How to add data to Many to Many association with extra column using JPA, Hibernate

I have a User table and a Book table that I would like to connect.

So I created third table Borrow that has foreign key (book_id, user_id) and takenDate and broughtDate fields.

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());
    }
}

My MySql database design looks like this:

I am trying to add data to Borrow table something like this:

EDITED

@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();
    }

I have't finished it because I am not sure how :/
Any tip is appreciated!

You are almost there. You just have to keep in mind two things:

1) You have to fetch the Book via repository as well (you only fetch the User currently)

2) All three operation have to be within the same transactional context:

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

TIP: You can put all these inside a Service and mark it as @Transactional or mark the @Post method as @Transactional. I would suggest first option, but it is up to you.

EDIT:

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

Also, it seems adequate to use @EmbeddedId instead of @IdClass here as ids are actual foreign entities:

@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;
}

and then in the Borrow class:

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

and in the Post method:

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

    borrow.setBorrowId(borrowId);

    borrowRepository.save(borrow);