problem deleting child categories when parent category is deleted

i had a Category pojo


public class Category {
protected List<Category> childCategories;

/**
* @return the childCategories
* @hibernate.list name="child_categories"
* @hibernate.key column="parent_id"
* @hibernate.one-to-many class="com.xxx.apps.xxx.model.Category" cascade="all-delete-orphan"
* @hibernate.list-index column="display_order"
*/
public List<Category> getChildCategories() {
return this.childCategories;
}
}

when i delete the parent all the childs should also be deleted.
childs with current id as parent_id shold be deleted,
but the update query is being generated by the hibernate instead of delete query

the query generated by hibernate is as follows

Hibernate: update hc_category set parent_id=null, display_order=null where parent_id=?

so it is giving the following error

ERROR http-8080-2 org.hibernate.util.JDBCExceptionReporter - ERROR: null value in column "parent_id" violates not-null constraint
ERROR http-8080-2 org.hibernate.event.def.AbstractFlushingEventListener - Could not synchronize database state with session
org.hibernate.exception.ConstraintViolationException: could not delete collection: [com.hvca.apps.catalog.model.Category.childCategories#16]
at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:71)
at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:43)
at org.hibernate.persister.collection.AbstractCollectionPersister.remove(AbstractCollectionPersister.java:1071)
at org.hibernate.action.CollectionRemoveAction.execute(CollectionRemoveAction.java:28)
at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:248)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:232)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:141)
at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:298)
at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:27)
at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1000)
at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:338)
at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:106)
at org.springframework.orm.hibernate3.HibernateTransactionManager.doCommit(HibernateTransactionManager.java:558)
at org.springframework.transaction.support.AbstractPlatformTransactionManager.processCommit(AbstractPlatformTransactionManager.java:540)
at org.springframework.transaction.support.AbstractPlatformTransactionManager.commit(AbstractPlatformTransactionManager.java:510)
at org.springframework.transaction.interceptor.TransactionAspectSupport.commitTransactionAfterReturning(TransactionAspectSupport.java:310)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:117)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:209)
at $Proxy22.removeCategory(Unknown Source)
at com.hvca.apps.catalog.service.dwr.impl.DWRCategoryManagerImpl.removeCategory(DWRCategoryManagerImpl.java:68)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at uk.ltd.getahead.dwr.impl.ExecuteQuery.execute(ExecuteQuery.java:239)
at uk.ltd.getahead.dwr.impl.DefaultExecProcessor.handle(DefaultExecProcessor.java:48)
at uk.ltd.getahead.dwr.impl.DefaultProcessor.handle(DefaultProcessor.java:81)
at uk.ltd.getahead.dwr.AbstractDWRServlet.doPost(AbstractDWRServlet.java:162)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:118)
at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.tuckey.web.filters.urlrewrite.UrlRewriteFilter.doFilter(UrlRewriteFilter.java:375)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:264)


please reply to this asp.