Spring Boot JPA中關聯表的使用

文章目錄

• 添加依賴
• 構建Entity
• 構建Repository
• 構建初始數據
• 測試

Spring Boot JPA中關聯表的使用

本文中，我們會將會通過一個Book和Category的關聯關系，來講解如何在JPA中使用。

添加依賴

我們還是使用H2內存數據庫來做測試：

        <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>

構建Entity

下面我們構建兩個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;
}

上面我們定義了兩個Entity，Category和Book是一對多的關系。我們通過@ManyToOne和@OneToMany來定義相應的關系。

構建Repository

我們接下來構建相應的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> {}

構建初始數據

為了方便測試，我們先構建需要的數據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');

測試

我們看一下怎么從Book中刪除一條數據：

    @Test
    public void whenDeleteByIdFromRepository_thenDeletingShouldBeSuccessful() {
        assertThat(bookRepository.count()).isEqualTo(2);
        bookRepository.deleteById(1L);
        assertThat(bookRepository.count()).isEqualTo(1);
    }

再看一下category的刪除：

    @Test
    public void whenDeletingCategories_thenBooksShouldAlsoBeDeleted() {
        categoryRepository.deleteAll();
        assertThat(bookRepository.count()).isEqualTo(0);
        assertThat(categoryRepository.count()).isEqualTo(0);
    }

再看一下book的刪除：

    @Test
    public void whenDeletingBooks_thenCategoriesShouldAlsoBeDeleted() {
        bookRepository.deleteAll();
        assertThat(bookRepository.count()).isEqualTo(0);
        assertThat(categoryRepository.count()).isEqualTo(1);
    }

因為我們只在Category中指定了cascade = CascadeType.ALL， 所以刪除category的時候可以刪除相關聯的Book，但是刪除Book的時候不會刪除相關聯的category。

本文的例子可以參考https://github.com/ddean2009/learn-springboot2/tree/master/springboot-jpa-relation

更多教程請參考 flydean的博客