JPA and Spring Data repositories

import jakarta.persistence.*;

@Entity
@Table(name = "product")
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)   // the database generates the id
    private Long id;

    @Column(nullable = false, length = 120)
    private String name;

    private String city;
    private long pricePaise;
    private boolean inStock;

    protected Product() { }                               // JPA needs a no-arg constructor
    public Product(String name, String city, long pricePaise) { this.name = name; this.city = city; this.pricePaise = pricePaise; this.inStock = true; }
    // getters …
}

public interface ProductRepository extends JpaRepository<Product, Long> {

    List<Product> findByCity(String city);                                   // derived query
    List<Product> findByCityAndPricePaiseLessThan(String city, long max);
    Optional<Product> findFirstByNameIgnoreCase(String name);
    long countByInStockFalse();

    @Query("select p from Product p where p.inStock = true order by p.pricePaise desc")
    List<Product> findAvailableMostExpensiveFirst();                         // JPQL (uses class and field names)
}

// Spring generates the implementation at start-up:
productRepository.save(new Product("Steel bottle", "Pune", 34_900));
productRepository.findById(1L);          // Optional<Product>
productRepository.findAll();
productRepository.deleteById(1L);

@Entity
public class Customer {
    @Id @GeneratedValue private Long id;
    private String name;
    @OneToMany(mappedBy = "customer")
    private List<Order> orders = new ArrayList<>();
}

@Entity
@Table(name = "orders")                       // "order" is a reserved SQL word
public class Order {
    @Id @GeneratedValue private Long id;
    @ManyToOne(fetch = FetchType.LAZY)        // many orders belong to one customer
    private Customer customer;
    private long amountPaise;
}

@Query("select c from Customer c left join fetch c.orders where c.city = :city")
List<Customer> findWithOrdersByCity(@Param("city") String city);   // one query instead of N+1