We didn’t want to create an overly large schema for storing objects so when we started getting into subclasses we looked for a solution to handle subclasses and store them in the same table as the superclass. Luckily Hibernate can handle this as well. The only downside to our approach was that we ended up with a table that has columns for all the properties of each subclass with not-null set to false. This means your data integrity will have to reside in your manager objects.

An example hibernate mapping for a super/subclass relationship looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
<class name="somepackage.model.DataItem" table="DATA_ITEM" polymorphism="implicit">
<id name="id" column="DATA_ITEM_ID" unsaved-value="0">
<!-- The "increment" generator would hose us in a clustered environment. -->
<generator class="increment" />
</id>
<discriminator column="DISCRIMINATOR" type="string" />

<subclass name="somepackage.model.ReportDataItem" discriminator-value="Report" />

<subclass name="somepackage.model.KpiDataItem" discriminator-value="KPI">
<property name="symbolID" column="SYMBOL_ID" not-null="false" />
</subclass>

</class>
</hibernate-mapping>

As you can see – set the polymorphism attribute to implicit and use the tags to create the mapping. You also need to define the discriminator to tell Hibernate what class to instantiate. In your DAO’s, pull back a list collection and you will have a collection of instances of classes based on your mapping.