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As you can see, the mappedBy element identifies the name of the association field in the owning side of the relationship. In a bidirectional relationship, the owning side is the entity that stores the relationship in its underlying table. In our example, the USERS table stores the relationship in the USER_BILLING_ID field and thus is the relationship owner. The one-to-one relationship in BillingInfo has the mappedBy element specified as billingInfo, which is the relationship field defined in the User entity that contains the definition for @JoinColumn. Note that you do not have to define @JoinColumn in the entities of both sides of one-to-one relationships. Next we ll discuss how you define the one-to-one relationship when the foreign key is in the table to which the child entity is mapped. Using @PrimaryKeyJoinColumn In the more likely case that the foreign key reference exists in the table to which the referenced entity is mapped, the @PrimaryKeyJoinColumn would be used instead (figure 8.5).
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Figure 8.5 User has a one-to-one unidirectional relationship with BillingInfo. The User and BillingInfo entities are mapped to the USERS and BILLING_INFO tables, respectively, and the BILLING_INFO and USERS tables share the same primary key; the primary key of the BILLING_INFO table is also a foreign key referencing the primary key of the USERS table. Such associations are mapped using @PrimaryKeyJoinColumn.
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Typically, @PrimaryKeyJoinColumn is used in one-to-one relationships when both the referenced and referencing tables share the primary key of the referencing table. In our example, as shown in figure 8.5, the BILLING_INFO table would contain a foreign key reference named BILLING_USER_ID pointing to the USER_ID primary key of the USERS table. In addition, BILLING_USER_ID would be the primary key of the BILLING_INFO table. The relationship would be implemented as shown in listing 8.8.
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Listing 8.8 Mapping a one-to-one relationship using @PrimaryKeyJoinColumn
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@Entity @Table(name="USERS") public class User { @Id @Column(name="USER_ID") protected Long userId; ... @OneToOne @PrimaryKeyJoinColumn(name="USER_ID", referencedColumnName="BILLING_USER_ID") protected BillingInfo billingInfo; } @Entity @Table(name="BILLING_INFO") public class BillingInfo { @Id @Column(name="BILLING_USER_ID") protected Long userId; ... }
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The @PrimaryKeyJoinColumn annotation s name element refers to the primary key column of the table storing the current entity. On the other hand, the referencedColumnName element refers to the foreign key in the table holding the referenced entity. In our case, the foreign key is the BILLING_INFO table s BILLING_ USER_ID column, and it points to the USERS.USER_ID primary key. If the names of both the primary key and foreign key columns are the same, you may omit the referencedColumnName element since this is what the JPA provider will assume by default. In our example, if we rename the foreign key in the BILLING_INFO table from BILLING_USER_ID to USER_ID to match the name of the primary key in the USERS table, we may omit the referencedColumnName value so that the provider can default it correctly.
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Object-relational mapping
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If you have a composite primary key in the parent table (which is rare if you are using surrogate keys), you should use the @PrimaryKeyJoinColumns annotation instead. We encourage you to explore this annotation on your own. You ll learn how to map one-to-many and many-to-one relationships next.
8.3.2 One-to-many and many-to-one As we mentioned in the previous chapter, one-to-many and many-to-one relationships are the most common in enterprise systems and are implemented using the @OneToMany and @ManyToOne annotations. For example, the Item-Bid relationship in the ActionBazaar system is one-to-many, since an Item holds references to a collection of Bids placed on it and a Bid holds a reference to the Item it was placed on. The beauty of EJB 3 persistence mapping is that the same two annotations we used for mapping one-to-one relationships are also used for one-to-many relationships. This is because both relation types are implemented as a primary-key/ foreign-key association in the underlying database. Let s see how to do this by implementing the Item-Bid relationship shown in listing 8.9.
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