Question

I am facing a strange problem and am unable to find a solution. I am trying to load a set of objects in Java using Hibernate from MySQl db.

These is a simplified version of my hibernate mappings and code:

<class name="org.Foo.Class1" table="class_profile" >
    <cache usage="read-write"/>
            <id name="id" column="id">
                <generator class="native"/>
            </id>
       <property name="amount" column="amount"/>
</class>
<class name="org.Foo.Class2" table="class_profile" >
    <cache usage="read-write"/>
            <id name="id" column="id">
                <generator class="native"/>
            </id>
       <property name="amount" column="amount"/>
</class>

This is my code to access, the objects:

public List<Class1> loadProfiles(final List<Integer> pIds)
{
  return (List<Class1>)getHibernateTemplate().executeFind(new HibernateCallback() {
     public Object doInHibernate(Session session) throws HibernateException {
    return session.createQuery("from Class1 il where il.id in (:idList)")
        .setParameterList("idList", pIds)
        .list();
    }
  });
}

Now, when I run my code

List<Class1> profiles = fooService.loadProfiles(Arrays.asList(3,4));

I get FOUR objects (instead of 2) in the list profiles – TWO Class1 objects and TWO Class2 objects. Where are the TWO Class2 objects coming from?

Answer 1

When you have inheritance between two entities, Hibernate needs to be able to tell which class a particular database row is supposed to represent. You do this by adding a discriminator class to the table that tells it which class to build to represent the row.

Class2 needs to be declared using a <subclass/> element nested inside Class1 that specifies what value of what column is used to tell them apart.

Detials here: http://docs.jboss.org/hibernate/orm/3.5/reference/en/html/inheritance.html#inheritance-tableperclass