Why is the Entity Framework generating a new record in the database?

Question

Next, I’m working on a new project Asp.net-mvc, and I have 3 classes.

public class Anuncio
{
    public int AnuncioID { get; set; }

    public string UsuarioID { get; set; }

    [Required]
    [MaxLength(255)]
    [Display(Name="Título")]
    public string Titulo { get; set; }

    [Required]
    [MaxLength(2000)]
    [Display(Name="Descrição")]
    public string Descricao { get; set; }

    [Display(Name="Data do anúncio")]
    public DateTime DataAnunciado { get; set; }

    public Contato Contato { get; set; }

    [Display(Name="Endereço")]
    public Endereco Endereco { get; set; }

    public TipoCategoria Categoria { get; set; }

    public ICollection<Imagem> Imagens { get; set; }

}

public class Endereco
{
    [Key]
    public int EnderecoID { get; set; }

    [Required]
    public Estado Estado { get; set; }

    [Required,Display(Name="Município")]
    public Municipio Municipio { get; set; }

    public string Bairro { get; set; }

    public string Rua { get; set; }

    [Display(Name="Número")]
    public string Numero { get; set; }

    public string Cep { get; set; }
    public string Complemento { get; set; }
}

public class Estado
{
    [Key]
    public int EstadoID { get; set; }
    public string Nome { get; set; }
    public string Sigla { get; set; }
}

So, as you may notice, Anuncio has 1 Endereco, which in turn has 1 Estado.

After I run the application, I include the states, so for example, I have a Estado with EstadoID = 1, Nome = Paraná, Sigla = PR.

When I’m gonna register a new one Anuncio, even defining the instance of Estado on the bench at Anuncio, for example:

public ActionResult NovoAnuncio(Anuncio anuncio)
{        
    if (ModelState.IsValid)
    {             
        Estado estado = db.Estado.Find(1);
        anuncio.Endereco.Estado = estado;

        db.Anuncio.Add(anuncio);
        db.SaveChanges();

        return RedirectToAction("Index", "Home");     
    }

    return View(anuncio);
}

It CREATES another record in the BD.

Why he does not reference the State already registered in the comic?

Answer 1

As I replied on this other issue you are loading the record twice in context and the Entity Framework understands that it is two different records. In your case, this applies to Endereco which does not have a correct reference to Estado.

Modify the following to avoid this:

var estado = db.Estado.Find(1);
anuncio.Endereco.EstadoId = estado.EstadoId;

EDIT

According to comments, two properties are missing for the Entity Framework to work correctly:

Models Endereco.Cs

public class Endereco
{
    ...
    public int EstadoID { get; set; }
    ...
}

Models Anuncio.Cs

public class Anuncio
{
    ...
    public int EnderecoID { get; set; }
    ...
}