one association -->
<many-to-one name="parent_category"
class="com.example.client.ShopCategory"
access="field"
foreign-key="FK_CATEGORY_PARENT_ID">
<column name="parent_category_id"
not-null="false"/>
</many-to-one>

</class>
</hibernate-mapping>

I had a look at the CaveatEmptor example you mentionned, they are doing things a little bit differently there since they use a bag... like this:

<!-- The inverse side of the one-to-many/many-to-one association, we
use a bag mapping, iteration order of this collection is by
category name. -->

<bag name="childCategories"
cascade="save-update, merge"
inverse="true"
order-by="CATEGORY_NAME asc">
<key column="PARENT_CATEGORY_ID"/>
<one-to-many class="Category"/>
</bag>

Can anyone tell me what's wrong with my mapping?

Jean-Noël

EDIT: I forgot to ask that with this mapping, at run time I get the exception and following stack trace:

ERROR http-8080-Processor25 org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/Shop] - Exception while dispatching incoming RPC call
java.lang.ClassCastException: java.util.ArrayList
at org.hibernate.type.SetType.wrap(SetType.java:39)
at org.hibernate.event.def.WrapVisitor.processArrayOrNewCollection(WrapVisitor.java:84)
at org.hibernate.event.def.WrapVisitor.processCollection(WrapVisitor.java:51)
at org.hibernate.event.def.AbstractVisitor.processValue(AbstractVisitor.java:101)
at org.hibernate.event.def.WrapVisitor.processValue(WrapVisitor.java:98)
at org.hibernate.event.def.AbstractVisitor.processEntityPropertyValues(AbstractVisitor.java:55)
at org.hibernate.event.def.AbstractSaveEventListener.visitCollectionsBeforeSave(AbstractSaveEventListener.java:333)
at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:250)
at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:167)
at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:101)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:186)
at org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:33)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:175)
at org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:27)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
at org.hibernate.impl.SessionImpl.fireSave(SessionImpl.java:530)
at org.hibernate.impl.SessionImpl.save(SessionImpl.java:518)
at org.hibernate.impl.SessionImpl.save(SessionImpl.java:514)
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:585)
at org.hibernate.context.ThreadLocalSessionContext$TransactionProtectionWrapper.invoke(ThreadLocalSessionContext.java:301)
at $Proxy0.save(Unknown Source)
at com.example.server.ShopServicesImplementation.test(ShopServicesImplementation.java:80)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at com.google.gwt.user.server.rpc.RemoteServiceServlet.processCall(RemoteServiceServlet.java:281)
at com.google.gwt.user.server.rpc.RemoteServiceServlet.doPost(RemoteServiceServlet.java:167)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Thread.java:595)

I got it working. Turned out I was using a List on the Java side, when I used a Set in the mapping.

That's a good thing you added the exception. Could you provide your beans? I guess you specified a set mapping and are actually using a List or even an ArrayList in the code => ClassCastException...

Yes, as I posted just before you posted (you probably did not read that), I solved the problem, and it was due to me using a List on the Java side.