public static string ExportCategoryStatistics(FastFoodDbContext context, string categoriesString)
        {
            string[] categories = categoriesString.Split(new[] { ',' }, StringSplitOptions.RemoveEmptyEntries);

            var categoriesWithItem = context.Categories
                                    .Where(c => categories.Contains(c.Name))
                                    .Select(c => new
                                    {
                                        Name = c.Name,
                                        MostPopularItem = c.Items
                                                    .OrderByDescending(i => i.Price * i.OrderItems.Sum(oi => oi.Quantity))
                                                    .Select(i => new
                                                    {
                                                        Name = i.Name,
                                                        TotalMade = i.Price * i.OrderItems.Sum(oi => oi.Quantity),
                                                        TimesSold = i.OrderItems.Sum(oi => oi.Quantity)
                                                    }).FirstOrDefault()
                                    })
                                    .OrderByDescending(c => c.MostPopularItem.TotalMade)
                                    .ThenByDescending(c => c.MostPopularItem.TimesSold)
                                    .ToArray();

            XElement[] elements = categoriesWithItem
                                  .Select(c => new XElement("Category",
                                                            new XElement("Name", c.Name),
                                                            new XElement("MostPopularItem",
                                                                         new XElement("Name", c.MostPopularItem.Name),
                                                                         new XElement("TotalMade", c.MostPopularItem.TotalMade.ToString("f2")),
                                                                         new XElement("TimesSold", c.MostPopularItem.TimesSold)
                                                                         ))).ToArray();
            XElement root = new XElement("Categories", elements);
            XDocument xDocument = new XDocument(root);
            string result = xDocument.ToString();
            return result;
        }