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Table per concrete class with implicit polymorphism
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Suppose we stick with the simplest approach suggested. You can use exactly one table for each (nonabstract) class. All properties of a class, including inherited properties, can be mapped to columns of this table, as shown in figure 5.1.
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Mapping all concrete classes to an independent table
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You don t have to do anything special in Hibernate to enable polymorphic behavior. The mapping for CreditCard and BankAccount is straightforward, each in its own entity <class> element, as we have done already for classes without a superclass (or persistent interfaces). Hibernate still knows about the superclass (or any interface) because it scans the persistent classes on startup. The main problem with this approach is that it doesn t support polymorphic associations very well. In the database, associations are usually represented as foreign key relationships. In figure 5.1, if the subclasses are all mapped to different tables, a polymorphic association to their superclass (abstract BillingDetails in this example) can t be represented as a simple foreign key relationship. This would be problematic in our domain model, because BillingDetails is associated with User; both subclass tables would need a foreign key reference to the USERS table. Or, if User had a many-to-one relationship with BillingDetails, the USERS table would need a single foreign key column, which would have to refer both concrete subclass tables. This isn t possible with regular foreign key constraints. Polymorphic queries (queries that return objects of all classes that match the interface of the queried class) are also problematic. A query against the superclass must be executed as several SQL SELECTs, one for each concrete subclass. For a query against the BillingDetails class Hibernate uses the following SQL:
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select CREDIT_CARD_ID, OWNER, NUMBER, EXP_MONTH, EXP_YEAR ... from CREDIT_CARD select BANK_ACCOUNT_ID, OWNER, ACCOUNT, BANKNAME, ... from BANK_ACCOUNT
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Notice that a separate query is needed for each concrete subclass. On the other hand, queries against the concrete classes are trivial and perform well only one of the statements is needed.
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(Also note that here, and in other places in this book, we show SQL that is conceptually identical to the SQL executed by Hibernate. The actual SQL may look superficially different.) A further conceptual problem with this mapping strategy is that several different columns, of different tables, share exactly the same semantics. This makes schema evolution more complex. For example, a change to a superclass property results in changes to multiple columns. It also makes it much more difficult to implement database integrity constraints that apply to all subclasses. We recommend this approach (only) for the top level of your class hierarchy, where polymorphism isn t usually required, and when modification of the superclass in the future is unlikely. Also, the Java Persistence interfaces don t support full polymorphic queries; only mapped entities (@Entity) can be officially part of a Java Persistence query (note that the Hibernate query interfaces are polymorphic, even if you map with annotations). If you re relying on this implicit polymorphism, you map concrete classes with @Entity, as usual. However, you also have to duplicate the properties of the superclass to map them to all concrete class tables. By default, properties of the superclass are ignored and not persistent! You need to annotate the superclass to enable embedding of its properties in the concrete subclass tables:
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@MappedSuperclass public abstract class BillingDetails { @Column(name = "OWNER", nullable = false) private String owner; ... }
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Now map the concrete subclasses:
@Entity @AttributeOverride(name = "owner", column = @Column(name = "CC_OWNER", nullable = false) ) public class CreditCard extends BillingDetails { @Id @GeneratedValue @Column(name = "CREDIT_CARD_ID") private Long id = null; @Column(name = "NUMBER", nullable = false) private String number; ... }
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