How to persist an entity having a ManyToMany association with other entity in JPA?

I am not able to add new Book with existing Genre in SpringBoot from json in Postman. It says 'constraint ["PRIMARY KEY ON PUBLIC.GENRE(GENRE_ID)'.

Here is my model:

@Entity
@NoArgsConstructor
@AllArgsConstructor
@Data
@Table(name = "book")
public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name="book_id")
    private Integer Id;

    private String title;

    private String imgurl;

    private String description;
    private String bookpath;
    @ManyToOne
    private Author author;


    @ManyToMany(cascade = CascadeType.ALL)
    @JoinTable(name = "book_genre",
            joinColumns = @JoinColumn(name="book_id"),
            inverseJoinColumns = @JoinColumn(name="genre_id"))
    private List<Genre> genres;

    public void addGenre(Genre genre){
        this.genres.add(genre);
        genre.getBooks().add(this);
    }



}





@NoArgsConstructor
@AllArgsConstructor
@Entity
@Data
@Table(name = "genre")
public class Genre {
    @Id
    @Column(name="genre_id")
    private Integer id;

    private String name;


    @JsonIgnore
    @ManyToMany(cascade = CascadeType.ALL)
    @JoinTable(name="book_genre",
            joinColumns=@JoinColumn(name="genre_id"),
            inverseJoinColumns=@JoinColumn(name="book_id"))
    private List<Book> books;

    public List<Book> getBooks(){
        return this.books;
    }


}

I am working with jpa Repository. Here is my book controller post method:

@PostMapping("/books")
    Book createBook(@RequestBody Book book){
        return this.bookRepository.save(book);
    }

In postman I have this:

{
    "title": "Evgenij Onegin",
    "imgurl": "https://biblioteka.msu.edu.mk/images/covers/2018/September/5bab75a75007c/821.806.jpeg",
    "description": "desctiption",
    "bookpath": "Books\\book1.pdf",
    "author": {
        "name": "Alexandar",
        "surname": "Puskin",
        "birthdate": "1799-06-05",
        "description": "Russian author",
        "location": "Russia",
        "category": {
            "name": "Romantism",
            "id": 1
        },
        "id": 1
    },
    "genres": [
        {
            "id": 1,
            "name": "Trailer"
        },
        {
            "id": 2,
            "name": "Comic"
        }
    ]
}

And the error I am getting:

constraint [\"PRIMARY KEY ON PUBLIC.GENRE(GENRE_ID)'.

So it seems like the problem is in ManyToMany relationship. Without adding Genre to Book it works perfect, but I want to add it.

You are doing the ManyToMany association incorrect. You are defining the JoinTable on the both end

public class Book {
 ...
  @ManyToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE } )
  @JoinTable(name = "book_genre",
    joinColumns = @JoinColumn(name = "book_id"),
    inverseJoinColumns = @JoinColumn(name = "genre_id")
  )
  private List<Genre> genres;
 ...

}

And on the other side in Genre you just do this

public class Genre {
  ...
  @ManyToMany(mappedBy = "genres")
  private List<Book> books;
  ...
}

This is a very good article about The best way to use the @ManyToMany annotation with JPA and Hibernate This will help you understading the association mappings.

You should have used CascadeType.PERSIST in both of your @ManyToMany mappings for Books and Genre.
Also, books in Genre entity should be mapped by genres collection.

// Book entity:
    @ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.PERSIST)
    @JoinTable(name = "book_genre",
            joinColumns = @JoinColumn(name="book_id"),
            inverseJoinColumns = @JoinColumn(name="genre_id"))
    private List<Genre> genres;

// Genre entity:
    @ManyToMany(mappedBy = "genres", fetch = FetchType.EAGER, cascade = CascadeType.PERSIST)
    private List<Book> books;

By: https://www.javaworld.com/article/3387643/java-persistence-with-jpa-and-hibernate-part-2-many-to-many-relationships.html