Question

我有一个ManyToMany关系，该关系分为2个OneToMany关系。一本书可以属于多个类别，一个类别可以包含多个书籍。当我查询Book时，categories列表只是空的。如何获得一本书所属的所有类别的列表？我想念什么吗？

@Entity
public class Book {
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Integer id;

  @NotBlank(message = "Please input book title")
  private String title;

  private Integer publishYear;

  private String publisher;

  private String language;

  private Integer numberOfPages;

  private String avatarUrl;

  @OneToMany(targetEntity = BookCategory.class, cascade = CascadeType.ALL)
  @JoinColumn(name = "category", nullable = false, insertable = false, updatable = false)
  private Set<BookCategory> categories = new LinkedHashSet<>();
}

@Entity
public class Category {
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Integer id;

  @Column(nullable = false)
  @NotBlank(message = "Please input category name")
  private String name;
}

@Entity
public class BookCategory {
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Integer id;

  @ManyToOne
  @JoinColumn(name = "book", nullable = false)
  private Book book;

  @ManyToOne
  @JoinColumn(name = "category", nullable = false)
  private Category category;
}

Answer 1

尝试将mappedBy属性与OneToMany关系的“一个”一侧的@OneTOMany注释一起使用。

或者您也可以尝试使用以下内容：

@ManyToMany
@JoinTable(
  name = “book”_category, 
  joinColumns = @JoinColumn(name = “book_id”), 
  inverseJoinColumns = @JoinColumn(name = “category_id))

了解更多： https://vladmihalcea.com/the-best-way-to-use-the-manytomany-annotation-with-jpa-and-hibernate/

Spring数据JPA填充2个一对多关系的数据

1 个答案: