N+1 problem. Fetch types. @Transactional annotation

 @OneToMany ve @ManyToMany - de eger fetch type EAGER -dirse o zaman N + 1 problemi ortaya cixir. N + 1 o demkdirki Many terefde ne qeder obyekt varsa hamisini cekib getirir ve her birisi ucun ayrica query icra olunur. Bunun qarshisini almaq ucun fetch type LAZY qoyulur ve @Transactional annotation istifade olunur. 

1-ci situasiyada butun query-ler iwleyir sonra cap olunur.

2-ci situasiyada ise lazim olduqca query-ler iwleyir.

Solutions for N + 1 problem.

1. Join Fetch

public interface CourseRepository extends JpaRepository<Course, Long> {

    // JPQL java persistence query language
    @Query(value = "select c from Course c join fetch c.students s")
    List<Course> getAllCourses();
}

Bu zaman cemi bir defe Hibernate query generate edecek. 

2. NamedEntityGraph

EntityGraphType iki cur olur: FETCH ve LOAD

FETCH - qeyd olunubsa o zaman value = "something" qeyd olunanlari EAGER weklinde getirir,

qalanlarini ise LAZY weklinde getirir. Hetta hemin fieldlere EAGER verilse bele yene lazy 

olacaq.

LOAD - qeyd olunubsa o zaman value = "something" qeyd olunanlari EAGER weklinde getirir, 

qalanlari ise nece qeyd olunubsa hemin wekilde getir. 

@Entity
@Table(name = "courses")
@Data
@FieldDefaults(level = AccessLevel.PRIVATE)
//@ToString(exclude = "students")
@NamedEntityGraph(name = "course-students-entity-graph",
        attributeNodes = {@NamedAttributeNode("students")})
public class Course {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    Long id;
    String name;

    @OneToMany(mappedBy = "course", cascade = CascadeType.PERSIST, fetch = FetchType.EAGER)
    List<Student> students;

    public Student getStudent() {
        return students.stream().findFirst().orElseThrow();
    }
}
// NamedEntityGraph
@EntityGraph(type = EntityGraph.EntityGraphType.FETCH, value = "course-students-entity-graph")
List<Course> getAllBy();
Burada diqqet edeceyimiz meqam: getAllBy() weklinde yazmaliyiq eks halda iwlemeyecek.

3. EntityGraph
// EntityGraph
@EntityGraph(type = EntityGraph.EntityGraphType.FETCH, attributePaths = "students")
List<Course> getAllBy();