Spring + Hibernate：在 OneToMany 关系中创建映射到虚构父级的子级

Question

I have these 2 entities:

@AllArgsConstructor
@NoArgsConstructor
@EqualsAndHashCode
@Getter
@Entity
@Table(name = "BOOKS")
public class Book {

    @Id
    @GeneratedValue
    @Column(name = "ID")
    private long id;

    @NotNull
    @Column(name = "TITLE")
    private String title;

    @NotNull
    @Column(name = "AUTHOR")
    private String author;

    @NotNull
    @Column(name = "PUBLICATION_YEAR")
    private int publicationYear;

    @JsonBackReference
    @EqualsAndHashCode.Exclude
    @OneToMany(
            targetEntity = BookCopy.class,
            mappedBy = "book",
            cascade = CascadeType.ALL,
//            orphanRemoval = true,
            fetch = FetchType.LAZY
    )
    private List<BookCopy> bookCopies = new ArrayList<>();

    public Book(String title, String author, int publicationYear) {
        this.title = title;
        this.author = author;
        this.publicationYear = LocalDate.of(publicationYear, 1, 1).getYear();
    }
}

@AllArgsConstructor
@NoArgsConstructor
@EqualsAndHashCode
@Getter
@Setter
@Entity
@Table(name = "BOOK_COPIES")
public class BookCopy {

    @Id
    @GeneratedValue
    @Column(name = "ID")
    private long id;

    @JsonManagedReference
    @NotNull
    @EqualsAndHashCode.Exclude
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "TITLE_ID")
    private Book book;

    @Enumerated(EnumType.STRING)
    @Column(name = "RENT_STATUS")
    private RentStatus rentStatus;

    public BookCopy(Book book, RentStatus rentStatus) {
        this.book = book;
        this.rentStatus = rentStatus;
    }

    public void setBook(Book book) {
        this.book = book;
        book.getBookCopies().add(this);
    }
}

Now, through Hibernate I create and save a new BookCopy item, by sending through to my Spring Web system a JSON like this one:

{
    "book":{
        "id": 99999
    },
    "rentStatus": "HIRED"
}

The thing is, that book by that id does not exist in the Book table. Is there a way, in which I can have this BookCopy save operation first verify the existence of its parent relationship?

UPDATE: Adding classes taking care of saving in DB:

First, the DBService class:

@Repository
public class BookCopyDbService {
    @Autowired
    private BookCopyRepository bookCopyRepository;

    public BookCopy saveBookCopy(BookCopy bookCopy) {
        return bookCopyRepository.save(bookCopy);
    }
}

And the CrudRepository class, interfacing with the DB:

@Transactional
@Repository
public interface BookCopyRepository extends CrudRepository<BookCopy, Long> {
}

Probably, I could put some logic in BookCopyDbService's saveBookCopy(), which would first try to find that book by ID in the Book table, and save BookCopy only if a book was found in that search, but I was wondering if Hibernate actually had some mechanism in place which did that for me.

Answer 1

You need to first fetch the Book entity by the id and then try to set the BookCopy. If book doesn't exsits, then you can throw exception.

@Service
public class BookService {
    @Autowired
    private BookRepository bookRepository;

    @Transactional
    public BookCopy saveBookCopy(BookCopy bookCopy,Long bookId) {
      Book book =bookRepository.findById(bookId).orElseThrow(()-> new 
       BookNotFoundException("Book not found."));
      book.getBookCopies().add(bookCopy);

        return book.getBookCopy();
    }

}

@Transactional will take care of inserting the BookCopy into the table.