[Table("DirDiscountTab")]
    public class DirDiscountTab
    {
        [Display(Name = "Скидка")]
        [Required]
        public int DirDiscountID { get; set; }
        [ForeignKey("DirDiscountID")]
        public virtual Dir.DirDiscount dirDiscount { get; set; }

        [Required]
        public decimal SumBegin { get; set; }
        [Required]
        public decimal Discount { get; set; }
    }

DELETE FROM DirDiscountTab 
WHERE DirDiscountID=@DirDiscountID

                //Получаем коллекцию записей
                var query =
                    (
                    from dirDiscountTabs in db.DirDiscountTabs
                    where dirDiscountTabs.DirDiscountID == dirDiscount.DirDiscountID
                    select dirDiscountTabs
                    ).ToList();

                //Удаляем каждую запись отдельно
                foreach (var dirDiscountTab in query)
                {
                    try
                    {
                        db.Entry(dirDiscountTab).State = EntityState.Deleted;
                        db.SaveChanges();
                    }
                    catch (Exception ex) { ... }
                }