public Purchase(Product product, string sessionId)
 {
     Product = product;
     Status = "prepared";
     NeedPolling = 0;
     SessionId = sessionId;
 }
 private Purchase CreatePurchase(Product product, string sessionId)
 {
     Purchase purchase = new Purchase(product, HttpContext.Session.SessionID);
     db.Purchases.Add(purchase);
     db.SaveChanges();
     return purchase;
 }