Optional one to many relationship with code first update

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CodeFirst

entity-framework

Hi!! I have a problem to update my db entities using code first approach. This is the model scenario:

public class Book
   {
       public int Id { get; set; }
       public string Title { get; set; }

       public virtual Encyclopedia Encyclopedia { get; set; }

   }

public class Encyclopedia
   {
       public int Id { get; set; }
       public string Name { get; set; }
       public string Publisher { get; set; }
       public List<Book> Books { get; set; }
   }

and this is the update method:

//Controller
public static void UpdateEncyclopedia(Encyclopedia encyclopedia)
{
    using (var context = new LibraryContext())
    {
        var encicloRepo = new EncyclopediaRepository(context);
        encicloRepo.UpdateEncyclopedia(encyclopedia);
        encicloRepo.Save();
    }
}

 //Repository
public void UpdateEncyclopedia(Encyclopedia en)
    {
        _context.Encyclopedies.Attach(en);
        foreach (Book book in en.Books)
        {
            _context.Books.Attach(book);
            var b = _context.Entry(book);
            b.State = EntityState.Modified;
        }

        var entry = _context.Entry(en);
        entry.State = EntityState.Modified;
    }

When I add a book in encyclopedia the framework does not add the relationship between the book and the encylopedia. This is the view:

private void butUpdateBooks_Click(object sender, RoutedEventArgs e)
      {
          Encyclopedia en = lstEncyclopedia.SelectedItem as Encyclopedia;
          //Encyclopedia get = EncyclopediaController.ReadEncyclopediaByPrimaryKey(en.Id);

          if (en == null)
              MessageBox.Show("Seleziona una enciclopedia", "Attenzione!");
          else
          {
              if (MessageBox.Show("Aggiornare i libri dell'enciclopedia"+en.Name+"?", "Attenzione!", MessageBoxButton.YesNo, MessageBoxImage.Warning) == MessageBoxResult.Yes)
              {
                  foreach (Book b in ViewModel.Books)
                  {
                      if (!en.Books.Contains(b))
                         en.Books.Add(b);
                  }
                  EncyclopediaController.UpdateEncyclopedia(en);
              }
          }
      }