Chapter 9. Inheritance Mapping

9.1. The Three Strategies

Hibernate supports the three basic inheritance mapping strategies:
  • table per class hierarchy
  • table per subclass
  • table per concrete class
In addition, Hibernate supports a fourth, slightly different kind of polymorphism:
  • implicit polymorphism
It is possible to use different mapping strategies for different branches of the same inheritance hierarchy, and then make use of implicit polymorphism to achieve polymorphism across the whole hierarchy. However, Hibernate does not support mixing <subclass>, and <joined-subclass> and <union-subclass> mappings under the same root <class> element. It is possible to mix together the table per hierarchy and table per subclass strategies, under the the same <class> element, by combining the <subclass> and <join> elements (see below).
It is possible to define subclass, union-subclass, and joined-subclass mappings in separate mapping documents, directly beneath hibernate-mapping. This allows you to extend a class hierachy just by adding a new mapping file. You must specify an extends attribute in the subclass mapping, naming a previously mapped superclass. Note: Previously this feature made the ordering of the mapping documents important. Since Hibernate3, the ordering of mapping files does not matter when using the extends keyword. The ordering inside a single mapping file still needs to be defined as superclasses before subclasses.
 <hibernate-mapping>
     <subclass name="DomesticCat" extends="Cat" discriminator-value="D">
          <property name="name" type="string"/>
     </subclass>
 </hibernate-mapping>

9.1.1. Table per class hierarchy

Suppose we have an interface Payment, with implementors CreditCardPayment, CashPayment, ChequePayment. The table per hierarchy mapping would look like:
<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="PAYMENT_TYPE" type="string"/>
    <property name="amount" column="AMOUNT"/>
    ...
    <subclass name="CreditCardPayment" discriminator-value="CREDIT">
        <property name="creditCardType" column="CCTYPE"/>
        ...
    </subclass>
    <subclass name="CashPayment" discriminator-value="CASH">
        ...
    </subclass>
    <subclass name="ChequePayment" discriminator-value="CHEQUE">
        ...
    </subclass>
</class>
Exactly one table is required. There is one big limitation of this mapping strategy: columns declared by the subclasses, such as CCTYPE, may not have NOT NULL constraints.

9.1.2. Table per subclass

A table per subclass mapping would look like:
<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="AMOUNT"/>
    ...
    <joined-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">
        <key column="PAYMENT_ID"/>
        <property name="creditCardType" column="CCTYPE"/>
        ...
    </joined-subclass>
    <joined-subclass name="CashPayment" table="CASH_PAYMENT">
        <key column="PAYMENT_ID"/>
        ...
    </joined-subclass>
    <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
        <key column="PAYMENT_ID"/>
        ...
    </joined-subclass>
</class>
Four tables are required. The three subclass tables have primary key associations to the superclass table (so the relational model is actually a one-to-one association).

9.1.3. Table per subclass, using a discriminator

Note that Hibernate's implementation of table per subclass requires no discriminator column. Other object/relational mappers use a different implementation of table per subclass which requires a type discriminator column in the superclass table. The approach taken by Hibernate is much more difficult to implement but arguably more correct from a relational point of view. If you would like to use a discriminator column with the table per subclass strategy, you may combine the use of <subclass> and <join>, as follow:
<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="PAYMENT_TYPE" type="string"/>
    <property name="amount" column="AMOUNT"/>
    ...
    <subclass name="CreditCardPayment" discriminator-value="CREDIT">
        <join table="CREDIT_PAYMENT">
            <key column="PAYMENT_ID"/>
            <property name="creditCardType" column="CCTYPE"/>
            ...
        </join>
    </subclass>
    <subclass name="CashPayment" discriminator-value="CASH">
        <join table="CASH_PAYMENT">
            <key column="PAYMENT_ID"/>
            ...
        </join>
    </subclass>
    <subclass name="ChequePayment" discriminator-value="CHEQUE">
        <join table="CHEQUE_PAYMENT" fetch="select">
            <key column="PAYMENT_ID"/>
            ...
        </join>
    </subclass>
</class>
The optional fetch="select" declaration tells Hibernate not to fetch the ChequePayment subclass data using an outer join when querying the superclass.

9.1.4. Mixing table per class hierarchy with table per subclass

You may even mix the table per hierarchy and table per subclass strategies using this approach:
<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="PAYMENT_TYPE" type="string"/>
    <property name="amount" column="AMOUNT"/>
    ...
    <subclass name="CreditCardPayment" discriminator-value="CREDIT">
        <join table="CREDIT_PAYMENT">
            <property name="creditCardType" column="CCTYPE"/>
            ...
        </join>
    </subclass>
    <subclass name="CashPayment" discriminator-value="CASH">
        ...
    </subclass>
    <subclass name="ChequePayment" discriminator-value="CHEQUE">
        ...
    </subclass>
</class>
For any of these mapping strategies, a polymorphic association to the root Payment class is mapped using <many-to-one>.
<many-to-one name="payment" column="PAYMENT_ID" class="Payment"/>

9.1.5. Table per concrete class

There are two ways we could go about mapping the table per concrete class strategy. The first is to use <union-subclass>.
<class name="Payment">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="sequence"/>
    </id>
    <property name="amount" column="AMOUNT"/>
    ...
    <union-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">
        <property name="creditCardType" column="CCTYPE"/>
        ...
    </union-subclass>
    <union-subclass name="CashPayment" table="CASH_PAYMENT">
        ...
    </union-subclass>
    <union-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
        ...
    </union-subclass>
</class>
Three tables are involved for the subclasses. Each table defines columns for all properties of the class, including inherited properties.
The limitation of this approach is that if a property is mapped on the superclass, the column name must be the same on all subclass tables. (We might relax this in a future release of Hibernate.) The identity generator strategy is not allowed in union subclass inheritance, indeed the primary key seed has to be shared accross all unioned subclasses of a hierarchy.
If your superclass is abstract, map it with abstract="true". Of course, if it is not abstract, an additional table (defaults to PAYMENT in the example above) is needed to hold instances of the superclass.

9.1.6. Table per concrete class, using implicit polymorphism

An alternative approach is to make use of implicit polymorphism:
<class name="CreditCardPayment" table="CREDIT_PAYMENT">
    <id name="id" type="long" column="CREDIT_PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="CREDIT_AMOUNT"/>
    ...
</class>

<class name="CashPayment" table="CASH_PAYMENT">
    <id name="id" type="long" column="CASH_PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="CASH_AMOUNT"/>
    ...
</class>

<class name="ChequePayment" table="CHEQUE_PAYMENT">
    <id name="id" type="long" column="CHEQUE_PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="CHEQUE_AMOUNT"/>
    ...
</class>
Notice that nowhere do we mention the Payment interface explicitly. Also notice that properties of Payment are mapped in each of the subclasses. If you want to avoid duplication, consider using XML entities (e.g. [ <!ENTITY allproperties SYSTEM "allproperties.xml"> ] in the DOCTYPE declartion and & allproperties; in the mapping).
The disadvantage of this approach is that Hibernate does not generate SQL UNION s when performing polymorphic queries.
For this mapping strategy, a polymorphic association to Payment is usually mapped using <any>.
<any name="payment" meta-type="string" id-type="long">
    <meta-value value="CREDIT" class="CreditCardPayment"/>
    <meta-value value="CASH" class="CashPayment"/>
    <meta-value value="CHEQUE" class="ChequePayment"/>
    <column name="PAYMENT_CLASS"/>
    <column name="PAYMENT_ID"/>
</any>

9.1.7. Mixing implicit polymorphism with other inheritance mappings

There is one further thing to notice about this mapping. Since the subclasses are each mapped in their own <class> element (and since Payment is just an interface), each of the subclasses could easily be part of another inheritance hierarchy! (And you can still use polymorphic queries against the Payment interface.)
<class name="CreditCardPayment" table="CREDIT_PAYMENT">
    <id name="id" type="long" column="CREDIT_PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="CREDIT_CARD" type="string"/>
    <property name="amount" column="CREDIT_AMOUNT"/>
    ...
    <subclass name="MasterCardPayment" discriminator-value="MDC"/>
    <subclass name="VisaPayment" discriminator-value="VISA"/>
</class>

<class name="NonelectronicTransaction" table="NONELECTRONIC_TXN">
    <id name="id" type="long" column="TXN_ID">
        <generator class="native"/>
    </id>
    ...
    <joined-subclass name="CashPayment" table="CASH_PAYMENT">
        <key column="PAYMENT_ID"/>
        <property name="amount" column="CASH_AMOUNT"/>
        ...
    </joined-subclass>
    <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
        <key column="PAYMENT_ID"/>
        <property name="amount" column="CHEQUE_AMOUNT"/>
        ...
    </joined-subclass>
</class>
Once again, we don't mention Payment explicitly. If we execute a query against the Payment interface - for example, from Payment - Hibernate automatically returns instances of CreditCardPayment (and its subclasses, since they also implement Payment), CashPayment and ChequePayment but not instances of NonelectronicTransaction.