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CHAPTER 4 INHERITANCE AND CUSTOM MAPPING
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Table 4-1. Polymorphic Associations Supported by Various Inheritance Strategies
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Inheritance Strategy
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Table per class hierarchy Table per subclass Table per concrete class with unions
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Polymorphic Many-to-One
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<many-to-one>
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<one-to-one>
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Polymorphic Many-to-Many
<many-to-many>
<many-to-one>
<one-to-one>
<one-to-many>
<many-to-many>
<many-to-one>
<one-to-one>
<one-to-many> (for Inverse="true" only) Not supported
<many-to-many>
Table per concrete class with implicit polymorphism
<any>
Not supported
<many-to-any>
You ve seen how inheritance mapping enables polymorphic queries; Table 4-2 summarizes the polymorphic query capabilities supported by various inheritance strategies. You ve also learned about the basic data types that Hibernate supports and the implementation of some custom data types. Table 4-2. Polymorphic Queries Supported by Various Inheritance Strategies
Inheritance Strategy
Table per Class hierarchy Table per subclass Sub class Table per concrete class with unions Table per concrete class with implicit polymorphism
Polymorphic load()/get()
Supported
Polymorphic Queries
Supported
Polymorphic Joins
Supported
Outer Join Fetching
Supported
Supported
Supported
Supported
Supported
Supported
Supported
Supported
Supported
Supported
Supported
Not supported
Not supported
CHAPTER 5
Many-to-One and One-to-One Mapping
In general, entities are related or associated to each other. For example, a Customer is associated with an Address. A Customer can have one or more than one Address (for example, Billing and Shipping). These relationships or associations are represented differently in a relational model and the domain/object model. In a relational model, a foreign key column represents the dependency of one table on another. The Customer table has an AddressId as the foreign key column, which is the primary key of the Address table. From the Customer perspective, because a Customer can have multiple addresses, so it s a many-toone relationship. On the other hand, from the Address perspective, one address can belong to only one customer, so it s a one-to-one relationship. The same relationship is represented in terms of classes and attributes in an object model. The Customer class has an attribute to hold multiple addresses belonging to that Customer (in Java, this is represented as a collection List, Set, and so on). The Address class has an attribute of Customer type. In the case of Customer and Address example, you consider the association from the view of both the Customer entity and the Address entity, so it s a bidirectional association. If you use the association from only one entity, it s a unidirectional association. In an object/relational mapping (ORM) framework, these relationships are represented using metadata in the configuration file as well using the entities/objects. This chapter shows you how these associations are represented in Hibernate and also discusses the following features: Various ways to use the many-to-one and one-to-one mappings How to make unidirectional associations into bidirectional associations How to use features like lazy initialization and cascade How to map an association or a join table
5.1 Using Many-To-One Associations
Problem
Coming back to the online bookshop example, a publisher can publish many books. This relationship between the books and the publisher is a many-to-one relationship. How do you map objects in this
CHAPTER 5 MANY-TO-ONE AND ONE-TO-ONE MAPPING
case using the many-to-one association How do you use the cascade option to persist the class hierarchy
Solution
As already mentioned, the association from books to publisher is a many-to-one association. Because this association is navigable from book to publisher only, it s also a unidirectional association. You use the <many-to-one> element to establish this association.
How It Works
Your application has a Publisher class that hasn t been mapped to the database. Following the best practice of object identifiers discussed in 4, you can add an autogenerated id property on the class. Figure 5-1 shows the entity relationship drawing for a many-to-one unidirectional association.
Figure 5-1. Entity-relationship drawing showing a many-to-one unidirectional association public class Publisher implements Serializable{ private Long publisher_id; private String code; private String name; private String address; // getters and setters } <hibernate-mapping package="com.hibernaterecipes.chapter5"> <class name="Publisher" table="Publisher" schema="BOOK5"> <id name="Publisher_id" type="long" column="PUBLISHER_ID" > <generator class="native"> </generator> </id> <property name="code" type="string"> <column name="CODE" length="4" not-null="true" unique="true" /> </property> <property name="name" type="string"> <column name="PUBLISHER_NAME" length="100" not-null="true" /> </property> <property name="address" type="string">
CHAPTER 5 MANY-TO-ONE AND ONE-TO-ONE MAPPING
<column name="ADDRESS" length="200" /> </property> </class> </hibernate-mapping> Because you add a new persistent object to your application, you need to specify it in the Hibernate configuration file: <mapping resource="com/hibernaterecipes/chapter5/Publisher.xml" /> For the Book class, you already have a Publisher property of type Publisher, which isn t used in the previous examples: public class Book_5_1 implements Serializable{ private Long book_id; private String isbn; private String name; private Publisher publisher; private Date publishDate; private Integer price; // getters and setters } <hibernate-mapping package="com.hibernaterecipes.chapter5"> <class name="Book_5_1" table="Book" schema="BOOK5"> <id name="book_id" type="long" column="BOOK_ID" > <generator class="native"> </generator> </id> <property name="isbn" type="string"> <column name="ISBN" length="50" not-null="true" unique="true" /> </property> <property name="name" type="string"> <column name="BOOK_NAME" length="100" not-null="true" /> </property> <property name="publishDate" type="date" column="PUBLISH_DATE" /> <property name="price" type="int" column="PRICE" /> <many-to-one name="publisher" class="Publisher" column="PUBLISHER_ID"/> </class> </hibernate-mapping> After you created a new book object together with a new publisher object, you want to save them into the database. Will Hibernate save the publisher object when you save the book object Unfortunately, an exception occurs if you save only the book object. That means you must save them one by one: Session session = factory.openSession(); Transaction tx = null; try {
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