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CHAPTER 4 OBJECT RELATIONAL MAPPING WITH HIBERNATE
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Figure 4-18. A hierarchical view of a Hibernate mapping
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Figure 4-19. Polymorphic query to retrieve all User objects We can also issue queries against the subclasses of User as shown in Figure 4-20 for the class, Attendee.
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Figure 4-20. Query to retrieve all Attendee objects Similarly, we can check the contents of the Address class by issuing the query shown in Figure 4-21.
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CHAPTER 4 OBJECT RELATIONAL MAPPING WITH HIBERNATE
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Figure 4-21. Query to retrieve all Address objects Let s try a more interesting query against the User hierarchy. The query shown in Figure 4-22 finds all User objects containing an Address object for which the City property is equal to Columbus .
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Figure 4-22. A more complex polymorphic query As seen previously, you can also use some of the familiar SQL syntax in HQL queries as shown in Figure 4-23, a query to find all Users with an email address ending in acme.com .
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Figure 4-23. HQL query using the like and % operators
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CHAPTER 4 OBJECT RELATIONAL MAPPING WITH HIBERNATE
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Inheritance: Table Per Subclass
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In this example, the PricingRule hierarchy is mapped using the Table-Per-Subclass strategy. Pricing rules are applied at registration time when a conference attendee pays to attend the conference. The PricingRule hierarchy is composed of the interface PricingRule and the classes GroupPricingRule and RegistrationDatePricingRule, which implement the PricingRule interface. The PricingRule hierarchy is shown in Figure 4-24.
Figure 4-24. PricingRule class hierarchy The three corresponding tables are shown in Listing 4-28. Notice that tables holding the subclasses data, GROUPPRICINGRULE and REGISTRATIONDATEPRICINGRULE, both have foreign key constraints referencing the PRICINGRULE table (the root of the class hierarchy). Listing 4-28. Pricing Rules Tables CREATE TABLE PRICINGRULE ( PK_ID INTEGER NOT NULL PRIMARY KEY AUTO_INCREMENT, NAME VARCHAR(16) NOT NULL, PRIORITY INTEGER, ACTIVE BOOLEAN, FK_CONFERENCE_ID INTEGER NOT NULL, CONSTRAINT PRICINGRULE_CONFERENCE_FK FOREIGN KEY(FK_CONFERENCE_ID) REFERENCES CONFERENCE(PK_ID), CONSTRAINT UNIQUE_PRICINGRULE_NAME UNIQUE(NAME) );
CHAPTER 4 OBJECT RELATIONAL MAPPING WITH HIBERNATE
CREATE TABLE GROUPPRICINGRULE ( PK_ID INTEGER NOT NULL PRIMARY KEY, MINIMUMATTENDEES INTEGER, MAXIMUMATTENDEES INTEGER, DISCOUNTRATE DOUBLE, CONSTRAINT GROUPPRICINGRULE_PRICINGRULE_FK FOREIGN KEY(PK_ID) REFERENCES PRICINGRULE(PK_ID) ); CREATE TABLE REGISTRATIONDATEPRICINGRULE ( PK_ID INTEGER NOT NULL PRIMARY KEY, STARTDATE DATETIME NOT NULL, ENDDATE DATETIME NOT NULL, PRICE DOUBLE NOT NULL, CONSTRAINT REGISTRATIONDATEPRICINGRULE_PRICINGRULE_FK FOREIGN KEY(PK_ID) REFERENCES PRICINGRULE(PK_ID) ); In this strategy the data model mirrors the object model. Hibernate will rely on matching the primary keys to find all the data belonging to a particular subclass and will use it to perform joins when querying the PricingRule hierarchy. To map these three tables to the object model, we use joined-subclass element in the HBM file as shown in Listing 4-29. The id element mapped to the column PK_ID is the primary key of the parent table, while the key elements inside each joined-subclass mapping provide the linkage between the parent table and the subclass tables. Listing 4-29. Table-Per-Subclass Mapping for Pricing Rules Hierarchy < xml version="1.0" > <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="com.integrallis.techconf.domain"> <class name="PricingRule" table="pricingrule"> <id column="PK_ID" name="Id" type="integer" > <generator class="identity" /> </id> <property name="Name" not-null="true" /> ...
CHAPTER 4 OBJECT RELATIONAL MAPPING WITH HIBERNATE
<property column="FK_CONFERENCE_ID" name="ConferenceId" not-null="true" type="integer" /> <!-- GroupPricingRule Subclass --> <joined-subclass name="GroupPricingRule" table="grouppricingrule"> <key column="PK_ID"/> <property name="MinimumAttendees" type="integer" /> <property name="DiscountRate" type="java.lang.Double" /> <property name="MaximumAttendees" type="integer" /> </joined-subclass> <!-- RegistrationDatePricingRule Subclass --> <joined-subclass name="RegistrationDatePricingRule" table="registrationdatepricingrule"> <key column="PK_ID"/> <property name="StartDate" not-null="true" type="timestamp" /> <property name="EndDate" not-null="true" type="timestamp" /> <property name="Price" not-null="true" type="java.lang.Double" /> </joined-subclass> </class> </hibernate-mapping> Testing the Table-Per-Subclass mapping is similar to the testing previously performed for the Table-Per-Class-Hierarchy strategy, and it is left as an exercise for you.
Summary
In this chapter you have learned the foundations of object-relational mapping using Hibernate, the prevalent and by far richest mapping service available. Now that you know how to map simple classes and collections and how to deal with inheritance, you ll be ready to tackle the development of the TechConf application Data Access Objects (DAO) in subsequent chapters. We will continue to learn about Hibernate features, including more advanced features such as caching and other facilities that will come into play when we transition into the world of J2EE by having our Hibernate-mapped POJOs working inside an EJB container. Also, in the Testing chapter, you ll learn how to apply some of the techniques of unit and integration testing to the testing of Hibernate-mapped POJOs and Hibernate-powered DAOs.
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