Add tests for Navigation issues that no longer reproduce

Add test for #3804 - Select Navigation following Skip or Take
Add test for #3674 - GroupBy with Where on Navigation Property
Add test for #3676 - GroupBy Let on Navigation Property (disabled)
Add test for #3913 - Sum + Count inside Select

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

@@ -105,6 +105,25 @@ from dynamic l2oItem in l2oItems
                 entryCount: 1);
         }
 
+        [ConditionalFact]
+        public virtual void Take_Select_Navigation()
+        {
+            AssertQuery<Customer>(
+                cs => cs.Take(2)
+                    .Select(c => c.Orders.FirstOrDefault()));
+        }
+
+        [ConditionalFact]
+        public virtual void Skip_Select_Navigation()
+        {
+            AssertQuery<Customer>(
+                cs => cs.OrderBy(c => c.CustomerID)
+                    .Skip(20)
+                    .Select(c => c.Orders
+                        .OrderBy(o => o.OrderID)
+                        .FirstOrDefault()));
+        }
+
         [ConditionalFact]
         public virtual void Select_Where_Navigation_Null()
         {
@@ -180,6 +199,15 @@ public virtual void Select_Where_Navigation_Included()
                 entryCount: 15);
         }
 
+        [ConditionalFact]
+        public virtual void Select_count_plus_sum()
+        {
+            AssertQuery<Order>(os => os.Select(o => new
+            {
+                Total = o.OrderDetails.Sum(od => od.Quantity) + o.OrderDetails.Count()
+            }));
+        }
+
         [ConditionalFact]
         public virtual void Singleton_Navigation_With_Member_Access()
         {
@@ -644,6 +672,51 @@ join efItem in efItems on l2oItem.Key equals efItem.Key
             });
         }
 
+        [ConditionalFact]
+        public virtual void Where_nav_prop_group_by()
+        {
+            AssertQuery<OrderDetail, IGrouping<short, OrderDetail>>(
+                ods => from od in ods
+                      where od.Order.CustomerID == "ALFKI"
+                      group od by od.Quantity,
+                asserter: (l2oItems, efItems) =>
+                    {
+                        foreach (var pair in
+                            from l2oItem in l2oItems
+                            join efItem in efItems on l2oItem.Key equals efItem.Key
+                            select new { l2oItem, efItem })
+                        {
+                            Assert.Equal(
+                                pair.l2oItem.Select(i => i.OrderID).OrderBy(i => i),
+                                pair.efItem.Select(i => i.OrderID).OrderBy(i => i));
+                        }
+                    });
+        }
+
+        // issue #3676
+        //// [ConditionalFact]
+        public virtual void Let_group_by_nav_prop()
+        {
+            AssertQuery<OrderDetail, IGrouping<string, OrderDetail>>(
+                ods => from od in ods
+                       let customer = od.Order.CustomerID
+                       group od by customer
+                       into odg
+                       select odg,
+                asserter: (l2oItems, efItems) =>
+                {
+                    foreach (var pair in
+                        from l2oItem in l2oItems
+                        join efItem in efItems on l2oItem.Key equals efItem.Key
+                        select new { l2oItem, efItem })
+                    {
+                        Assert.Equal(
+                            pair.l2oItem.Select(i => i.OrderID).OrderBy(i => i),
+                            pair.efItem.Select(i => i.OrderID).OrderBy(i => i));
+                    }
+                });
+        }
+
         protected QueryNavigationsTestBase(TFixture fixture)
         {
             Fixture = fixture;

src/Microsoft.EntityFrameworkCore.Specification.Tests/TestModels/Northwind/NorthwindData.cs

@@ -26,40 +26,34 @@ static NorthwindData()
             _orders = CreateOrders();
             _orderDetails = CreateOrderDetails();
 
+            foreach (var customer in _customers)
+            {
+                customer.Orders = new List<Order>();
+            }
+
+            foreach (var product in _products)
+            {
+                product.OrderDetails = new List<OrderDetail>();
+            }
+
             foreach (var order in _orders)
             {
+                order.OrderDetails = new List<OrderDetail>();
+
                 var customer = _customers.Where(c => c.CustomerID == order.CustomerID).First();
                 order.Customer = customer;
-
-                if (customer.Orders == null)
-                {
-                    customer.Orders = new List<Order>();
-                }
-
                 customer.Orders.Add(order);
             }
 
             foreach (var orderDetail in _orderDetails)
             {
                 var order = _orders.Where(o => o.OrderID == orderDetail.OrderID).First();
-                var product = _products.Where(p => p.ProductID == orderDetail.ProductID).First();
                 orderDetail.Order = order;
-                orderDetail.Product = product;
-
-                if (order.OrderDetails == null)
-                {
-                    order.OrderDetails = new List<OrderDetail>();
-                }
-
                 order.OrderDetails.Add(orderDetail);
 
-                if (product.OrderDetails == null)
-                {
-                    product.OrderDetails = new List<OrderDetail>();
-                }
-
+                var product = _products.Where(p => p.ProductID == orderDetail.ProductID).First();
+                orderDetail.Product = product;
                 product.OrderDetails.Add(orderDetail);
-
             }
 
             foreach (var employee in _employees)

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

@@ -36,6 +36,44 @@ FROM [Order Details] AS [od]
                 Sql);
         }
 
+        public override void Take_Select_Navigation()
+        {
+            base.Take_Select_Navigation();
+
+            Assert.StartsWith(
+                @"@__p_0: 2
+
+SELECT [t].[CustomerID]
+FROM (
+    SELECT TOP(@__p_0) [c0].*
+    FROM [Customers] AS [c0]
+) AS [t]
+
+SELECT [o].[OrderID], [o].[CustomerID], [o].[EmployeeID], [o].[OrderDate]
+FROM [Orders] AS [o]
+",
+                Sql);
+        }
+
+        public override void Skip_Select_Navigation()
+        {
+            base.Skip_Select_Navigation();
+
+            Assert.StartsWith(
+                @"@__p_0: 20
+
+SELECT [c].[CustomerID]
+FROM [Customers] AS [c]
+ORDER BY [c].[CustomerID]
+OFFSET @__p_0 ROWS
+
+SELECT [o].[OrderID], [o].[CustomerID], [o].[EmployeeID], [o].[OrderDate]
+FROM [Orders] AS [o]
+ORDER BY [o].[OrderID]
+",
+                Sql);
+        }
+
         public override void Select_Where_Navigation_Included()
         {
             base.Select_Where_Navigation_Included();
@@ -121,6 +159,24 @@ FROM [Orders] AS [o]
                 Sql);
         }
 
+        public override void Select_count_plus_sum()
+        {
+            base.Select_count_plus_sum();
+
+            Assert.StartsWith(
+                @"SELECT (
+    SELECT SUM([od0].[Quantity])
+    FROM [Order Details] AS [od0]
+    WHERE [o].[OrderID] = [od0].[OrderID]
+) + (
+    SELECT COUNT(*)
+    FROM [Order Details] AS [o1]
+    WHERE [o].[OrderID] = [o1].[OrderID]
+)
+FROM [Orders] AS [o]",
+                Sql);
+        }
+
         public override void Singleton_Navigation_With_Member_Access()
         {
             base.Singleton_Navigation_With_Member_Access();
@@ -641,6 +697,28 @@ FROM [Orders] AS [o]
                 Sql);
         }
 
+        public override void Where_nav_prop_group_by()
+        {
+            base.Where_nav_prop_group_by();
+
+            Assert.Equal(
+                @"SELECT [od].[OrderID], [od].[ProductID], [od].[Discount], [od].[Quantity], [od].[UnitPrice]
+FROM [Order Details] AS [od]
+INNER JOIN [Orders] AS [od.Order] ON [od].[OrderID] = [od.Order].[OrderID]
+WHERE [od.Order].[CustomerID] = N'ALFKI'
+ORDER BY [od].[Quantity]",
+                Sql);
+        }
+
+        public override void Let_group_by_nav_prop()
+        {
+            base.Let_group_by_nav_prop();
+
+            Assert.Equal(
+                @"",
+                Sql);
+        }
+
         protected override void ClearLog() => TestSqlLoggerFactory.Reset();
 
         private static string Sql => TestSqlLoggerFactory.Sql;