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In listing 7.4, the embeddable Address object b is embedded inside a User entity D and shares the identity marked with the @Id annotation C. It is illegal for an @Embeddable object to have an identity. Also, the EJB 3 API does not support nested embedded objects. In most cases, embedded objects are stored in the same database record as the entity and are only materialized in the OO world. We ll show you how this works in chapter 8. Our discussion of embedded objects rounds out our coverage of domain objects. We ll take a look at domain object relationships next.
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7.3 Entity relationships
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Earlier in the chapter, we explored the concepts of domain relationships, direction, and multiplicity. As a review, we ll summarize those concepts here before diving into the details of how to specify domain relationships using JPA. As you might have noted in our domain object code samples, a relationship essentially means that one entity holds an object reference to another. For example, the Bid object
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Implementing domain models
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holds a reference to the Item object the bid was placed on. Therefore, a relationship exists between the Bid and Item domain objects. Recall that relationships can be either unidirectional or bidirectional. The relationship between Bidder and Bid in figure 7.3 is unidirectional, since the Bidder object has a reference to Bid but the Bid object has no reference to the Bidder. The Bid-Item relationship, on the other hand, is bidirectional, meaning both the Bidder and Item objects have references to each other. Relationships can be one-to-one, one-to-many, many-toone, or many-to-many. Each of these relationship types is expressed in JPA through an annotation. Table 7.2 lists the relationship annotations we ll discuss in the following sections.
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Table 7.2 Domain relation types and corresponding annotations Annotation @OneToOne @OneToMany @ManyToOne @ManyToMany
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Type of relationship One-to-one One-to-many Many-to-one Many-to-many
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We explore each annotation using examples next.
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7.3.1 @OneToOne
The @OneToOne annotation is used to mark uni- and bidirectional one-to-one relationships. Although in most systems one-to-one relationships are rare, they make perfect sense for domain modeling. In fact, our ActionBazaar example in figure 7.3 has no one-to-one relationship. However, we can imagine that the User domain object parent to both Seller and Bidder has a one-to-one relationship with a BillingInfo object. The BillingInfo object might contain billing data on a user s credit card, bank account, and so on. Let s start by seeing what a unidirectional relationship would look like. Unidirectional one-to-one For the time, being, let s assume that the User object has a reference to the BillingInfo but not vice versa. In other words, the relationship is unidirectional, as shown in figure 7.5.
Entity relationships
Figure 7.5 A one-to-one relationship between the User and BillingInfo entities. A User may have at most one instance of the BillingInfo object and the BillingInfo object cannot exist without a User.
Listing 7.5 illustrates this relationship.
Listing 7.5 Unidirectional one-to-one relationship
@Entity public class User { @Id protected String userId; protected String email; One-to-one relationship between @OneToOne User and BillingInfo protected BillingInfo billingInfo; }
@Entity public class BillingInfo { @Id protected Long billingId; protected String creditCardType; protected String creditCardNumber; protected String nameOnCreditCard; protected Date creditCardExpiration; protected String bankAccountNumber; protected String bankName; protected String routingNumber; }
In listing 7.5, the User class holds a BillingInfo reference in the persisted billingInfo field. Since the billingInfo variable holds only one instance of the BillingInfo class, the relationship is one-to-one. The @OneToOne annotation indicates that the persistence provider should maintain this relationship in the database b. Let s take a closer look at the definition of the @OneToOne annotation to better understand its features:
@Target({METHOD, FIELD}) @Retention(RUNTIME) public @interface OneToOne { Class targetEntity() default void.class; CascadeType[] cascade() default {}; FetchType fetch() default EAGER;
Implementing domain models
boolean optional() default true; String mappedBy() default ""; }
First, note that this annotation can be applied to either fields or properties since the Target is specified to be METHOD, FIELD. We are using field-based persistence for the examples to keep things simple. The targetEntity element tells the persistence provider what the related entity class is. In most cases, this is redundant since the container can infer the class from the class of the field or the return type of the property getter and setter. However, you can specify it explicitly anyway if you prefer. You ll see a case in which this element is indispensable when we explore one-to-many relations. The cascade and fetch parameters are best discussed after we introduce object-relational mapping in the next chapter. For now, suffice it to say that cascade controls what happens to related data when the relationship is altered or deleted and fetch specifies when and how the related fields are populated from database tables. Listing 7.6 shows an example of how the @OneToOne annotation might be applied to a property instead of a field.
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