public class User
{
    public int UserId { get; set; }
    public string Name { get; set; }
    public ICollection<UserRole> UserRoles { get; set; }
}

public class Role
{
    public int RoleId { get; set; }
    public string Name { get; set; }
    public ICollection<UserRole> UserRoles { get; set; }
}

public class UserRole
{
    public int UserId { get; set; }
    public User User { get; set; }
    
    public int RoleId { get; set; }
    public Role Role { get; set; }
    
    public DateTime AssignedDate { get; set; } // 额外属性
}

// DbContext配置
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<UserRole>()
        .HasKey(ur => new { ur.UserId, ur.RoleId });
    
    modelBuilder.Entity<UserRole>()
        .HasOne(ur => ur.User)
        .WithMany(u => u.UserRoles)
        .HasForeignKey(ur => ur.UserId);
    
    modelBuilder.Entity<UserRole>()
        .HasOne(ur => ur.Role)
        .WithMany(r => r.UserRoles)
        .HasForeignKey(ur => ur.RoleId);
}

public class Student
{
    public int StudentId { get; set; }
    public string Name { get; set; }
    public ICollection<StudentCourse> StudentCourses { get; set; }
    public ICollection<StudentClub> StudentClubs { get; set; }
}

public class Course
{
    public int CourseId { get; set; }
    public string Title { get; set; }
    public ICollection<StudentCourse> StudentCourses { get; set; }
}

public class Club
{
    public int ClubId { get; set; }
    public string Name { get; set; }
    public ICollection<StudentClub> StudentClubs { get; set; }
}

public class StudentCourse
{
    public int StudentId { get; set; }
    public Student Student { get; set; }
    public int CourseId { get; set; }
    public Course Course { get; set; }
    public int? Grade { get; set; }
}

public class StudentClub
{
    public int StudentId { get; set; }
    public Student Student { get; set; }
    public int ClubId { get; set; }
    public Club Club { get; set; }
    public DateTime JoinDate { get; set; }
}

// 在DbContext中配置
builder.Entity<User>().Ignore(u => u.UserRoles);
// 或者使用DTO进行序列化

// 确保先保存主实体
context.Users.Add(user);
context.Roles.Add(role);
context.SaveChanges();

// 然后保存连接实体
var userRole = new UserRole { UserId = user.UserId, RoleId = role.RoleId };
context.UserRoles.Add(userRole);
context.SaveChanges();

var usersWithRoles = context.Users
    .Include(u => u.UserRoles)
    .ThenInclude(ur => ur.Role)
    .ToList();