Hibernate Mapping Parent/Child Relationship

Question

I have a database table which contains a simple self parent/child relationship (categories) like this:

+------------+----------+--------------------+
| categoryid | parentid | category_name      |
+------------+----------+--------------------+
|          1 |        0 | Java               |
|          2 |        0 | SKM                |
|          3 |        0 | Neuigkeiten        |
|          4 |        0 | Neue Versionen     |
|          5 |        0 | PlugIn             |
..
|          9 |        2 | Subversion         |
|         10 |        2 | DVCS               |
|         11 |        2 | SVK                |
|         12 |        2 | Bazaar             |
|         13 |        2 | CVS                |
|         14 |        2 | SpectrumSCM        |
|         15 |        2 | Plastic SCM        |
|         16 |        2 | Monotone           |
|         17 |        2 | Mercurial          |
|         18 |        2 | ClearCase          |

Now I've created a Hibernate mapping class CategoryBO with the above Table like this (AbstractBaseBO contains only two public methods getId() and setId()..):

@Entity
@Table(name = TabellenNamen.CATEGORY)
public class CategoryBO extends AbstractBaseBO {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "categoryid", nullable = false, unique = true)
    private Long Id;

    @Column(name = "category_name")
    private String categoryName;

    @ManyToOne( cascade = { CascadeType.ALL } )
    @JoinColumn(name = "parentid")
    private CategoryBO parent;

    @OneToMany(mappedBy = "parent")
    private ArrayList<CategoryBO> subCategories = new ArrayList<CetegoryBO>();


    ...getter/setters

}

But if I try to get the list of category entries from the database i get the following error message:

2013-05-30 16:58:57,329|DEBUG|main|hibernate.cfg.AnnotationBinder||Processing annotations of com.soebes.casestudy.bo.CategoryBO.subCategories
2013-05-30 16:58:57,329|DEBUG|main|hibernate.cfg.Ejb3Column||Binding column null. Unique false. Nullable true.
2013-05-30 16:58:57,329|DEBUG|main|hibernate.cfg.Ejb3Column||Binding column subCategories. Unique false. Nullable true.
2013-05-30 16:58:57,331|DEBUG|main|hibernate.cfg.Ejb3Column||Binding column null. Unique false. Nullable true.
FAILED: testGet
org.hibernate.AnnotationException: Illegal attempt to map a non collection as a @OneToMany, @ManyToMany or @CollectionOfElements: com.soebes.casestudy.bo.CategoryBO.subCategories
    at org.hibernate.cfg.annotations.CollectionBinder.getCollectionBinder(CollectionBinder.java:266)
    at org.hibernate.cfg.AnnotationBinder.processElementAnnotations(AnnotationBinder.java:1448)
    at org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:754)
    at org.hibernate.cfg.AnnotationConfiguration.processArtifactsOfType(AnnotationConfiguration.java:546)
    at org.hibernate.cfg.AnnotationConfiguration.secondPassCompile(AnnotationConfiguration.java:291)
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1319)
    at org.hibernate.cfg.AnnotationConfiguration.buildSessionFactory(AnnotationConfiguration.java:867)
    at com.soebes.casestudy.hibernate.HibernateUtil.getSession(HibernateUtil.java:36)
    at com.soebes.casestudy.hibernate.HibernateUtil.beginTransaction(HibernateUtil.java:48)

which i don't understand.

So where is my misunderstanding or my problem (I assume something really stupid)...

Update:

After I have updated the code accordingly to the suggestion in the answer of Jimmy T like this:

@OneToMany(mappedBy = "parent")
private List<CategoryBO> subCategories;

I got the following error messages:

2013-05-30 19:08:33,363|DEBUG|main|hibernate.loader.Loader||done entity load
2013-05-30 19:08:33,364|INFO|main|event.def.DefaultLoadEventListener||Error performing load command
org.hibernate.ObjectNotFoundException: No row with the given identifier exists: [com.soebes.casestudy.bo.CategoryBO#0]
    at org.hibernate.impl.SessionFactoryImpl$2.handleEntityNotFound(SessionFactoryImpl.java:449)
    at org.hibernate.event.def.DefaultLoadEventListener.load(DefaultLoadEventListener.java:233)
    at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:285)
    at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:152)
    at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:1080)
    at org.hibernate.impl.SessionImpl.internalLoad(SessionImpl.java:1028)
    at org.hibernate.type.EntityType.resolveIdentifier(EntityType.java:623)
    at org.hibernate.type.EntityType.resolve(EntityType.java:431)
    at org.hibernate.engine.TwoPhaseLoad.initializeEntity(TwoPhaseLoad.java:140)
    at org.hibernate.loader.Loader.initializeEntitiesAndCollections(Loader.java:898)
    at org.hibernate.loader.Loader.doQuery(Loader.java:773)
    at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:270)
    at org.hibernate.loader.Loader.doList(Loader.java:2449)
    at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2192)
    at org.hibernate.loader.Loader.list(Loader.java:2187)
    at org.hibernate.loader.criteria.CriteriaLoader.list(CriteriaLoader.java:119)
    at org.hibernate.impl.SessionImpl.list(SessionImpl.java:1706)
    at org.hibernate.impl.CriteriaImpl.list(CriteriaImpl.java:347)
    at com.soebes.casestudy.dao.HibernateDAO.findAll(HibernateDAO.java:68)
    at com.soebes.casestudy.dao.HibernateDAO.get(HibernateDAO.java:75)
    at com.soebes.casestudy.dao.IdDAO.get(IdDAO.java:23)
    at com.soebes.casestudy.CategoryBOTest.testGet(CategoryBOTest.java:26)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:80)
    at org.testng.internal.Invoker.invokeMethod(Invoker.java:714)
    at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:901)
    at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1231)
    at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:128)
    at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:111)
    at org.testng.TestRunner.privateRun(TestRunner.java:767)
    at org.testng.TestRunner.run(TestRunner.java:617)
    at org.testng.SuiteRunner.runTest(SuiteRunner.java:334)

which I assume is caused by the rows where parentId is 0. Which makes sense, cause no category with the categoryid of 0 exists. But the question is: How can i map that to the meaning not having a parent category?

Update

So after updating my definitions in code like this:

@ManyToOne( cascade = { CascadeType.ALL } )
@JoinColumn(name = "parentid")
@NotFound(action = NotFoundAction.IGNORE)
private CategoryBO parent;

@OneToMany(mappedBy = "parent")
@NotFound(action = NotFoundAction.IGNORE)
private List<CategoryBO> subCategories;

Now i get a list of categories.

Answer 1

You should use Listinstead of ArrayList.

Hibernate may put in another List-implementation to support lazy loading.

EDIT: You also have to use NULL instead of 0 because otherwise Hibernate searches for the object with the id 0.