MVC - Fix error - The result of a query cannot be enumerated more than once

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to fix it convert function that loads view
replace
        public ActionResult Test()
        {
            OrderClass model = new OrderClass();
            model.or = db.SelectOrders().FirstOrDefault();
            model.pr = db.SelectProducts();
            return View(model);
        }

with
        public ActionResult Test()
        {
            OrderClass model = new OrderClass();
            model.or = db.SelectOrders().FirstOrDefault();
            model.pr = db.SelectProducts().ToList();
            return View(model);
        }