Spring JPA difficult delete - jpa

Question

Can someone help with the following model, implemented, but unable to delete as desired
Author OneToMany with Book
Book ManyToMany with Genre
Genre ManyToMany with Book
I can create the db and add records. I believe my cascade designation is wrong as I cannot do the following
From Author, delete a book(yes odd,copyright issues:) have it deleted from Books BUT also have it removed from Genre
The problem is depending on what cascade I either get FK violation on Genre delete or FK violation on Book delete. Is it a deadlock? It is a trivial pattern and must be solveable. Thank you

Answer 1

Ultimately I think that cascade is best used to handle the case when all child relations are deleted but still I think it's a complicated annotation and not well understood by most. I think that if the Book entity owns the authors and genres relationships then the ORM has no choice but to delete the relations when a book is deleted.
When I setup a simple springboot project it seems to take case of all this.
#Entity
#Data
#Builder
#AllArgsConstructor
#NoArgsConstructor
public class Author {
#Id #GeneratedValue(strategy = GenerationType.IDENTITY) private Long id;
#ManyToMany(mappedBy = "authors")
#lombok.ToString.Exclude
private Set<Book> books;
}
#Entity
#Data
#Builder
#AllArgsConstructor
#NoArgsConstructor
public class Genre {
#Id #GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#ManyToMany(mappedBy = "genres")
#lombok.ToString.Exclude
private Set<Book> books;
}
#Entity
#Data
#Builder
#AllArgsConstructor
#NoArgsConstructor
public class Book {
#Id #GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#ManyToMany
private Set<Author> authors;
#ManyToMany
private Set<Genre> genres;
}
And an simple example:
Author author = authorRepository.save(Author.builder().build());
Genre genre = genreRepository.save(Genre.builder().build());
Book book = bookRepository.save(Book.builder().authors(Collections.singleton(author)).genres(Collections.singleton(genre)).build());
System.out.println(book);
bookRepository.delete(bookRepository.getOne(book.getId()));
And the logs seem to indicate that authors and genres are cleaned up when a book is deleted.
Hibernate: insert into author (id) values (null)
Hibernate: insert into genre (id) values (null)
Hibernate: insert into book (id) values (null)
Hibernate: insert into book_authors (books_id, authors_id) values (?, ?)
Hibernate: insert into book_genres (books_id, genres_id) values (?, ?)
Book(id=1, authors=[Author(id=1)], genres=[Genre(id=1)])
Hibernate: select book0_.id as id1_1_0_ from book book0_ where book0_.id=?
Hibernate: delete from book_authors where books_id=?
Hibernate: delete from book_genres where books_id=?
Hibernate: delete from book where id=?
I think this is doing what your questioned is about.