Using Spring Data JPA - how to save ONLY UNIQUE items in child entity while saving parent entity and using CascadeType=PERSIST - spring

Question

I am preparing simple Spring app. I have 2 entities :
Book.class (parent) and Author.class (child): with #OneToMany from Author view and #ManyToOne(cascade=CascadeType.PERSIST) from Book view relations. While saving new Book - also Author is being saved and added to DB( mySql)- which is what I want. But I cannot understand why Spring adds Author - if such item already exists. How to change the code to make sure that only unique Authors will be added to DB and there will be no duplicates in author table in DB?
I've added hashCode and equals methods to Author class but it did not help.
I've tried to change also Cascade.Type but also did not help.
The Author.class(part of code):
#Entity
public class Author {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String firstName;
private String lastName;
#OneToMany(mappedBy = "author")
#JsonManagedReference(value = "book-author")
private Set<Book> books = new HashSet<Book>();
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Author author = (Author) o;
return Objects.equals(getFirstName(), author.getFirstName()) &&
Objects.equals(getLastName(), author.getLastName());
}
#Override
public int hashCode() {
return Objects.hash(getFirstName(), getLastName());
}
And the Book.class(part of code):
#Entity
public class Book {
#Id
#GeneratedValue(strategy= GenerationType.IDENTITY)
private Long id;
private String title;
#ManyToOne( cascade=CascadeType.PERSIST)
#JoinColumn(name = "author_id", unique = true)
#JsonBackReference(value="book-author")
private Author author;
Edit 1
BookServiceImpl.class
#Override
public BookDto addBookDto(BookDto bookDto) {
Book book = bookConverter.apply(bookDto);
bookRepository.save(book);
return bookDtoConverter.apply(book);
}
AuthorServiceImpl.class
#Override
public Author findAuthor(String firstName, String lastName) {
Optional<Author> authorByNameOptional = authorRepository.findByFirstNameAndLastName(firstName, lastName);
if (authorByNameOptional.isPresent()) {
return authorByNameOptional.get();
} else {
Author newAuthor = new Author();
newAuthor.setFirstName(firstName);
newAuthor.setLastName(lastName);
return newAuthor;
}
And BookWebController.class
#PostMapping("/addWebBook")
public String addBook(#ModelAttribute(name = "addedBook") BookDto addedBook, Model model) {
Author author1 = addedBook.getAuthor();
Author author = authorService.findAuthor(author1.getFirstName(), author1.getLastName());
addedBook.setAuthor(author);
bookService.addBookDto(addedBook);
return "redirect:/message?msg";
}
Would be greatful for any hint as I am quite new to this area :-)

Answer 1

Let me suggest that your Author object in Book has empty primary key field? Hibernate's logic in this case: empty id means new row to insert. It may work correctly if you set a primary key to author. For example, user can find author (with it's PK) or add new (without PK) and call save(book) method which will cascadely persists only new author. In most cases that usually works like that.
Another moment to pay attention, that if you wanna keep author's uniqueness in database, than you must to put constraint on the author's entity.
For example if each author must have unique first name and last name it may look something like this:
#Entity
#Table(uniqueConstraints= #UniqueConstraint(columnNames={"first_name", "last_name"}))
public class Author {
...
After that, DataIntegrityViolationException will be thrown on duplicating value insertion and your database will stay clean of duplicates.