Entity Framework one to many relation not updated

Question

I have the following entities:

BIEntities db = new BIEntities();

public class SALES
{
   public int IDs {get; set;}
   public string product { get; set; }
   ICollection<COUNTRIES> COUNTRIES { get; set; }
}
public class COUNTRIES
{
   public int IDc { get; set; }
   public string country { get; set; }
   public virtual SALES SALES { get; set; }
}

And a ViewModel:

public class myViewModel
{
   public int ID { get; set; }
   public string product { get; set; }
   List<string> country { get; set; }
}

I am trying to update (edit) all entities from myViewModel:

SALES sUpdate = db.SALES.FirstOrDefault(s => s.IDs == myModel.ID);
if (sUpdate != null)
{
   //This works fine!
   db.Entry(sUpdate).CurrentValues.SetValues(myModel);
}

var cUpdate = from c in db.COUNTRIES where c.IDc == myModel.ID select c;
if (cUpdate != null)
{
   foreach (var item in cUpdate)
      {
         item.country = myModel.country;
         item.IDc = myModel.ID;

         //What should I add here to update the records??

      }
}
db.SaveChanges();

This can update the main table "SALES" but table "COUNTRIES" is not updated.