如何使用 JPA、Hibernate 将数据添加到与额外列的多对多关联

Question

我有一个 User 表和一个 Book 表要连接。

所以我创建了第三个表 Borrow，它具有外键 (book_id, user_id) 和 takenDate 和 broughtDate 字段。

User.java

@Entity
@Table(name = "Users")
public class User {


    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

    private String name;
    private String surname;
    private String username;
    private String email;
    private String password;

    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Borrow> borrow;
    ....

Book.java

@Entity
@Table(name = "Books")
public class Book {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

    private String title;
    private String ISBN;
    private String author;
    private String issuer;
    private Integer dateOfIssue;
    private Boolean IsRented;


    @OneToMany(mappedBy = "book", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Borrow> borrow;
    .....

Borrow.java

@Entity
@Table(name = "Borrows")
@IdClass(BorrowId.class)
public class Borrow {

    private Date takenDate;
    private Date broughtDate;

    //lazy means it will get details of book
    // only if we call GET method

    @Id
    @JsonIgnore
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "book_id", referencedColumnName = "id")
    private Book book;


    @Id
    @JsonIgnore
    @ManyToOne
    @JoinColumn(name = "user_id", referencedColumnName = "id")
    private User user;
    ....

BorowId.java

public class BorrowId implements Serializable {

    private int book;
    private int user;

    // getters/setters and most importantly equals() and hashCode()

    public int getBook() {
        return book;
    }

    public void setBook(int book) {
        this.book = book;
    }

    public int getUser() {
        return user;
    }

    public void setUser(int user) {
        this.user = user;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof BorrowId)) return false;
        BorrowId borrowId = (BorrowId) o;
        return getBook() == borrowId.getBook() &&
                getUser() == borrowId.getUser();
    }

    @Override
    public int hashCode() {
        return Objects.hash(getBook(), getUser());
    }
}

MySql 数据库设计如下：

我正在尝试将数据添加到 Borrow 表中，如下所示：

已编辑

@Transactional
    @PostMapping("/addUser/{id}/borrow")
    public ResponseEntity<Object> createItem(@PathVariable int id, @RequestBody Borrow borrow, @RequestBody Book book){
        Optional<User> userOptional = userRepository.findById(id);
        Optional<Book> bookOptional = bookRepository.findById(book.getId());


        if(!userOptional.isPresent()){
            throw new UserNotFoundException("id-" + id);
        }

        User user = userOptional.get();
        borrow.setUser(user);
        borrow.setBook(book);

        borrowRepository.save(borrow);

        URI location = ServletUriComponentsBuilder.fromCurrentRequest().path("/{id}").buildAndExpand(borrow.getId()).toUri();

                return ResponseEntity.created(location).build();
    }

我还没有完成它，因为我不确定如何：/ 任何提示表示赞赏！

Answer 1

你快到了。你只需要记住两件事：

1)您还必须通过存储库获取Book（目前您只获取User）

2) 这三个操作必须在同一个事务上下文中：

   fetching of `User`, fetching of `Book` and save `Borrow` entity.

提示：您可以将所有这些放在 Service 中并将其标记为 @Transactional 或将 @Post 方法标记为 @Transactional。我会建议第一个选项，但这取决于你。

编辑：

Optional<Book> bookOptional = bookRepository.findById(book.getId());

此外，这里使用 @EmbeddedId 而不是 @IdClass 似乎就足够了，因为 id 是实际的外国实体：

@Embeddable
public class BorrowId{
   
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "book_id", referencedColumnName = "id")
    private Book book;


    @ManyToOne
    @JoinColumn(name = "user_id", referencedColumnName = "id")
    private User user;
}

然后在 Borrow 类中：

@Entity class Borrow{
  @EmbeddedId BorrwId borrowId;
  ...
}

在 Post 方法中：

    BorrowId borrowId = new BorrowId();
    borrowId.setUser(user);
    borrowId.setBook(book);

    borrow.setBorrowId(borrowId);

    borrowRepository.save(borrow);