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Single-valued entity associations Let s change the mapping from a User to an Address. Instead of the shared primary key, you now add a SHIPPING_ADDRESS_ID column in the USERS table: The mapping element in XML for this association is not , as you might have expected. The reason is simple: You don t care what s on the target side of the association, so you can treat it like a to-one association without the many part. All you want is to express This entity has a property that is a reference to an instance of another entity and use a foreign key field to represent that relationship. The database schema for this mapping is shown in figure 7.3. Figure 7.3 A one-to-one foreign key association between USERS and ADDRESS An additional constraint enforces this relationship as a real one to one. By making the SHIPPING_ADDRESS_ID column unique, you declare that a particular address can be referenced by at most one user, as a shipping address. This isn t as strong as the guarantee from a shared primary key association, which allows a particular address to be referenced by at most one user, period. With several foreign key columns (let s say you also have unique HOME_ADDRESS_IDand BILLING_ADDRESS_ID), you can reference the same address target row several times. But in any case, two users can t share the same address for the same purpose. Let s make the association from User to Address bidirectional. Inverse property reference The last foreign key association was mapped from Userto Addresswith and a unique constraint to guarantee the desired multiplicity. What mapping
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CHAPTER 7 Advanced entity association mappings instead if you map with composite primary keys. In a JPA XML descriptor, a one-toone mapping looks like this: …
The JPA specification doesn t include a standardized method to deal with the problem of shared primary key generation, which means you re responsible for setting the identifier value of an Address instance correctly before you save it (to the identifier value of the linked User instance). Hibernate has an extension annotation for custom identifier generators which you can use with the Address entity (just like in XML): @Entity @Table(name = “ADDRESS”) public class Address { @Id @GeneratedValue(generator = “myForeignGenerator”) @org.hibernate.annotations.GenericGenerator( name = “myForeignGenerator”, strategy = “foreign”, parameters = @Parameter(name = “property”, value = “user”) ) @Column(name = “ADDRESS_ID”) private Long id; … private User user; } Shared primary key one-to-one associations aren t uncommon but are relatively rare. In many schemas, a to-one association is represented with a foreign key field and a unique constraint. 7.1.2 One-to-one foreign key associations Instead of sharing a primary key, two rows can have a foreign key relationship. One table has a foreign key column that references the primary key of the associated table. (The source and target of this foreign key constraint can even be the same table: This is called a self-referencing relationship.)
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Single-valued entity associations
user … This mapping seems strange at first. Read it as follows: When an Address is saved, the primary key value is taken from the user property. The user property is a reference to a User object; hence, the primary key value that is inserted is the same as the primary key value of that instance. Look at the table structure in figure 7.2. Figure 7.2 The USERSand ADDRESStables have the same primary keys. The code to save both objects now has to consider the bidirectional relationship, and it finally works: User newUser = new User(); Address shippingAddress = new Address(); newUser.setShippingAddress(shippingAddress); shippingAddress.setUser(newUser); // Bidirectional session.save(newUser); Let s do the same with annotations. Shared primary key with annotations JPA supports one-to-one entity associations with the @OneToOne annotation. To map the association of shippingAddress in the User class as a shared primary key association, you also need the @PrimaryKeyJoinColumn annotation: @OneToOne @PrimaryKeyJoinColumn private Address shippingAddress; This is all that is needed to create a unidirectional one-to-one association on a shared primary key. Note that you need @PrimaryKeyJoinColumns (plural)
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CHAPTER 7 Advanced entity association mappings User newUser = new User(); Address shippingAddress = new Address(); newUser.setShippingAddress(shippingAddress); session.save(newUser); Hibernate inserts a row into the USERS table and a row into the ADDRESS table. But wait, this doesn t work! How can Hibernate possibly know that the record in the ADDRESS table needs to get the same primary key value as the USERS row? At the beginning of this section, we intentionally didn t show you any primary-key generator in the mapping of Address. You need to enable a special identifier generator. The foreign identifier generator If an Address instance is saved, it needs to get the primary key value of a User object. You can t enable a regular identifier generator, let s say a database sequence. The special foreign identifier generator for Address has to know where to get the right primary key value. The first step to create this identifier binding between Address and User is a bidirectional association. Add a new user property to the Address entity: public class Address { … private User user; // Getters and setters } Map the new user property of an Address in Address.hbm.xml: This mapping not only makes the association bidirectional, but also, with constrained=”true”, adds a foreign key constraint linking the primary key of the ADDRESS table to the primary key of the USERS table. In other words, the database guarantees that an ADDRESS row s primary key references a valid USERS primary key. (As a side effect, Hibernate can now also enable lazy loading of users when a shipping address is loaded. The foreign key constraint means that a user has to exist for a particular shipping address, so a proxy can be enabled without hitting the database. Without this constraint, Hibernate has to hit the database to find out if there is a user for the address; the proxy would then be redundant. We ll come back to this in later chapters.) You can now use the special foreign identifier generator for Address objects:
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Single-valued entity associations
We assume you won t have any difficulty creating the same mapping with annotations or changing the Java class to an entity, with an identifier property this is the only change you have to make. Now let s create the association mappings from other entities to that class. There are several choices, the first being a primary key one-to-one association. Figure 7.1 Address as an entity with two associations referencing the same instance 7.1.1 Shared primary key associations Rows in two tables related by a primary key association share the same primary key values. The main difficulty with this approach is ensuring that associated instances are assigned the same primary key value when the objects are saved. Before we try to solve this problem, let s see how you map the primary key association. Mapping a primary key association with XML The XML mapping element that maps an entity association to a shared primary key entity is . First you need a new property in the User class: public class User { … private Address shippingAddress; // Getters and setters } Next, map the association in User.hbm.xml: You add a cascading option that is natural for this model: If a User instance is made persistent, you usually also want its shippingAddress to become persistent. Hence, the following code is all that is needed to save both objects:
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CHAPTER 7 Advanced entity association mappings When we use the word associations, we always refer to relationships between entities. In the previous chapter, we demonstrated a unidirectional many-to-one association, made it bidirectional, and finally turned it into a parent/children relationship (one-to-many and many-to-one with cascading options). One reason we discuss more advanced entity mappings in a separate chapter is that quite a few of them are considered rare, or at least optional. It s absolutely possible to only use component mappings and many-to-one (occasionally one-to-one) entity associations. You can write a sophisticated application without ever mapping a collection! Of course, efficient and easy access to persistent data, by iterating a collection for example, is one of the reasons why you use full object/relational mapping and not a simple JDBC query service. However, some exotic mapping features should be used with care and even avoided most of the time. We ll point out recommended and optional mapping techniques in this chap ter, as we show you how to map entity associations with all kinds of multiplicity, with and without collections. 7.1 Single-valued entity associations Let s start with one-to-one entity associations. We argued in chapter 4 that the relationships between User and Address (the user has a billingAddress, homeAddress, and shippingAddress) are best represented with a mapping. This is usually the simplest way to represent one-to-one relationships, because the lifecycle is almost always dependent in such a case, it s either an aggregation or a composition in UML. But what if you want a dedicated table for Address, and you map both User and Address as entities? One benefit of this model is the possibility for shared references another entity class (let s say Shipment) can also have a reference to a particular Address instance. If a User has a reference to this instance, as their shippingAddress, the Address instance has to support shared references and needs its own identity. In this case, User and Address classes have a true one-to-one association. Look at the revised class diagram in figure 7.1. The first change is a mapping of the Address class as a stand-alone entity:
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Advanced entity association mappings This chapter covers Mapping one-to-one and many-to-one entity associations Mapping one-to-many and many-to-many entity associations Polymorphic entity associations 277
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CHAPTER 6 Mapping collections and entity associations Table 6.1 Hibernate and JPA comparison chart for chapter 6 (continued) Automatic deletion of orphaned entity instances is built in. Hibernate Annotations is required for automatic deletion of orphaned entity instances. Hibernate Core Java Persistence and EJB 3.0 We ve covered only a tiny subset of the entity association options in this chapter. The remaining options we explore in detail in the next chapter are either rare or variations of the techniques we ve just described.
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Summary … Note that the Hibernate extension for cascade orphan deletion isn t available in this case. 6.5 Summary You re probably a little overwhelmed by all the new concepts we introduced in this chapter. You may have to read it a few times, and we encourage you to try the code (and watch the SQL log). Many of the strategies and techniques we ve shown in this chapter are key concepts of object/relational mapping. If you master collection mappings, and once you ve mapped your first parent/children entity association, you ll have the worst behind you. You ll already be able to build entire applications! Table 6.1 summarizes the differences between Hibernate and Java Persistence related to concepts discussed in this chapter. Table 6.1 Hibernate and JPA comparison chart for chapter 6 Hibernate Core Java Persistence and EJB 3.0 Hibernate provides mapping support for sets, lists, maps, bags, identifier bags, and arrays. All JDK collection interfaces are supported, and extension points for custom persistent collections are available. Standardized persistent sets, lists, maps, and bags are supported. Collections of value types and components are supported. Hibernate Annotations is required for collections of value types and embeddable objects. Parent/children entity relationships are supported, with transitive state cascading on associations per operation. You can map entity associations and enable transitive state cascading on associations per operation.
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CHAPTER 6 Mapping collections and entity associations and the row can be removed safely. You may have removed all other references manually. Or, if you didn t, the database constraints prevent any inconsistency, and you see a foreign key constraint exception. Hibernate offers you a way to declare this guarantee for collections of entity references. You can tell Hibernate, If I remove an element from this collection, it will be an entity reference, and it s going to be the only reference to that entity instance. You can safely delete it. The code that worked for deletion with a collection of components works with collections of entity references. This option is called cascade orphan delete. You can enable it on a collection mapping in XML as follows: … With annotations, this feature is available only as a Hibernate extension: public class Item { … @OneToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE }, mappedBy = “item”) @org.hibernate.annotations.Cascade( value = org.hibernate.annotations.CascadeType.DELETE_ORPHAN ) private Set bids = new HashSet(); … } Also note that this trick works only for collections of entity references in a one-tomany association; conceptually, no other entity association mapping supports it. You should ask yourself at this point, with so many cascading options set on your collection, whether a simple collection of components may be easier to handle. After all, you ve enabled a dependent lifecycle for objects referenced in this collection, so you may as well switch to the implicit and fully dependent lifecycle of components. Finally, let s look at the mapping in a JPA XML descriptor: …
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