Spring-Boot查询SELECT*from book（MANY to MANY rel with user）无法正常工作

Question

我试图从我的数据库中返回所有的书籍，但是一旦我将值插入到名为COLLECTION的连接表中（用于解决BOOK和USER之间的多对多Rel），我收到的输出看起来像一个循环（用Postman测试）。

图书实体的实施：

@Getter
@Setter
@ToString
@EqualsAndHashCode
@NoArgsConstructor
@AllArgsConstructor

@Entity
public class Book {

    @Id
    private Integer id;
    private String title;
    private String author;
    private String description;

    @OneToMany(mappedBy = "book", cascade = CascadeType.ALL)
    private Set<Collection> collections;

    public Book(String title, String author, String description) {
        this.title = title;
        this.author = author;
        this.description = description; 
}

    public Book(String title, String author Collection... collections){
        this.title = title;
        this.author = author;

        for (Collection collection : collections){
            collection.setBook(this);
        }
        this.collections = Stream.of(collections).collect(Collectors.toSet());
    }
}

用户实体的实现：

@Getter
@Setter
@NoArgsConstructor

@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private String email;
    private String password;
    private String username;

    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL)
    private Set<Collection> collections = new HashSet<>();

    public User(String username, String password) {
        this.username = username;
        this.password = password;
    }
}

集合实体：

@Getter
@Setter
@NoArgsConstructor

@Entity
public class Collection implements Serializable {
    @Id
    @ManyToOne
    @JoinColumn
    private Book book;

    @Id
    @ManyToOne
    @JoinColumn
    private User user;

 public Collection(User user){
        this.user = user;
}
 @Override
    public boolean equals(Object o){
        if(this == o) return true;
        if(!(o instanceof Collection)) return false;
        Collection that = (Collection) o;
        return  Objects.equals(book.getTitle(), that.book.getTitle()) &&
                Objects.equals(book.getAuthor(), that.book.getAuthor()) &&
                Objects.equals(user.getUsername(), that.user.getUsername());
}
 @Override
    public int hashCode(){
        return Objects.hash(book.getTitle(), book.getAuthor(), user.getUsername());
    }
}

最后是BookRepository：

@Transactional
@Repository
public interface BookRepository extends JpaRepository<Book, Integer> {
// this causes sql syntax error
//    @Query(value = "SELECT b.id, b.title, b.author FROM book b", nativeQuery =  true)
//    List<Book> getAllBooks();

// this returns the loopy output from postman detailed below 
     List<Book> findAll();
}

我在Postman中收到的输出看起来像一个循环：

[{"id":1,"title":"Pride and Prejudice","author":"Jane Austen","users":[{"id":1,"email":"[e-mail]","password":"$2a$10$BVXUCumzWyec9zEUeCv1r.m2pFwvAe7Cp1dLjiGfXuEEIHkhn3jHO","username":"user","books":[{"id":1,"title":"Pride and Prejudice","author":"Jane Austen", "users":[{"id":1,"email":"[e-mail]","password":"$2a$10$BVXUCumzWyec9zEUeCv1r.m2pFwvAe7Cp1dLjiGfXuEEIHkhn3jHO, "username":"user","books": .......

预期的输出应该是我在数据库中的所有书籍的列表。

Answer 1

要设置book和user之间的关系，请使用@ManyToMany映射设置直接关系。作为参考，您可以选择以下链接-

https://www.callicoder.com/hibernate-spring-boot-jpa-many-to-many-mapping-example/

此外，您不需要显式定义findAll（），Jpa存储库本身提供了这些通用方法。