JPA Patterns Skill

// ❌ BAD: N+1 queries
@Entity
public class Author {
    @Id private Long id;
    private String name;

    @OneToMany(mappedBy = "author", fetch = FetchType.LAZY)
    private List<Book> books;
}

// This innocent code...
List<Author> authors = authorRepository.findAll();  // 1 query
for (Author author : authors) {
    System.out.println(author.getBooks().size());   // N queries!
}
// Result: 1 + N queries (if 100 authors = 101 queries)

// ✅ GOOD: Single query with JOIN FETCH
public interface AuthorRepository extends JpaRepository<Author, Long> {

    @Query("SELECT a FROM Author a JOIN FETCH a.books")
    List<Author> findAllWithBooks();
}

// Usage - single query
List<Author> authors = authorRepository.findAllWithBooks();

// ✅ GOOD: EntityGraph for declarative fetching
public interface AuthorRepository extends JpaRepository<Author, Long> {

    @EntityGraph(attributePaths = {"books"})
    List<Author> findAll();

    // Or with named graph
    @EntityGraph(value = "Author.withBooks")
    List<Author> findAllWithBooks();
}

// Define named graph on entity
@Entity
@NamedEntityGraph(
    name = "Author.withBooks",
    attributeNodes = @NamedAttributeNode("books")
)
public class Author {
    // ...
}

// ✅ GOOD: Batch fetching (Hibernate-specific)
@Entity
public class Author {

    @OneToMany(mappedBy = "author")
    @BatchSize(size = 25)  // Fetch 25 at a time
    private List<Book> books;
}

// Or globally in application.properties
spring.jpa.properties.hibernate.default_batch_fetch_size=25

# Enable SQL logging to detect N+1
spring:
  jpa:
    show-sql: true
    properties:
      hibernate:
        format_sql: true

logging:
  level:
    org.hibernate.SQL: DEBUG
    org.hibernate.type.descriptor.sql.BasicBinder: TRACE

@Entity
public class Order {

    // LAZY: Load only when accessed (default for collections)
    @OneToMany(mappedBy = "order", fetch = FetchType.LAZY)
    private List<OrderItem> items;

    // EAGER: Always load immediately (default for @ManyToOne, @OneToOne)
    @ManyToOne(fetch = FetchType.EAGER)  // ⚠️ Usually bad
    private Customer customer;
}

// ✅ GOOD: Always use LAZY, fetch when needed
@Entity
public class Order {

    @ManyToOne(fetch = FetchType.LAZY)  // Override EAGER default
    private Customer customer;

    @OneToMany(mappedBy = "order", fetch = FetchType.LAZY)
    private List<OrderItem> items;
}

// ❌ BAD: Accessing lazy field outside transaction
@Service
public class OrderService {

    public Order getOrder(Long id) {
        return orderRepository.findById(id).orElseThrow();
    }
}

// In controller (no transaction)
Order order = orderService.getOrder(1L);
order.getItems().size();  // 💥 LazyInitializationException!

// ✅ Fetch needed associations in query
@Query("SELECT o FROM Order o JOIN FETCH o.items WHERE o.id = :id")
Optional<Order> findByIdWithItems(@Param("id") Long id);

// ✅ Keep transaction open while accessing
@Service
public class OrderService {

    @Transactional(readOnly = true)
    public OrderDTO getOrderWithItems(Long id) {
        Order order = orderRepository.findById(id).orElseThrow();
        // Access within transaction
        int itemCount = order.getItems().size();
        return new OrderDTO(order, itemCount);
    }
}

// ✅ BEST: Return only what you need
public interface OrderSummary {
    Long getId();
    String getStatus();
    int getItemCount();
}

@Query("SELECT o.id as id, o.status as status, SIZE(o.items) as itemCount " +
       "FROM Order o WHERE o.id = :id")
Optional<OrderSummary> findOrderSummary(@Param("id") Long id);

# Keeps session open during view rendering
# ⚠️ Can mask N+1 problems, use with caution
spring:
  jpa:
    open-in-view: true  # Default is true

@Service
public class OrderService {

    // Read-only: Optimized, no dirty checking
    @Transactional(readOnly = true)
    public Order findById(Long id) {
        return orderRepository.findById(id).orElseThrow();
    }

    // Write: Full transaction with dirty checking
    @Transactional
    public Order createOrder(CreateOrderRequest request) {
        Order order = new Order();
        // ... set properties
        return orderRepository.save(order);
    }

    // Explicit rollback
    @Transactional(rollbackFor = Exception.class)
    public void processPayment(Long orderId) throws PaymentException {
        // Rolls back on any exception, not just RuntimeException
    }
}

@Service
public class OrderService {

    @Autowired
    private PaymentService paymentService;

    @Transactional
    public void placeOrder(Order order) {
        orderRepository.save(order);

        // REQUIRED (default): Uses existing or creates new
        paymentService.processPayment(order);

        // If paymentService throws, entire order is rolled back
    }
}

@Service
public class PaymentService {

    // REQUIRES_NEW: Always creates new transaction
    // If this fails, order can still be saved
    @Transactional(propagation = Propagation.REQUIRES_NEW)
    public void processPayment(Order order) {
        // Independent transaction
    }

    // MANDATORY: Must run within existing transaction
    @Transactional(propagation = Propagation.MANDATORY)
    public void updatePaymentStatus(Order order) {
        // Throws if no transaction exists
    }
}

// ❌ BAD: Calling @Transactional method from same class
@Service
public class OrderService {

    public void processOrder(Long id) {
        updateOrder(id);  // @Transactional is IGNORED!
    }

    @Transactional
    public void updateOrder(Long id) {
        // Transaction not started because called internally
    }
}

// ✅ GOOD: Inject self or use separate service
@Service
public class OrderService {

    @Autowired
    private OrderService self;  // Or use separate service

    public void processOrder(Long id) {
        self.updateOrder(id);  // Now transaction works
    }

    @Transactional
    public void updateOrder(Long id) {
        // Transaction properly started
    }
}

// ✅ GOOD: Bidirectional with proper mapping
@Entity
public class Author {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @OneToMany(mappedBy = "author", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Book> books = new ArrayList<>();

    // Helper methods for bidirectional sync
    public void addBook(Book book) {
        books.add(book);
        book.setAuthor(this);
    }

    public void removeBook(Book book) {
        books.remove(book);
        book.setAuthor(null);
    }
}

@Entity
public class Book {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "author_id")
    private Author author;
}

// ✅ GOOD: ManyToMany with Set (not List) to avoid duplicates
@Entity
public class Student {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinTable(
        name = "student_course",
        joinColumns = @JoinColumn(name = "student_id"),
        inverseJoinColumns = @JoinColumn(name = "course_id")
    )
    private Set<Course> courses = new HashSet<>();

    public void addCourse(Course course) {
        courses.add(course);
        course.getStudents().add(this);
    }

    public void removeCourse(Course course) {
        courses.remove(course);
        course.getStudents().remove(this);
    }
}

@Entity
public class Course {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToMany(mappedBy = "courses")
    private Set<Student> students = new HashSet<>();
}

// ✅ GOOD: Use business key or ID carefully
@Entity
public class Book {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @NaturalId  // Hibernate annotation for business key
    @Column(unique = true, nullable = false)
    private String isbn;

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Book book)) return false;
        return isbn != null && isbn.equals(book.isbn);
    }

    @Override
    public int hashCode() {
        return Objects.hash(isbn);  // Use business key, not ID
    }
}

// ✅ GOOD: Always paginate large result sets
public interface OrderRepository extends JpaRepository<Order, Long> {

    Page<Order> findByStatus(OrderStatus status, Pageable pageable);

    // With sorting
    @Query("SELECT o FROM Order o WHERE o.status = :status")
    Page<Order> findByStatusSorted(
        @Param("status") OrderStatus status,
        Pageable pageable
    );
}

// Usage
Pageable pageable = PageRequest.of(0, 20, Sort.by("createdAt").descending());
Page<Order> orders = orderRepository.findByStatus(OrderStatus.PENDING, pageable);

// ✅ GOOD: Fetch only needed columns

// Interface-based projection
public interface OrderSummary {
    Long getId();
    String getCustomerName();
    BigDecimal getTotal();
}

@Query("SELECT o.id as id, o.customer.name as customerName, o.total as total " +
       "FROM Order o WHERE o.status = :status")
List<OrderSummary> findOrderSummaries(@Param("status") OrderStatus status);

// Class-based projection (DTO)
public record OrderDTO(Long id, String customerName, BigDecimal total) {}

@Query("SELECT new com.example.dto.OrderDTO(o.id, o.customer.name, o.total) " +
       "FROM Order o WHERE o.status = :status")
List<OrderDTO> findOrderDTOs(@Param("status") OrderStatus status);

// ✅ GOOD: Bulk update instead of loading entities
public interface OrderRepository extends JpaRepository<Order, Long> {

    @Modifying
    @Query("UPDATE Order o SET o.status = :status WHERE o.createdAt < :date")
    int updateOldOrdersStatus(
        @Param("status") OrderStatus status,
        @Param("date") LocalDateTime date
    );

    @Modifying
    @Query("DELETE FROM Order o WHERE o.status = :status AND o.createdAt < :date")
    int deleteOldOrders(
        @Param("status") OrderStatus status,
        @Param("date") LocalDateTime date
    );
}

// Usage
@Transactional
public void archiveOldOrders() {
    LocalDateTime threshold = LocalDateTime.now().minusYears(1);
    int updated = orderRepository.updateOldOrdersStatus(
        OrderStatus.ARCHIVED,
        threshold
    );
    log.info("Archived {} orders", updated);
}

// ✅ GOOD: Use @Version for optimistic locking
@Entity
public class Order {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Version
    private Long version;

    private OrderStatus status;
    private BigDecimal total;
}

// When two users update same order:
// User 1: loads order (version=1), modifies, saves → version becomes 2
// User 2: loads order (version=1), modifies, saves → OptimisticLockException!

@Service
public class OrderService {

    @Transactional
    public Order updateOrder(Long id, UpdateOrderRequest request) {
        try {
            Order order = orderRepository.findById(id).orElseThrow();
            order.setStatus(request.getStatus());
            return orderRepository.save(order);
        } catch (OptimisticLockException e) {
            throw new ConcurrentModificationException(
                "Order was modified by another user. Please refresh and try again."
            );
        }
    }

    // Or with retry
    @Retryable(value = OptimisticLockException.class, maxAttempts = 3)
    @Transactional
    public Order updateOrderWithRetry(Long id, UpdateOrderRequest request) {
        Order order = orderRepository.findById(id).orElseThrow();
        order.setStatus(request.getStatus());
        return orderRepository.save(order);
    }
}

// ❌ BAD: CascadeType.ALL on @ManyToOne
@Entity
public class Book {
    @ManyToOne(cascade = CascadeType.ALL)  // Dangerous!
    private Author author;
}
// Deleting a book could delete the author!

// ✅ GOOD: Cascade only from parent to child
@Entity
public class Author {
    @OneToMany(mappedBy = "author", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Book> books;
}

// ❌ BAD: Frequent queries on non-indexed column
@Query("SELECT o FROM Order o WHERE o.customerEmail = :email")
List<Order> findByCustomerEmail(@Param("email") String email);

// ✅ GOOD: Add index
@Entity
@Table(indexes = @Index(name = "idx_order_customer_email", columnList = "customerEmail"))
public class Order {
    private String customerEmail;
}

// ❌ BAD: toString includes lazy collection
@Entity
public class Author {
    @OneToMany(mappedBy = "author", fetch = FetchType.LAZY)
    private List<Book> books;

    @Override
    public String toString() {
        return "Author{id=" + id + ", books=" + books + "}";  // Triggers lazy load!
    }
}

// ✅ GOOD: Exclude lazy fields from toString
@Override
public String toString() {
    return "Author{id=" + id + ", name='" + name + "'}";
}