Question

I want to implement a one-to-many relation using Hibernate (in Java). I have two entities:

• Experiment
• ExperimentGroup

Experiment has many ExperimentGroups. I tried to configure this one-to-many relation, the way the official Hibernate document recommends: http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html/example-parentchild.html#example-parentchild-cascades

My config file:

<class name="ExperimentImpl" table="Experiment">
    <id name="id" type="int" column="id">
        <generator class="increment" />
    </id>
    <!-- One to Many   -->
    <set name="experimentGroups" table="ExperimentGroup" 
                lazy="false" fetch="select" cascade="all" inverse="true">
            <key>
                <column name="Experiment_id" not-null="true" />
            </key>
            <one-to-many class="ExperimentGroupImpl" />
    </set>  
</class>
<class name="ExperimentGroupImpl" table="ExperimentGroup">
    <id name="id" type="int" column="id">
        <generator class="increment" />
    </id>
    <many-to-one name="experiment" class="ExperimentImpl" fetch="select">
            <column name="Experiment_id" not-null="true" />
    </many-to-one>
</class>

Adding a new ExperimentGroup to an Experiment works fine, but deleting an Experiment (and all of its ExperimentGroups) causes an exception:

Cannot delete or update a parent row: a foreign key constraint fails
(testdb.ExperimentGroup, CONSTRAINT FK9C71D86238B5500C FOREIGN
KEY (Experiment_id) REFERENCES Experiment (id))

My ExperimentDAO code looks like this:

public void deleteExperiment(Experiment experiment) 
         throws DAOException
    {
        Session session = null;
        Transaction t = null;
        try{
            session = HibernateUtil.getSessionFactory().openSession();
            t = session.beginTransaction();
                session.delete(experiment);
            t.commit();
            session.flush();
            session.close();
        }catch (HibernateException e)
        {
            e.printStackTrace();
            if (t != null)
                t.rollback();

            if (session != null)
                session.close();

            throw new DAOException(e, "Could not delete Experiment");
        }
    }

EDIT:
The DAO code to create an Experiment and ExperimentGroup:

public Experiment createExperiment(String name, String description, RegisteredUser owner)
{
    Transaction tx = null;
    Session session = null;
    try
    {
        session = HibernateUtil.getSessionFactory().openSession();

        tx= session.beginTransaction();
        ExperimentImpl e = new ExperimentImpl();

        e.setName(name);
        e.setDescription(description);
        e.setOwner(owner);

        owner.getOwnedExperiments().add(e);

        session.save(e);
        tx.commit();
        session.flush();
        session.close();

        return e;
    } catch (HibernateException ex )
    {
        if (tx != null)
            tx.rollback();

        if (session!=null)
            session.close();

        throw ex;
    }
}




public ExperimentGroup createAndAddExperimentGroup(Experiment experiment, String experimentGroupName, Date startDate, Date endDate) throws ArgumentException
    {

        Transaction t = null;
        Session session = null;

        try
        {
            session = HibernateUtil.getSessionFactory().openSession();
            t = session.beginTransaction();
                ExperimentGroupImpl g = new ExperimentGroupImpl();
                g.setEndDate(endDate);
                g.setStartDate(startDate);
                g.setName(experimentGroupName);
                g.setExperiment(experiment);

                experiment.getExperimentGroups().add(g);
                g.setExperiment(experiment);

                session.save(g);
                session.update(experiment);
            t.commit();
            session.flush();
            session.close();

            return g;

        }catch(HibernateException e)
        {
            if (t!=null)
                t.rollback();

            if (session!= null)
                session.close();

            throw e;
        }
    }

There are many other properties for an Experiment, like the name, description etc., which I have removed from the config file, because they don’t have an impact on the one-to-many relation.

Any suggestion? What did I wrong?

Answer 1

I am under the impression that you are trying to delete a detached entity that you manually created. I would try to use the primary key of the Experiment you want to delete, and load before actually deleting it:

session = HibernateUtil.getSessionFactory().openSession();
t = session.beginTransaction();
ExperimentImpl persistentExperiment = session.load(ExperimentImpl.class, experiment.getId());
session.delete(persistentExperiment);
t.commit();