我想在注册时添加多个角色，使用EF模型建立多对多关系

public class User
{
    public int UserId { get; set; }
    public string UserName { get; set; }
    // 其他用户属性

    public ICollection<UserRole> UserRoles { get; set; }
}

public class Role
{
    public int RoleId { get; set; }
    public string RoleName { get; set; }
    // 其他角色属性

    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; }
}

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);
}