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118 CHAPTER 6 n MAPPING WITH ANNOTATIONS CHAPTER 6 n MAPPING WITH ANNOTATIONS Listing 6-16. Mapping a Many-to-One Relationship from the PublisherEntity to the Book Entity @ManyToOne @JoinColumn(name = “publisher_id”) public Publisher getPublisher() { return publisher; } The @ManyToOne annotation takes a similar set of attributes to @OneToMany. The following list describes the attributes, all of which are optional. cascade indicates the appropriate cascade policy for operations on the association; it defaults to none. fetch indicates the fetch strategy to use; it defaults to LAZY. optional indicates whether the value can be null; it defaults to true. targetEntity indicates the entity that stores the primary key this is normally inferred from the type of the field or property (Publisherin the preceding example). We have also supplied the optional @JoinColumnattribute to name the foreign key column required by the association something other than the default (publisher) this is not necessary, but it illustrates the use of the annotation. When a unidirectional one-to-many association is to be formed, it is possible to express the relationship using a link table. This is achieved by adding the @JoinTable annotation as shown in Listing 6-17.2 Listing 6-17. A Simple Unidirectional One-to-Many Association with a Join Table @OneToMany(cascade = ALL) @JoinTable public Set getBooks() { return books; } The @JoinTable annotation provides attributes that allow various aspects of the link table to be controlled. These attributes are as follows: name is the name of the join table to be used to represent the association. catalog is the name of the catalog containing the join table. schema is the name of the schema containing the join table. joinColumns is an array of @JoinColumn attributes representing the primary key of the entity at the one end of the association. inverseJoinColumns is an array of @JoinColumn attributes representing the primary key of the entity at the many end of the association. 2. When a join table is being used, the foreign key relationship is maintained within the join table itself it is therefore not appropriate to combine the mappedBy attribute of the @OneToMany annotation with the use of an @JoinTable annotation.
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