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Inheritance and custom types
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public abstract class BillingDetails { @Id @GeneratedValue @Column(name = "BILLING_DETAILS_ID") private Long id = null; @Column(name = "OWNER", nullable = false) private String owner; ... }
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If you don t specify a discriminator column in the superclass, its name defaults to DTYPE and its type to string. All concrete classes in the inheritance hierarchy can have a discriminator value; in this case, BillingDetails is abstract, and CreditCard is a concrete class:
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@Entity @DiscriminatorValue("CC") public class CreditCard extends BillingDetails { @Column(name = "CC_NUMBER") private String number; ... }
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Without an explicit discriminator value, Hibernate defaults to the fully qualified class name if you use Hibernate XML files and the entity name if you use annotations or JPA XML files. Note that no default is specified in Java Persistence for nonstring discriminator types; each persistence provider can have different defaults. This is the equivalent mapping in JPA XML descriptors:
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<entity-mappings> <entity class="auction.model.BillingDetails" access="FIELD"> <inheritance strategy="SINGLE_TABLE"/> <discriminator-column name="BILLING_DETAILS_TYPE" discriminator-type="STRING"/> ... </entity> <entity class="auction.model.CreditCard" access="FIELD"> <discriminator-value>CC</discriminator-value> ... </entity> </entity-mappings>
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Sometimes, especially in legacy schemas, you don t have the freedom to include an extra discriminator column in your entity tables. In this case, you can apply a formula to calculate a discriminator value for each row:
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<discriminator formula="case when CC_NUMBER is not null then 'CC' else 'BA' end" type="string"/> ... <subclass name="CreditCard" discriminator-value="CC"> ...
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This mapping relies on an SQL CASE/WHEN expression to determine whether a particular row represents a credit card or a bank account (many developers never used this kind of SQL expression; check the ANSI standard if you aren t familiar with it). The result of the expression is a literal, CC or BA, which in turn is declared on the <subclass> mappings. Formulas for discrimination aren t part of the JPA specification. However, you can apply a Hibernate annotation:
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@Entity @Inheritance(strategy = InheritanceType.SINGLE_TABLE) @org.hibernate.annotations.DiscriminatorFormula( "case when CC_NUMBER is not null then 'CC' else 'BA' end" ) public abstract class BillingDetails { ... }
The disadvantages of the table per class hierarchy strategy may be too serious for your design after all, denormalized schemas can become a major burden in the long run. Your DBA may not like it at all. The next inheritance mapping strategy doesn t expose you to this problem.
Table per subclass
The fourth option is to represent inheritance relationships as relational foreign key associations. Every class/subclass that declares persistent properties including abstract classes and even interfaces has its own table. Unlike the table per concrete class strategy we mapped first, the table here contains columns only for each noninherited property (each property declared by the subclass itself) along with a primary key that is also a foreign key of the superclass table. This approach is shown in figure 5.3. If an instance of the CreditCard subclass is made persistent, the values of properties declared by the BillingDetails superclass are persisted to a new row of the BILLING_DETAILS table. Only the values of properties declared by the subclass are persisted to a new row of the CREDIT_CARD table. The two rows are linked together
Inheritance and custom types
Mapping all classes of the hierarchy to their own table
by their shared primary key value. Later, the subclass instance may be retrieved from the database by joining the subclass table with the superclass table. The primary advantage of this strategy is that the SQL schema is normalized. Schema evolution and integrity constraint definition are straightforward. A polymorphic association to a particular subclass may be represented as a foreign key referencing the table of that particular subclass. In Hibernate, you use the <joined-subclass> element to create a table per subclass mapping. See listing 5.3.
Listing 5.3 Hibernate <joined-subclass> mapping
<hibernate-mapping> <class name="BillingDetails" table="BILLING_DETAILS"> <id name="id" column="BILLING_DETAILS_ID" type="long"> <generator class="native"/>
Mapping class inheritance
</id> <property name="owner" column="OWNER" type="string"/> ...
<joined-subclass name="CreditCard" table="CREDIT_CARD"> <key column="CREDIT_CARD_ID"/>
<property name="number" column="NUMBER"/> <property name="expMonth" column="EXP_MONTH"/> <property name="expYear" column="EXP_YEAR"/> </joined-subclass> <joined-subclass name="BankAccount" table="BANK_ACCOUNT"> ... </class> </hibernate-mapping>
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