Relations JPA

Cas d'usage : Un auteur a plusieurs livres, un livre a un seul auteur.

@Entity
public class Author {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @OneToMany
    @JoinColumn(name = "author_id") // Crée une FK dans Book
    private List<Book> books = new ArrayList<>();
}

@Entity
public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    // Pas de référence à Author
}

@Entity
public class Author {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @OneToMany(
        mappedBy = "author",          // Référence au champ dans Book
        cascade = CascadeType.ALL,
        orphanRemoval = true,
        fetch = FetchType.LAZY         // Par défaut
    )
    private List<Book> books = new ArrayList<>();
    
    // Méthodes helper pour synchroniser les deux côtés
    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,       // Recommandé
        optional = false              // NOT NULL en base
    )
    @JoinColumn(name = "author_id")   // Nom de la FK
    private Author author;
}

Cas d'usage : Un étudiant suit plusieurs cours, un cours a plusieurs étudiants.

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @ManyToMany
    @JoinTable(
        name = "student_course",
        joinColumns = @JoinColumn(name = "student_id"),
        inverseJoinColumns = @JoinColumn(name = "course_id")
    )
    private Set<Course> courses = new HashSet<>();
}

@Entity
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    // Pas de référence à Student
}

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @ManyToMany(
        cascade = {CascadeType.PERSIST, CascadeType.MERGE},
        fetch = FetchType.LAZY
    )
    @JoinTable(
        name = "student_course",
        joinColumns = @JoinColumn(name = "student_id"),
        inverseJoinColumns = @JoinColumn(name = "course_id")
    )
    private Set<Course> courses = new HashSet<>();
    
    public void enrollCourse(Course course) {
        courses.add(course);
        course.getStudents().add(this);
    }
    
    public void unenrollCourse(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<>();
}