Loading a property after attaching an object

I tried this before asking here, and I did not find any solution.

I have these two classes and one one-to-many mapping

I am trying to attach a new object

    public class MyContext: DbContext
    {
        public IDbSet Operacoes {get; set; }
        public IDbSet Apontamentos {get; set; }
    }

    public class Operacao
    {
        public string Filial {get; set; }
        public string Codigo {get; set; }
        public string Descricao {get; set; }
    }

    public class apontamento
    {
        public int Id {get; set; }
        public string Filial {get; set; }
        public string OperacaoCodigo {get; set; }
        public virtual Operacao Operacao {get; set; }
    }

    public class OperacaoMap: EntityTypeConfiguration
    {
        public OperacaoMap ()
        {
            ToTable ("za6010");
            HasKey (x => new {x.Filial, x.Codigo})
                .Property (x => x.Codigo) .HasColumnName ("za6_cod");

            Property (x => x.Descricao) .HasColumnName ("za6_desc");
        }
    }



    public class ApontamentoMap: EntityTypeConfiguration
    {
        public ApontamentoMap ()
        {
            ToTable ("za4010");

            HasKey (x => new {x.Filial, x.Id});

            Property (x => x.OperacaoCodigo)
                .HasColumnName ("za4_oper");

            //
            HasRequired (x => x.Operacao)
                .WithMany ()
                .HasForeignKey (x => new {x.Filial, x.OperacaoCodigo})
                .WillCascadeOnDelete (false);
        }
    }

    public static class Program
    {
        static void main ()
        {
            // this not work, and I need it to work.
            var context = new MyContext ();
            var newObj = new Apontamento
            {
                Filial = "01",
                OperacaoCodigo = "001"
            };
            context.Apontamentos.Attach (newObj);
            var desc = newObj.Operacao.Descricao; // Here Operacao Property is null

            // this works
            var newObjTmp = new Apontamento
            {
                Filial = "01",
                OperacaoCodigo = "001"
            };
            var operacao = context.Operacoes.Where (x => x.Codigo == "001");
            context.Apontamentos.Attach (newObj);
            var descTmp = newObjTmp.Operacao.Descricao; // Operacao Property is ok.

        }
    }