Rule Definition
If you require polymorphic associations (an association to a superclass, hence to all classes in the hierarchy with dynamic resolution of the concrete class at runtime) or queries, and sub-classes declare relatively few properties (particularly if the main difference between subclasses is in their behavior), lean toward the table-per-class-hierarchy model.

Remediation
Use the table-per-class-hierarchy strategy to map the hierarchy.

Violation Code Sample

----> animal.hbm.xml

<class name="Animal" table="ANIMAL">
    <id name="id" type="long" column="ANIMAL_ID">
        <generator class="native"/>
    </id>

    <property name="amount" column="AMOUNT"/>

     // VIOLATION none of the subclass attributes count 
     // is greater or equal to 3 and 
     // table-per-class-hierarchy strategy is not used

    <joined-subclass name="Dog" table="DOG">
        <key column="DOG_ID"/>
        <property name="race" column="RACE"/>
    </joined-subclass>

    <joined-subclass name="Cat" table="CAT">
        <key column="CAT_ID"/>
        <property name="color" column="COLOR"/>
    </joined-subclass>
</class>

Fixed Code Sample

<class name="Animal" table="ANIMAL">
    <id name="id" type="long" column="ANIMAL_ID">
        <generator class="native"/>
    </id>
    <discriminator column="TYPE" type="string"/>
    <property name="amount" column="AMOUNT"/>

    // FIXED: table-per-class-hierarchy strategy is used
    <subclass name="Dog" discriminator-value="DOG">
        <property name="race" column="RACE"/>
    </subclass>

    <subclass name="Cat" discriminator-value="CAT">
        <property name="color" column="COLOR"/>
    </subclass>
</class>

http://www.developer.com/open/print.php/10930_3559931_5 Hibernate in Action (ISBN 1932394-15-X) p 105