using System;

using System.Linq;

using System.Collections.Generic;

​

// We need to know which product sold the most items per order on average, 

// as well as the average instances per order. As before we only want items that 

// are not discontinued. • E.g. if item X sold on average 5.2 items per order and 

// item Y sold on average 7.3, we would want “Item Y” and 7.3 to be returned

public class Program

{

    public static void Main()

    {

        var orders = new List<Order>();

        var orderItems = new List<OrderItem>();

        var product = new Product{

            ProductId = 1,

            Discontinued = false,

            OrderItems = null

        };

        var product3 = new Product{

            ProductId = 1,

            Discontinued = true,

            OrderItems = null

        };

        var product2 = new Product{

            ProductId = 2,

            Discontinued = false,

            OrderItems = null

        };

        orderItems.Add(new OrderItem {

            OrderItemId = 1,

            OrderId = 1,

            Order = null,

            ProductId = 2,

            Product = product2,

            Quantity = 5

        });

        orderItems.Add(new OrderItem {

            OrderItemId = 1,

            OrderId = 1,

            Order = null,

            ProductId = 1,

            Product = product,

            Quantity = 6

        });

        orderItems.Add(new OrderItem {

            OrderItemId = 1,

            OrderId = 1,

            Order = null,

            ProductId = 1,

            Product = product3,

            Quantity = 5

        });

        var order1 = new Order {

            OrderId = 1,

            OrderItems = orderItems

        };

        var order2 = new Order {

            OrderId = 2,

            OrderItems = orderItems

        };

        orders.Add(order1);

        // Most sold product for all orders

        // If you need any other behavior, replace .Max from the below line with other extension method that is more suitable for the specific use case

        var mostSoldProduct = orders.Max(order => {

            // key is the productId for individual groups

            var sameOrderItems = order.OrderItems.GroupBy(orderItem => orderItem.ProductId).ToList();

            var eachProductQuantity = sameOrderItems.Select(ois => {

                var notDiscontinuedOis = ois.Where(oi => !oi.Product.Discontinued);

                return new {

                    ProductId = ois.Key,

                    Quantity = notDiscontinuedOis.Sum(oi => oi.Quantity)

                };

            }).OrderByDescending(pro => pro.Quantity).ToList();

            return eachProductQuantity.FirstOrDefault();

        });

        Console.WriteLine(mostSoldProduct);

    }

    public class Order {

      public int OrderId {get;set;}

      public ICollection<OrderItem> OrderItems {get;set;}

    }

    public class OrderItem {

      public int OrderItemId {get;set;}

      public int OrderId {get;set;}

      public Order Order {get;set;}

      public int ProductId {get;set;}

      public Product Product {get;set;}

      public int Quantity {get;set;}

    }

    public class Product {

      public int ProductId {get;set;}

      public bool Discontinued {get;set;}

      public ICollection<OrderItem> OrderItems {get;set;}

    }

}