일대다 및 다대다 관계

JPA의 관계 유형

@ManyToOne: 주인 쪽

@Entity
public class Order {
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id", nullable = false)
    private User user;
}

@OneToMany: 반대쪽

@Entity
public class User {
    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Order> orders = new ArrayList<>();
}

양방향 일관성 도우미

public void addOrder(Order order) {
    orders.add(order);
    order.setUser(this); // keep both sides in sync
}
public void removeOrder(Order order) {
    orders.remove(order);
    order.setUser(null);
}

캐스케이드 유형

@OneToMany(mappedBy = "user",
           cascade = {CascadeType.PERSIST, CascadeType.MERGE},
           orphanRemoval = true)
private List<Order> orders;

orphanRemoval: 자식 자동 삭제

user.getOrders().remove(order); // triggers DELETE for the orphaned order on flush

@ManyToMany와 @JoinTable

@Entity
public class Student {
    @ManyToMany
    @JoinTable(
        name = "student_course",
        joinColumns = @JoinColumn(name = "student_id"),
        inverseJoinColumns = @JoinColumn(name = "course_id"))
    private Set<Course> courses = new HashSet<>();
}

조인 테이블의 추가 열

@Entity
public class Enrollment {
    @ManyToOne Student student;
    @ManyToOne Course course;
    private LocalDate enrolledAt;
}

FetchType: LAZY와 EAGER

// Always override to LAZY:
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "category_id")
private Category category;

N+1 쿼리 문제

// N+1 problem:
List<User> users = repo.findAll();
users.forEach(u -> u.getOrders().size()); // 1 + N queries!
// Fix:
@Query("SELECT u FROM User u LEFT JOIN FETCH u.orders")
List<User> findAllWithOrders();

컬렉션에서 Set과 List 비교

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