文章目录
Spring Boot JPA中关联表的使用
本文中，我们会将会通过一个Book和Category的关联关系，来讲解如何在JPA中使用。
添加依赖
我们还是使用H2内存数据库来做测试：
<dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-data-jpa</artifactId> </dependency> <dependency> <groupId>com.h2database</groupId> <artifactId>h2</artifactId> <scope>runtime</scope> </dependency>
构建Entity
下面我们构建两个Entity：
@Data @Entity public class Book { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; private String title; @ManyToOne private Category category; }
@Data @Entity public class Category { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; private String name; @OneToMany(mappedBy = "category", cascade = CascadeType.ALL) private List<Book> books; }
上面我们定义了两个Entity，Category和Book是一对多的关系。我们通过@ManyToOne和@OneToMany来定义相应的关系。
构建Repository
我们接下来构建相应的Repository：
public interface BookRepository extends CrudRepository<Book, Long> { long deleteByTitle(String title); @Modifying @Query("delete from Book b where b.title=:title") void deleteBooks(@Param("title") String title); }
public interface CategoryRepository extends CrudRepository<Category, Long> {}
构建初始数据
为了方便测试，我们先构建需要的数据schema.sql和data.sql:
CREATE TABLE book ( id BIGINT NOT NULL AUTO_INCREMENT, title VARCHAR(128) NOT NULL, category_id BIGINT, PRIMARY KEY (id) ); CREATE TABLE category ( id BIGINT NOT NULL AUTO_INCREMENT, name VARCHAR(128) NOT NULL, PRIMARY KEY (id) );
insert into book(id,title,category_id) values(1,'The Hobbit',1); insert into book(id,title,category_id) values(2,'The Rabbit',1); insert into category(id,name) values(1,'category');
测试
我们看一下怎么从Book中删除一条数据：
@Test public void whenDeleteByIdFromRepository_thenDeletingShouldBeSuccessful() { assertThat(bookRepository.count()).isEqualTo(2); bookRepository.deleteById(1L); assertThat(bookRepository.count()).isEqualTo(1); }
再看一下category的删除：
@Test public void whenDeletingCategories_thenBooksShouldAlsoBeDeleted() { categoryRepository.deleteAll(); assertThat(bookRepository.count()).isEqualTo(0); assertThat(categoryRepository.count()).isEqualTo(0); }
再看一下book的删除：
@Test public void whenDeletingBooks_thenCategoriesShouldAlsoBeDeleted() { bookRepository.deleteAll(); assertThat(bookRepository.count()).isEqualTo(0); assertThat(categoryRepository.count()).isEqualTo(1); }
因为我们只在Category中指定了cascade = CascadeType.ALL， 所以删除category的时候可以删除相关联的Book，但是删除Book的时候不会删除相关联的category。
本文的例子可以参考https://github.com/ddean2009/learn-springboot2/tree/master/springboot-jpa-relation
