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Entity relationships
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the Bid. Similar to the one-to-one case, we can mark the owning side of the relationship by using the mappedBy column on the entity that is not the owner of the relationship. We ll analyze these relationships further by actually coding the BidItem relationship (see listing 7.8).
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Listing 7.8 One-to-many bidirectional relationship
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@Entity public class Item { @Id protected Long itemId; protected String title; protected String description; protected Date postdate; ... @OneToMany(mappedBy="item") protected Set<Bid> bids; ... } @Entity public class Bid { @Id protected Long bidId; protected Double amount; protected Date timestamp; ... @ManyToOne protected Item item; ... }
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One-to-many relationship
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Corresponding many-to-one relationship
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One-to-many relationship Listing 7.8 shows that the Item domain object has a Set of Bid objects that it has references to. To signify this domain relationship, the bids field is marked with a @OneToMany annotation. There are a few nuances about the @OneToMany annotation we should talk about. To explore them, take a quick look at the definition of the annotation:
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@Target({METHOD, FIELD}) @Retention(RUNTIME) public @interface OneToMany { Class targetEntity() default void.class; CascadeType[] cascade() default {}; FetchType fetch() default LAZY; String mappedBy() default ""; }
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Implementing domain models
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As you ll notice, this is literally identical to the definition of the @OneToOne annotation, including the mappedBy element. As a matter of fact, the only element we need to discuss further is targetEntity. Remember that this element is used to specify the class of the related entity if it is not immediately obvious. In the @OneToMany annotation used in listing 7.8, this parameter is omitted since we are using Java generics to specify the fact that the bids variable stores a Set of Bid objects:
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@OneToMany(mappedBy="item") protected Set<Bid> bids;
Imagine, however, what would happen if we did not use generics on the Set. In this case, it would be impossible for the persistence provider to determine what entity the Item object has a relation to. This is exactly the situation the targetEntity parameter is designed for. We would use it to specify the entity at the other end of the one-to-many relationship as follows:
@OneToMany(targetEntity=Bid.class,mappedBy="item") protected Set bids;
Many-to-one as owning-side of relationship Also, note the mappedBy="item" value on the @OneToMany annotation. This value specifies the owning side of the bidirectional relationship as the items field of the Bid entity. Because the relationship is bidirectional, the Bid domain object has a reference to an Item through the item variable. The @ManyToOne annotation on the item variable tells the persistence provider that more than one Bid entity could hold references to the same Item instance. For bidirectional one-to-many relationships, ManyToOne is always the owning side of the relationship. Because of this fact, the mappedBy element does not exist in the definition of the @ManyToOne annotation:
@Target({METHOD, FIELD}) @Retention(RUNTIME) public @interface ManyToOne { Class targetEntity() default void.class; CascadeType[] cascade() default {}; FetchType fetch() default EAGER; boolean optional() default true; }
Other than this minor difference, all the other elements of the @ManyToOne annotation have the same purpose and functionality as the elements in the @OneToOne and @OneToMany annotations. The last type of domain relationship is many-to-many, which we ll discuss next.
Entity relationships
7.3.3 @ManyToMany
While not as common as one-to-many, many-to-many relationships occur quite frequently in enterprise applications. In this type of relationship, both sides might have multiple references to related entities. In our ActionBazaar example, the relationship between Category and Item is many-to-many, as shown in figure 7.7.
Figure 7.7 The relationship between Category and Item is many-to-many because every category may have one or more items, whereas each item may belong to more than one category.
That is, a category can contain multiple items and an item can belong to multiple categories. For example, a category named Sixties Fashion could contain items like Bellbottom Pants and Platform Shoes. Bellbottom Pants and Platform Shoes could also be listed under Uncomfortable and Outdated Clothing. Although many-to-many relationships can be unidirectional, they are often bidirectional because of their crossconnecting, mutually independent nature. Not too surprisingly, a bidirectional many-to-many relationship is often represented by @ManyToMany annotations on opposite sides of the relationship. As with the one-toone and one-many relationships, you can identify the owning side of the relationship by specifying mappedBy on the subordinate entity; you may have to use the targetEntity attribute if you re not using Java generics. The definition for @ManyToMany is identical to OneToMany and holds no special intricacies beyond those already discussed:
@Target({METHOD, FIELD}) @Retention(RUNTIME) public @interface ManyToMany { Class targetEntity() default void.class; CascadeType[] cascade() default {}; FetchType fetch() default LAZY; String mappedBy() default ""; }
To round off our discussion of many-to-many relationships, listing 7.9 shows how the Item-Category relationship might look.
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