public partial class syssujetos
{
    [Key]
    [Column(Order = 0)]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int SujetoId { get; set; }

    [StringLength(15)]
    public string cifdni { get; set; }

    [StringLength(255)]
    public string nombre { get; set; }
RECORTO LAS COLUMNAS QUE SON MUCHAS


public partial class sysusuarios
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int UsuarioId { get; set; }

    [StringLength(150)]
    public string username { get; set; }

    [StringLength(255)]
    public string Pass { get; set; }

    public bool? usuarioactivado { get; set; }

    public string comentarios { get; set; }

    public syssujetos Sujeto { get; set; }

}

public partial class sysroles
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    public int RolId { get; set; }

    [Required]
    [StringLength(50)]
    public string nombre { get; set; }

    [Required]
    [StringLength(150)]
    public string descripcion { get; set; }

    public bool? desistema { get; set; }

    public bool? deacceso { get; set; }
    public ICollection<syssujetos> Sujetos { get; set; }
    public sysroles()
    {
        Sujetos = new HashSet<syssujetos>();
    }
}

var idrol = Request.Form["idrol"];
        //objUser.Sujetos.id
        if (ModelState.IsValid)
        {
            // && a.Pass.Equals(objUser.Pass)
            using (testModel db = new testModel())
            {
                //var usuario = db.Set<sysusuarios>();
                syssujetos sujeto = new syssujetos();
                sysroles rol = db.sysroles.Where(r=>r.nombre=="USUARIO").FirstOrDefault();
                objUser.Sujeto.Roles.Add(rol);

                rol = db.sysroles.Where(r => r.RolId.ToString() == idrol).FirstOrDefault();
                objUser.Sujeto.Roles.Add(rol);

                db.syssujetos.Add(objUser.Sujeto);
                db.sysusuarios.Add(objUser);

                db.SaveChanges();
                return RedirectToAction("Home", "Index");
            }
        }
        return View(objUser);