private void RemovePurchase(ref Purchase purchase)
 {
     if (purchase.ZaypayPaymentId == 0)
     {
         db.Purchases.Remove(purchase);
         db.SaveChanges();
     }
 }
        // ========================================================================================
        // PRIVATE METHODS
        // ========================================================================================

       
        private Purchase CreatePurchase(Product product)
        {
            Purchase purchase = new Purchase(product);
            db.Purchases.Add(purchase);
            db.SaveChanges();
            return purchase;
        }