How to Map Nested Collection `Map<Key, List<Values>>` with Hibernate JPA Annotations: Maintain Order via Index Column & Cascade All

import javax.persistence.*;
 
@Entity
@Table(name = "books")
public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
 
    @Column(nullable = false)
    private String title;
 
    @Column(nullable = false)
    private String author;
 
    // Bidirectional relationship: Book belongs to one Library
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "library_id", nullable = false) // Foreign key to Library
    private Library library;
 
    // Constructors, Getters, Setters
    public Book() {}
 
    public Book(String title, String author) {
        this.title = title;
        this.author = author;
    }
 
    // Getters and Setters (omitted for brevity)
}

import javax.persistence.*;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
 
@Entity
@Table(name = "libraries")
public class Library {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
 
    @Column(nullable = false)
    private String name;
 
    // Nested collection: Map<Genre (String), List<Book>>
    @OneToMany(
        mappedBy = "library", // Bidirectional: Book.library owns the relationship
        cascade = CascadeType.ALL, // Cascade all operations (save, update, delete)
        orphanRemoval = true // Delete books removed from the list
    )
    @MapKeyColumn(name = "genre") // Column to store the map key (e.g., "Fiction")
    @OrderColumn(name = "book_index") // Column to maintain list order
    private Map<String, List<Book>> booksByGenre = new HashMap<>(); // Initialize to avoid NPE
 
    // Constructors, Getters, Setters
    public Library() {}
 
    public Library(String name) {
        this.name = name;
    }
 
    // Helper method to add a book to a genre (maintains bidirectional relationship)
    public void addBookToGenre(String genre, Book book) {
        // Initialize list if genre doesn't exist in the map
        List<Book> books = booksByGenre.computeIfAbsent(genre, k -> new ArrayList<>());
        books.add(book);
        book.setLibrary(this); // Set bidirectional link
    }
 
    // Getters and Setters (omitted for brevity)
}

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.2"
             xmlns="http://xmlns.jcp.org/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence
             http://xmlns.jcp.org/xml/ns/persistence/persistence_2_2.xsd">
 
    <persistence-unit name="LibraryPU" transaction-type="RESOURCE_LOCAL">
        <class>com.example.Library</class>
        <class>com.example.Book</class>
 
        <properties>
            <!-- H2 Database Config -->
            <property name="javax.persistence.jdbc.driver" value="org.h2.Driver"/>
            <property name="javax.persistence.jdbc.url" value="jdbc:h2:mem:librarydb;DB_CLOSE_DELAY=-1"/>
            <property name="javax.persistence.jdbc.user" value="sa"/>
            <property name="javax.persistence.jdbc.password" value=""/>
 
            <!-- Hibernate Config -->
            <property name="hibernate.hbm2ddl.auto" value="create-drop"/> <!-- Auto-creates tables -->
            <property name="hibernate.show_sql" value="true"/> <!-- Log SQL queries -->
            <property name="hibernate.dialect" value="org.hibernate.dialect.H2Dialect"/>
        </properties>
    </persistence-unit>
</persistence>

# H2 Config
spring.datasource.url=jdbc:h2:mem:librarydb
spring.datasource.driverClassName=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=
spring.h2.console.enabled=true # Access H2 console at /h2-console
 
# Hibernate Config
spring.jpa.hibernate.ddl-auto=create-drop
spring.jpa.show-sql=true
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.H2Dialect

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
 
public class LibraryTest {
    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("LibraryPU");
        EntityManager em = emf.createEntityManager();
 
        // 1. Create a Library and add books to genres
        em.getTransaction().begin();
 
        Library library = new Library("City Central Library");
        
        // Add Fiction books (order: 1984, To Kill a Mockingbird)
        library.addBookToGenre("Fiction", new Book("1984", "George Orwell"));
        library.addBookToGenre("Fiction", new Book("To Kill a Mockingbird", "Harper Lee"));
        
        // Add Science books (order: The Martian, Cosmos)
        library.addBookToGenre("Science", new Book("The Martian", "Andy Weir"));
        library.addBookToGenre("Science", new Book("Cosmos", "Carl Sagan"));
 
        em.persist(library); // Cascades to books due to CascadeType.ALL
        em.getTransaction().commit();
 
        // 2. Retrieve the Library and verify the nested map
        em.clear(); // Detach entities to test retrieval
        Library retrievedLibrary = em.find(Library.class, library.getId());
 
        // Check Fiction books (order should be preserved)
        List<Book> fictionBooks = retrievedLibrary.getBooksByGenre().get("Fiction");
        System.out.println("Fiction Books (Expected Order: 1984, To Kill a Mockingbird):");
        fictionBooks.forEach(book -> System.out.println("- " + book.getTitle()));
 
        // 3. Cleanup
        em.close();
        emf.close();
    }
}