perf(loan): change FetchType from EAGER to LAZY on Loan items and user

EAGER loading on collections always fetches all loan items regardless
of whether they are needed, causing unnecessary database queries and
potential N+1 problems at scale.

- Changed OneToMany(items) and ManyToOne(user) to FetchType.LAZY
- Added JOIN FETCH queries in LoanRepository for cases that need items
- Introduced findWithItemsOrThrow() helper in LoanService to centralize
  the fetch strategy and avoid LazyInitializationException
- findAll() and findByUser() now use dedicated JOIN FETCH queries
- findOverdue() and markOverdue() remain lightweight (no items needed)

Author: erichiroshi

src/main/java/com/example/library/loan/Loan.java

@@ -51,7 +51,7 @@ public class Loan extends BaseEntity {
 	@Enumerated(EnumType.STRING)
 	private LoanStatus status;
 
-	@ManyToOne
+    @ManyToOne(fetch = FetchType.LAZY)
 	@JoinColumn(name = "user_id", nullable = false)
 	private User user;
 

src/main/java/com/example/library/loan/LoanRepository.java

@@ -2,35 +2,69 @@
 
 import java.time.LocalDate;
 import java.util.List;
+import java.util.Optional;
 
 import org.springframework.data.jpa.repository.JpaRepository;
 import org.springframework.data.jpa.repository.Query;
 import org.springframework.data.repository.query.Param;
 
 public interface LoanRepository extends JpaRepository<Loan, Long> {
+	
+	 /**
+     * Busca um empréstimo pelo ID carregando itens e usuário via JOIN FETCH.
+     * Necessário após migração de EAGER para LAZY para evitar LazyInitializationException
+     * ao acessar items e user fora da sessão JPA.
+     */
+    @Query("""
+            SELECT l FROM Loan l
+            JOIN FETCH l.items i
+            JOIN FETCH i.book
+            JOIN FETCH l.user
+            WHERE l.id = :id
+            """)
+    Optional<Loan> findByIdWithItemsAndUser(@Param("id") Long id);
+    
+    /**
+     * Todos os empréstimos de um usuário específico, com itens e livros carregados.
+     */
+    @Query("""
+            SELECT DISTINCT l FROM Loan l
+            JOIN FETCH l.items i
+            JOIN FETCH i.book
+            WHERE l.user.id = :userId
+            """)
+    List<Loan> findByUserIdWithItems(@Param("userId") Long userId);
 
-	/**
-	 * Todos os empréstimos de um usuário específico.
-	 */
-	List<Loan> findByUserId(Long userId);
+	 /**
+     * Empréstimos vencidos: status WAITING_RETURN e dueDate antes de hoje.
+     * Não precisa de itens — apenas marca o status.
+     */
+    @Query("""
+            SELECT l FROM Loan l
+            WHERE l.status = com.example.library.loan.LoanStatus.WAITING_RETURN
+            AND l.dueDate < :today
+            """)
+    List<Loan> findOverdueLoans(@Param("today") LocalDate today);
+    
+    /**
+     * Todos os empréstimos com itens e livros carregados — uso exclusivo de ADMIN.
+     */
+    @Query("""
+            SELECT DISTINCT l FROM Loan l
+            JOIN FETCH l.items i
+            JOIN FETCH i.book
+            JOIN FETCH l.user
+            """)
+    List<Loan> findAllWithItems();
 
-	/**
-	 * Empréstimos vencidos: status WAITING_RETURN e dueDate antes de hoje.
-	 */
-	@Query("""
-			SELECT l FROM Loan l
-			WHERE l.status = com.example.library.loan.LoanStatus.WAITING_RETURN
-			AND l.dueDate < :today
-			""")
-	List<Loan> findOverdueLoans(@Param("today") LocalDate today);
-
-	/**
-	 * Empréstimos ativos de um usuário (para verificar limite de empréstimos).
-	 */
-	@Query("""
-			SELECT COUNT(l) FROM Loan l
-			WHERE l.user.id = :userId
-			AND l.status = com.example.library.loan.LoanStatus.WAITING_RETURN
-			""")
-	long countActiveByUserId(@Param("userId") Long userId);
+    /**
+     * Empréstimos ativos de um usuário (para verificar limite de empréstimos).
+     * Não precisa de itens — apenas conta.
+     */
+    @Query("""
+            SELECT COUNT(l) FROM Loan l
+            WHERE l.user.id = :userId
+            AND l.status = com.example.library.loan.LoanStatus.WAITING_RETURN
+            """)
+    long countActiveByUserId(@Param("userId") Long userId);
 }

src/main/java/com/example/library/loan/LoanService.java

@@ -24,6 +24,9 @@
 import com.example.library.user.UserRepository;
 import com.example.library.user.exception.UserNotFoundException;
 
+import lombok.RequiredArgsConstructor;
+
+@RequiredArgsConstructor
 @Service
 public class LoanService {
 
@@ -34,13 +37,6 @@ public class LoanService {
 	private final UserRepository userRepository;
 	private final LoanMapper mapper;
 
-	public LoanService(LoanRepository loanRepository, BookRepository bookRepository, UserRepository userRepository, LoanMapper mapper) {
-		this.loanRepository = loanRepository;
-		this.bookRepository = bookRepository;
-		this.userRepository = userRepository;
-		this.mapper = mapper;
-	}
-
     // ─────────────────────────────────────────────
     // CRIAR EMPRÉSTIMO
     // ─────────────────────────────────────────────
@@ -63,7 +59,6 @@ public LoanResponseDTO create(LoanCreateDTO dto) {
 		loan.setStatus(LoanStatus.WAITING_RETURN);
 
 		for (Long bookId : dto.booksId()) {
-
 			Book book = bookRepository.findById(bookId)
 					.orElseThrow(() -> new BookNotFoundException(bookId));
 
@@ -89,7 +84,8 @@ public LoanResponseDTO create(LoanCreateDTO dto) {
         log.info("Loan created: loanId={} user={} books={}",
                 saved.getId(), user.getEmail(), dto.booksId().size());
 
-		return mapper.toDTO(loan);
+        // Recarrega com JOIN FETCH para garantir que o mapper acessa itens dentro da transação
+        return mapper.toDTO(findWithItemsOrThrow(saved.getId()));
 	}
 
 	// ─────────────────────────────────────────────
@@ -100,7 +96,7 @@ public LoanResponseDTO create(LoanCreateDTO dto) {
 	public LoanResponseDTO returnLoan(Long loanId) {
 
         User user = getAuthenticatedUser();
-		Loan loan = find(loanId);
+		Loan loan = findWithItemsOrThrow(loanId);
 
         validateOwnershipOrAdmin(loan, user);
 
@@ -135,7 +131,7 @@ public LoanResponseDTO returnLoan(Long loanId) {
     public LoanResponseDTO cancelLoan(Long loanId) {
 
         User user = getAuthenticatedUser();
-        Loan loan = find(loanId);
+        Loan loan = findWithItemsOrThrow(loanId);
 
         validateOwnershipOrAdmin(loan, user);
 
@@ -177,32 +173,26 @@ public void markOverdue() {
     @Transactional(readOnly = true)
 	public LoanResponseDTO findById(Long loanId) {
     	User user = getAuthenticatedUser();
-        Loan loan = find(loanId);
-
+        Loan loan = findWithItemsOrThrow(loanId);
         validateOwnershipOrAdmin(loan, user);
 		return mapper.toDTO(loan);
 	}
 
     @Transactional(readOnly = true)
     public List<LoanResponseDTO> findMyLoans() {
-    	
         User user = getAuthenticatedUser();
-        
         log.debug("Fetching loans for user={}", user.getEmail());
-        
-        return loanRepository.findByUserId(user.getId())
+        return loanRepository.findByUserIdWithItems(user.getId())
                 .stream()
                 .map(mapper::toDTO)
                 .toList();
     }
 
     @Transactional(readOnly = true)
     public List<LoanResponseDTO> findByUser(Long userId) {
-    	
         userRepository.findById(userId)
                 .orElseThrow(() -> new UserNotFoundException(userId));
-        
-        return loanRepository.findByUserId(userId)
+        return loanRepository.findByUserIdWithItems(userId)
                 .stream()
                 .map(mapper::toDTO)
                 .toList();
@@ -218,7 +208,7 @@ public List<LoanResponseDTO> findOverdue() {
 
     @Transactional(readOnly = true)
     public List<LoanResponseDTO> findAll() {
-        return loanRepository.findAll()
+        return loanRepository.findAllWithItems()
                 .stream()
                 .map(mapper::toDTO)
                 .toList();
@@ -228,10 +218,14 @@ public List<LoanResponseDTO> findAll() {
     // HELPERS PRIVADOS
     // ─────────────────────────────────────────────
 
-	private Loan find(Long loanId) {
-		return loanRepository.findById(loanId)
-				.orElseThrow(() -> new LoanNotFoundException(loanId));
-	}
+    /**
+     * Busca um empréstimo pelo ID com itens e usuário já carregados via JOIN FETCH.
+     * Garante que o mapper acessa as coleções LAZY dentro da transação ativa.
+     */
+    private Loan findWithItemsOrThrow(Long loanId) {
+        return loanRepository.findByIdWithItemsAndUser(loanId)
+                .orElseThrow(() -> new LoanNotFoundException(loanId));
+    }
 
 	/**
      * Recupera o usuário autenticado direto do SecurityContext.
@@ -245,14 +239,15 @@ private User getAuthenticatedUser() {
 
     /**
      * Garante que apenas o dono do empréstimo ou um ADMIN pode operá-lo.
+     * Retorna 404 intencionalmente para não vazar que o empréstimo existe.
      */
     private void validateOwnershipOrAdmin(Loan loan, User user) {
         boolean isAdmin = user.getRoles().contains("ROLE_ADMIN");
         boolean isOwner = loan.getUser().getId().equals(user.getId());
 
         if (!isOwner && !isAdmin) {
             log.warn("Unauthorized loan access attempt: loanId={} userId={}", loan.getId(), user.getId());
-            throw new LoanUnauthorizedException(loan.getId()); // 404 intencional — não vazar que o loan existe
+            throw new LoanUnauthorizedException(loan.getId());
         }
     }
 }