Example for SO Q14368021

Q14368021.cs

using System.Linq;
using Raven.Client.Indexes;
using Raven.Tests.Helpers;
using Xunit;

namespace RavenTests.StackOverflow
{
    public class Q14368021 : RavenTestBase
    {
        public abstract class Product
        {
            public int Id { get; set; }
            public string Name { get; set; }
            public string Manufacturer { get; set; }
        }

        public class ProductA : Product
        {
            public string Foo { get; set; }
        }

        public class ProductB : Product
        {
            public string Bar { get; set; }
        }

        public class Offer
        {
            public int Id { get; set; }
            public string SellerUserId { get; set; }
            public Product Product { get; set; }
            public decimal Price { get; set; }
        }

        public class Products_ByLowestOfferPrice : AbstractIndexCreationTask<Offer, Products_ByLowestOfferPrice.Result>
        {
            public class Result
            {
                public Product Product { get; set; }
                public decimal Price { get; set; }
                public string ProductType { get; set; }
            }

            public Products_ByLowestOfferPrice()
            {
                Map = offers => from offer in offers
                                select new
                                    {
                                        offer.Product,
                                        offer.Price,
                                        ProductType = AsDocument(offer.Product)["$type"]
                                    };

                Reduce = results => from result in results
                                    group result by result.Product.Id
                                    into g
                                    select new
                                        {
                                            g.First().Product,
                                            Price = g.Min(x => x.Price),
                                            g.First().ProductType,
                                        };
            }
        }

        [Fact]
        public void Test()
        {
            using (var documentStore = NewDocumentStore())
            {
                documentStore.ExecuteIndex(new Products_ByLowestOfferPrice());

                using (var session = documentStore.OpenSession())
                {
                    var product1 = new ProductA { Id = 1, Name = "A1", Manufacturer = "XYZ", Foo = "ABC" };
                    var product2 = new ProductA { Id = 2, Name = "A2", Manufacturer = "XYZ", Foo = "ABC" };
                    var product3 = new ProductB { Id = 3, Name = "B3", Manufacturer = "XYZ", Bar = "DEF" };
                    var product4 = new ProductB { Id = 4, Name = "B4", Manufacturer = "XYZ", Bar = "DEF" };
                    session.Store(product1);
                    session.Store(product2);
                    session.Store(product3);
                    session.Store(product4);

                    session.Store(new Offer { Id = 1, Product = product1, Price = 10 });
                    session.Store(new Offer { Id = 2, Product = product1, Price = 20 });
                    session.Store(new Offer { Id = 3, Product = product2, Price = 10 });
                    session.Store(new Offer { Id = 4, Product = product2, Price = 20 });
                    session.Store(new Offer { Id = 5, Product = product3, Price = 10 });
                    session.Store(new Offer { Id = 6, Product = product3, Price = 20 });
                    session.Store(new Offer { Id = 7, Product = product4, Price = 10 });
                    session.Store(new Offer { Id = 8, Product = product4, Price = 20 });

                    session.SaveChanges();
                }

                using (var session = documentStore.OpenSession())
                {
                    var productType = documentStore.Conventions.GetClrTypeName(typeof(ProductA));

                    var results = session.Query<Products_ByLowestOfferPrice.Result, Products_ByLowestOfferPrice>()
                                         .Customize(x => x.WaitForNonStaleResults()) // only for testing purposes
                                         .Where(x => x.ProductType == productType)
                                         .ToList();

                    Assert.Equal(2, results.Count);
                    Assert.IsType<ProductA>(results[0].Product);
                    Assert.IsType<ProductA>(results[1].Product);
                    Assert.Equal(1, results[0].Product.Id);
                    Assert.Equal(2, results[1].Product.Id);
                    Assert.Equal(10, results[0].Price);
                    Assert.Equal(10, results[1].Price);
                }
            }
        }
    }
}