title: Data annotations MVC3 Required attribute
body: <p>I have the Model (User) below, I use it to add new users and to update existing users. 
  When I'm adding a new user it's required to enter the user name and the password, and when I'm updating it's required to enter only the user name because it's not allowed to change the password. Here is the problem, adding a new user everything works ok because I enter both name and password values so ModelState.IsValid returns true, but when updating an user there is no input to the password, so it always have the null value what makes the ModelState.IsValid returns false all the time.
  Is there a way to use the same model even if in the Add View the password is required and in the Update View it's not?
  please, any suggestions?</p>
  
  <pre><code>     public class User {
  
          public int ID { get; set; }
  
          [Display(Name = "Nome do Usuário")]
          [Required(ErrorMessage = "Digite o Nome do Usuário.")]
          public string name { get; set; }
  
          [Display(Name = "Senha")]
          [Required(ErrorMessage = "Digite a Senha.")]
          public string password { get; set; }
  
      }
  
  
  
  public ActionResult Add()
          {
              return View();
          }
  
        [HttpPost]
              public ActionResult Add(User user)
              {
                  UsuariosViewModel viewModel = new UsuariosViewModel();
  
  
                  if (ModelState.IsValid)
                  {
                      viewModel.Msg = new AdmUsuarios().CadastraUsuario(user);
                  }
                  return View(viewModel);
              }
  
  
  
      public ActionResult Update(int id)
              {
                  UsuariosViewModel viewModel = new UsuariosViewModel();
  
              using (var dataContext = new DBEntities()) 
              {
                  Repository&lt;User&gt; repository = new Repository&lt;User&gt;(dataContext);
                  viewModel.User = repository.GetById(id);
              }
  
                  return View(viewModel);
              }
  
          [HttpPost]
          public ActionResult Update(User user, int id)
          {          
              UsuariosViewModel viewModel = new UsuariosViewModel();
  
              if (ModelState.IsValid)
              {
                  viewModel.Msg = new AdmUsuarios().AlteraUsuario(user, id);
              }
              return View();
          }
  </code></pre>