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superclass's table or to its subclasses' tables, or it can have its own table. However,
one tricky problem is mapping interfaces such as
Coupon
. As figure 5.1 shows, the
Coupon
interface is implemented by the
FreeShippingCoupon
and
Percentage-
DiscountCoupon
classes.
We need to map the
Coupon
interface and its implementation classes to the
COU-
PON
table and map the many-to-one relationships
Order
-
Coupon
and
Pending-
Order
-
Coupon
to foreign keys in the
PLACED_ORDER
and
PENDING_ORDER
tables.
Unfortunately,
JDO
does not allow this to be done directly by mapping an interface
Domain Model
Database Schema
<<table>>
ORDER
Order
ORDER_ID <<pk>>
COUPON_ID <<fk>>
...
<<table>>
PENDING_ORDER
PendingOrder
PENDING_ORDER_ID <<pk>>
COUPON_ID <<fk>>
...
<<table>>
COUPON
<<interface>>
Coupon
COUPON_ID <<pk>>
TYPE
CODE <<unique>>
MINIMUM
PER_DISCOUNT
FreeShipping
Coupon
Percentage
DiscountCoupon
code
minimum
code
minimum
percentage
Figure 5.1
The object/relational mapping for the
Coupon
classes
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