Validate custom fields of generated classes

To prevent validations made with Data Annotations are lost if the table structure changes, it is necessary to create another class with the fields for validation and a class partial , path that this last class references the class generated by the Entity Framework . And these classes should look like these:

Metadata.cs

namespace BancoDeHoras.Models
{
  public class StatusMetadata
  {
    [Required(ErrorMessage="Nome deve ser preenchido.")]
    public string Nome {set;get;}
  }
}

PartialClasses.cs

namespace BancoDeHoras.Models
{
  [MetadataType(typeof(StatusMetadata))]
  public partial class Status{}
}

Status.cs (generated class)

namespace BancoDeHoras.Data
{
  //usings omitidos

  public partial class Status
  {    
    public int Id { get; set; }
    public string Nome { get; set; }
    public string Descricao { get; set; }
  }
}

Controller

[HttpPost]
public ActionResult Cadastrar(Status status)
{
    if (ModelState.IsValid)
    {
        try
        {
            using (BancoDeHorasEntities banco = new BancoDeHorasEntities())
            {
                banco.Entry(status).State = System.Data.Entity.EntityState.Added;
                banco.SaveChanges();
                TempData["s"] = "Cadastro realizado com sucesso!";
                ModelState.Clear();
                return View();
            }
        }
        catch (Exception)
        {
            TempData["e"] = "Não foi possível cadastrar!";
        }
    }
    return View(status);
}

However, even empty the Nome field, ModelState.isValid is always true . Why does it happen? ModelState.isValid should return false .