Persisting a domain model with JDO 2.0 - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 157

to a table in the same way as a class. You must instead change the class hierarchy by
introducing a common superclass that implements the interface and then map it
to the database table.
For example, to persist the Coupon hierarchy we define an AbstractCoupon-
Impl class that implements the Coupon interface. The concrete Coupon classes
such as FreeShippingCoupon and PercentageDiscountCoupon extend this class:

interface Coupon {..};
class AbstractCouponImpl implements Coupon {…};
class FreeShippingCoupon extends AbstractCouponImpl {…};
class PercentageDiscountCoupon extends AbstractCouponImpl {…};
Once we have made these changes to the domain model, we can then define the
XML metadata for the O/R mapping for the Coupon class hierarchy. Here is an
excerpt of the XML metadata that defines the mapping for the AbstractCoupon-
Impl and FreeShippingCoupon classes:

<class name="AbstractCouponImpl" table="COUPON" >
<implements name="net.chrisrichardson.foodToGo.domain.Coupon"/>
<inheritance strategy="new-table">
bbbb <discriminator strategy="value-map">
bbb <column name="COUPON_TYPE"/>
bbbb </discriminator>
bbbb </inheritance>
</class>

<class name="FreeShippingCoupon" persistence-capable-superclass=
➥

"net.chrisrichardson.foodToGo.domain.AbstractCouponImpl">
<inheritance strategy="superclass-table">
<discriminator value="FREE_SHIP"/>
</inheritance>
<field name="minimum">
<column name="FREE_SHIP_MINIMUM" allows-null="true"/>
</field>
</class>
The <implements> element specifies that the AbstractCouponImpl class imple-
ments the Coupon interface. This tells the JDO implementation that any field of
type Coupon is really a reference to an instance of this class or one of its subclasses.
The <inheritance> element of the AbstractCouponImpl specifies that the
AbstractCouponImpl class has its own table called COUPON and that the
COUPON_TYPE column stores the type of the coupon. The <inheritance> ele-
ment of the FreeShippingCoupon class specifies that the FreeShippingCoupon
is mapped to the COUPON table and that its type code is FREE_SHIP . The map-
ping for PercentageDiscountCoupon would be defined in a similar way.