Fix #6091 - Left Outer Join Results Difference Between EF Core 1.0 an…

…d EF 6.x

Turns out that we have fixed this indirectly (albeit inefficiently) via #5953d9 and #e514b49.

I have prototyped an efficient solution that uses ROW_NUMBER for group delineation. Created #6441 to
track the improvement.

src/Microsoft.EntityFrameworkCore.Relational/Query/RelationalQueryModelVisitor.cs

@@ -273,7 +273,7 @@ public virtual SelectExpression TryGetQuery([NotNull] IQuerySource querySource)
             SelectExpression selectExpression;
             return QueriesBySource.TryGetValue(querySource, out selectExpression)
                 ? selectExpression
-                : QueriesBySource.Values.SingleOrDefault(se => se.HandlesQuerySource(querySource));
+                : QueriesBySource.Values.LastOrDefault(se => se.HandlesQuerySource(querySource));
         }
 
         /// <summary>

src/Microsoft.EntityFrameworkCore.Specification.Tests/QueryTestBase.cs

@@ -4818,6 +4818,51 @@ join o in os on c.CustomerID equals o.CustomerID into orders
                 from o in orders
                 select new { c, o });
         }
+
+        [ConditionalFact]
+        public virtual void GroupJoin_outer_projection()
+        {
+            AssertQuery<Customer, Order>((cs, os) =>
+                cs.GroupJoin(os, c => c.CustomerID, o => o.CustomerID, (c, o) => new { c.City, o }),
+                asserter: (l2oResults, efResults) => { Assert.Equal(l2oResults.Count, efResults.Count); });
+        }
+
+        [ConditionalFact]
+        public virtual void GroupJoin_outer_projection2()
+        {
+            AssertQuery<Customer, Order>((cs, os) =>
+                cs.GroupJoin(os, c => c.CustomerID, o => o.CustomerID, (c, g) => new { c.City, g = g.Select(o => o.CustomerID) }),
+                asserter: (l2oResults, efResults) => { Assert.Equal(l2oResults.Count, efResults.Count); });
+        }
+
+        [ConditionalFact]
+        public virtual void GroupJoin_outer_projection_reverse()
+        {
+            AssertQuery<Customer, Order>((cs, os) =>
+                os.GroupJoin(cs, o => o.CustomerID, c => c.CustomerID, (o, c) => new { o.CustomerID, c }),
+                asserter: (l2oResults, efResults) => { Assert.Equal(l2oResults.Count, efResults.Count); });
+        }
+
+        [ConditionalFact]
+        public virtual void GroupJoin_outer_projection_reverse2()
+        {
+            AssertQuery<Customer, Order>((cs, os) =>
+                os.GroupJoin(cs, o => o.CustomerID, c => c.CustomerID, (o, g) => new { o.CustomerID, g = g.Select(c => c.City) }),
+                asserter: (l2oResults, efResults) => { Assert.Equal(l2oResults.Count, efResults.Count); });
+        }
+
+        [ConditionalFact]
+        public virtual void GroupJoin_subquery_projection_outer_mixed()
+        {
+            AssertQuery<Customer, Order>((cs, os) =>
+                from c in cs
+                from o0 in os.Take(1)
+                join o1 in os on c.CustomerID equals o1.CustomerID into orders
+                from o2 in orders
+                select new { A = c.CustomerID, B = o0.CustomerID, C = o2.CustomerID },
+                asserter:
+                    (l2oResults, efResults) => { Assert.Equal(l2oResults.Count, efResults.Count); });
+        }
 
         [ConditionalFact]
         public virtual void GroupJoin_DefaultIfEmpty()

test/Microsoft.EntityFrameworkCore.SqlServer.FunctionalTests/QueryBugsTest.cs

@@ -13,6 +13,7 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
 using Xunit;
+using Xunit.Abstractions;
 
 // ReSharper disable ClassNeverInstantiated.Local
 // ReSharper disable AccessToDisposedClosure
@@ -23,6 +24,106 @@ namespace Microsoft.EntityFrameworkCore.SqlServer.FunctionalTests
 {
     public class QueryBugsTest : IClassFixture<SqlServerFixture>
     {
+        private readonly SqlServerFixture _fixture;
+
+        public QueryBugsTest(SqlServerFixture fixture, ITestOutputHelper testOutputHelper)
+        {
+            _fixture = fixture;
+
+            //TestSqlLoggerFactory.CaptureOutput(testOutputHelper);
+        }
+
+        [Fact]
+        public void Left_outer_join_bug_6091()
+        {
+            using (var testStore = SqlServerTestStore.CreateScratch())
+            {
+                testStore.ExecuteNonQuery(@"
+CREATE TABLE [dbo].[Customers](
+    [CustomerID] [int] NOT NULL PRIMARY KEY,
+    [CustomerName] [varchar](120) NULL,
+    [PostcodeID] [int] NULL);
+
+CREATE TABLE [dbo].[Postcodes](
+    [PostcodeID] [int] NOT NULL PRIMARY KEY,
+    [PostcodeValue] [varchar](100) NOT NULL,
+    [TownName] [varchar](255) NOT NULL);
+
+INSERT [dbo].[Customers] ([CustomerID], [CustomerName], [PostcodeID]) VALUES (1, N'Sam Tippet', 5);
+INSERT [dbo].[Customers] ([CustomerID], [CustomerName], [PostcodeID]) VALUES (2, N'William Greig', 2);
+INSERT [dbo].[Customers] ([CustomerID], [CustomerName], [PostcodeID]) VALUES (3, N'Steve Jones', 3);
+INSERT [dbo].[Customers] ([CustomerID], [CustomerName], [PostcodeID]) VALUES (4, N'Jim Warren', NULL);
+INSERT [dbo].[Customers] ([CustomerID], [CustomerName], [PostcodeID]) VALUES (5, N'Andrew Smith', 5);
+
+INSERT [dbo].[Postcodes] ([PostcodeID], [PostcodeValue], [TownName]) VALUES (2, N'1000', N'Town 1');
+INSERT [dbo].[Postcodes] ([PostcodeID], [PostcodeValue], [TownName]) VALUES (3, N'2000', N'Town 2');
+INSERT [dbo].[Postcodes] ([PostcodeID], [PostcodeValue], [TownName]) VALUES (4, N'3000', N'Town 3');
+INSERT [dbo].[Postcodes] ([PostcodeID], [PostcodeValue], [TownName]) VALUES (5, N'4000', N'Town 4');
+");
+                var loggingFactory = new TestSqlLoggerFactory();
+                var serviceProvider = new ServiceCollection()
+                    .AddEntityFrameworkSqlServer()
+                    .AddSingleton<ILoggerFactory>(loggingFactory)
+                    .BuildServiceProvider();
+
+                using (var context = new Bug6091Context(serviceProvider, testStore.ConnectionString))
+                {
+                    var customers
+                        = from customer in context.Customers
+                          join postcode in context.Postcodes
+                              on customer.PostcodeID equals postcode.PostcodeID into custPCTmp
+                          from custPC in custPCTmp.DefaultIfEmpty()
+                          select new
+                          {
+                              customer.CustomerID,
+                              customer.CustomerName,
+                              TownName = custPC == null ? string.Empty : custPC.TownName,
+                              PostcodeValue = custPC == null ? string.Empty : custPC.PostcodeValue
+                          };
+
+                    var results = customers.ToList();
+
+                    Assert.Equal(5, results.Count);
+                    Assert.True(results[3].CustomerName != results[4].CustomerName);
+                }
+            }
+        }
+
+        private class Bug6091Context : DbContext
+        {
+            private readonly IServiceProvider _serviceProvider;
+            private readonly string _connectionString;
+
+            public Bug6091Context(IServiceProvider serviceProvider, string connectionString)
+            {
+                _serviceProvider = serviceProvider;
+                _connectionString = connectionString;
+            }
+
+            protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
+                => optionsBuilder.UseInternalServiceProvider(_serviceProvider).UseSqlServer(_connectionString);
+
+            protected override void OnModelCreating(ModelBuilder modelBuilder)
+                => modelBuilder.Entity<Customer>().ToTable("Customers");
+
+            public DbSet<Customer> Customers { get; set; }
+            public DbSet<Postcode> Postcodes { get; set; }
+
+            public class Customer
+            {
+                public int CustomerID { get; set; }
+                public string CustomerName { get; set; }
+                public int? PostcodeID { get; set; }
+            }
+
+            public class Postcode
+            {
+                public int PostcodeID { get; set; }
+                public string PostcodeValue { get; set; }
+                public string TownName { get; set; }
+            }
+        }
+
         [Fact]
         public async Task Multiple_optional_navs_should_not_deadlock_bug_5481()
         {
@@ -549,13 +650,6 @@ public class CustomerDetails_1742
             public string LastName { get; set; }
         }
 
-        private readonly SqlServerFixture _fixture;
-
-        public QueryBugsTest(SqlServerFixture fixture)
-        {
-            _fixture = fixture;
-        }
-
         [Fact]
         public void Customer_collections_materialize_properly_3758()
         {