Question

I have two classes, Employee and Manager. Manager extends Employee, as it is a type of employee. Employee has an instance of Manager, which represents the idea of an employee having one manager. Conversely, manager has a set of Employees. There are two tables, EMPLOYEE and MANAGER:

table EMPLOYEE
  long ID
  varchar NAME
  long MANAGERID

table MANAGER
  long ID
  long EMPLOYEEID //a join on this field enables inheritance

The classes look like this:

Employee.java (setters and getters omitted for brevity):

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@Table(name = "EMPLOYEE", schema = "TEST01")
public class Employee extends hata.util.Entity implements java.io.Serializable {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name = "ID", unique = true, nullable = false)   
    protected Long id;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "MANAGERID")
    protected Manager manager;

    @Column(name = "FIRSTNAME", nullable = false, length = 50)
    protected String firstname;

Manager.java

@Entity
@PrimaryKeyJoinColumn(name="EMPLOYEEID")
@Table(name = "MANAGER", schema = "TEST01")
public class Manager extends Employee implements java.io.Serializable {

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "manager")
    private Set<Employee> employees = new HashSet<Employee>(0);

I have filled the tables with a few employees, and all of those employees have the same manager, except for one employee – the one who is the manager. He himself has no manager. (If you’re following me correctly, then it should be obvious that I have 1 row in the manager table, with and EMPLOYEEID referring back to a row in the EMPLOYEE table that has null as it’s MANAGERID).

So, this setup looked correct to me, however when I tried to query all Employee objects with:

Query q = em.createQuery("select em from Employee em");
result = (List<Employee>) q.getResultList();

I get a nasty stack trace:

Caused by: org.hibernate.PropertyAccessException: could not set a field value by reflection setter of hata.staff.entity.Employee.manager
at org.hibernate.property.DirectPropertyAccessor$DirectSetter.set(DirectPropertyAccessor.java:151)
at org.hibernate.tuple.entity.AbstractEntityTuplizer.setPropertyValues(AbstractEntityTuplizer.java:586)
at org.hibernate.tuple.entity.PojoEntityTuplizer.setPropertyValues(PojoEntityTuplizer.java:231)
at org.hibernate.persister.entity.AbstractEntityPersister.setPropertyValues(AbstractEntityPersister.java:3824)
at org.hibernate.engine.TwoPhaseLoad.initializeEntity(TwoPhaseLoad.java:153)
at org.hibernate.loader.Loader.initializeEntitiesAndCollections(Loader.java:898)
at org.hibernate.loader.Loader.doQuery(Loader.java:773)
at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:270)
at org.hibernate.loader.Loader.doList(Loader.java:2294)
at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2172)
at org.hibernate.loader.Loader.list(Loader.java:2167)
at org.hibernate.loader.hql.QueryLoader.list(QueryLoader.java:448)
at org.hibernate.hql.ast.QueryTranslatorImpl.list(QueryTranslatorImpl.java:363)
at org.hibernate.engine.query.HQLQueryPlan.performList(HQLQueryPlan.java:196)
at org.hibernate.impl.SessionImpl.list(SessionImpl.java:1258)
at org.hibernate.impl.QueryImpl.list(QueryImpl.java:102)
at org.hibernate.ejb.QueryImpl.getResultList(QueryImpl.java:236)
… 108 more
Caused by: java.lang.IllegalArgumentException: Can not set hata.staff.entity.Manager field hata.staff.entity.Employee.manager to hata.staff.entity.Employee
at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:164)
at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:168)
at sun.reflect.UnsafeObjectFieldAccessorImpl.set(UnsafeObjectFieldAccessorImpl.java:81)
at java.lang.reflect.Field.set(Field.java:680)
at org.hibernate.property.DirectPropertyAccessor$DirectSetter.set(DirectPropertyAccessor.java:139)
… 124 more

Did I code this wrong, or can hibernate simply not handle this scenario? I’d appreciate any assistance.

Answer 1

The EMPLOYEEID column in manager shouldn’t be there. @PrimaryKeyJoinColumn means that the primary key is also the join column. So, the ID column in MANAGER is both the primary key and the foreign key to EMPLOYEE.

And the annotation should of course be changed to @PrimaryKeyJoinColumn(name="ID") (or it must be removed, since the default is to use the same column name as in the parent table)

See http://download.oracle.com/javaee/6/api/javax/persistence/PrimaryKeyJoinColumn.html.