[3.1] Fix to #12729 - Flatten Case expression

src/EFCore.Relational/Query/Internal/CaseWhenFlatteningExpressionVisitor.cs

@@ -0,0 +1,34 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Linq;
+using System.Linq.Expressions;
+using Microsoft.EntityFrameworkCore.Query.SqlExpressions;
+
+namespace Microsoft.EntityFrameworkCore.Query.Internal
+{
+    public class CaseWhenFlatteningExpressionVisitor : ExpressionVisitor
+    {
+        private readonly ISqlExpressionFactory _sqlExpressionFactory;
+
+        public CaseWhenFlatteningExpressionVisitor(ISqlExpressionFactory sqlExpressionFactory)
+        {
+            _sqlExpressionFactory = sqlExpressionFactory;
+        }
+
+        protected override Expression VisitExtension(Expression node)
+        {
+            // Only applies to 'CASE WHEN condition...' not 'CASE operand WHEN...'
+            if (node is CaseExpression caseExpression && caseExpression.Operand == null)
+            {
+                if (caseExpression.ElseResult is CaseExpression nestedCaseExpression && nestedCaseExpression.Operand == null)
+                {
+                    return VisitExtension(_sqlExpressionFactory.Case(
+                        caseExpression.WhenClauses.Union(nestedCaseExpression.WhenClauses).ToList(),
+                        nestedCaseExpression.ElseResult));
+                }
+            }
+            return base.VisitExtension(node);
+        }
+    }
+}

src/EFCore.Relational/Query/RelationalQueryTranslationPostprocessor.cs

@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
 using System.Linq.Expressions;
 using Microsoft.EntityFrameworkCore.Infrastructure;
 using Microsoft.EntityFrameworkCore.Query.Internal;
@@ -37,6 +38,11 @@ public override Expression Process(Expression query)
             query = new CollectionJoinApplyingExpressionVisitor().Visit(query);
             query = new TableAliasUniquifyingExpressionVisitor().Visit(query);
 
+            if (!(AppContext.TryGetSwitch("Microsoft.EntityFrameworkCore.Issue12729", out var enabled) && enabled))
+            {
+                query = new CaseWhenFlatteningExpressionVisitor(SqlExpressionFactory).Visit(query);
+            }
+
             if (!UseRelationalNulls)
             {
                 query = new NullSemanticsRewritingExpressionVisitor(SqlExpressionFactory).Visit(query);

test/EFCore.Specification.Tests/Query/SimpleQueryTestBase.Select.cs

@@ -1482,6 +1482,39 @@ public virtual Task Projection_with_parameterized_constructor_with_member_assign
                 elementAsserter: (e, a) => Assert.Equal(e.Customer, a.Customer));
         }
 
+        [ConditionalTheory]
+        [MemberData(nameof(IsAsyncData))]
+        public virtual Task Select_over_10_nested_ternary_condition(bool isAsync)
+        {
+            return AssertQuery(
+                isAsync,
+                os => from c in os.Set<Customer>()
+                      select
+                      c.CustomerID == "1"
+                        ? "01"
+                        : c.CustomerID == "2"
+                            ? "02"
+                            : c.CustomerID == "3"
+                                ? "03"
+                                : c.CustomerID == "4"
+                                    ? "04"
+                                    : c.CustomerID == "5"
+                                        ? "05"
+                                        : c.CustomerID == "6"
+                                            ? "06"
+                                            : c.CustomerID == "7"
+                                                ? "07"
+                                                : c.CustomerID == "8"
+                                                    ? "08"
+                                                    : c.CustomerID == "9"
+                                                        ? "09"
+                                                        : c.CustomerID == "10"
+                                                            ? "10"
+                                                            : c.CustomerID == "11"
+                                                                ? "11"
+                                                                : null);
+        }
+
         private class CustomerWrapper2
         {
             public CustomerWrapper2(Customer customer)

test/EFCore.SqlServer.FunctionalTests/Query/ComplexNavigationsQuerySqlServerTest.cs

@@ -4363,10 +4363,8 @@ FROM [LevelOne] AS [l]
 LEFT JOIN [LevelFour] AS [l2] ON [l1].[Id] = [l2].[Level3_Optional_Id]
 WHERE CASE
     WHEN [l0].[Id] IS NULL THEN NULL
-    ELSE CASE
-        WHEN [l1].[Id] IS NULL THEN NULL
-        ELSE [l2].[Name]
-    END
+    WHEN [l1].[Id] IS NULL THEN NULL
+    ELSE [l2].[Name]
 END = N'L4 01'");
         }
 

test/EFCore.SqlServer.FunctionalTests/Query/SimpleQuerySqlServerTest.Select.cs

@@ -1240,5 +1240,27 @@ FROM [Customers] AS [c]
 WHERE [c].[CustomerID] = N'ALFKI'
 ORDER BY [c].[CustomerID], [o].[OrderID], [o0].[OrderID]");
         }
+
+        public override async Task Select_over_10_nested_ternary_condition(bool isAsync)
+        {
+            await base.Select_over_10_nested_ternary_condition(isAsync);
+
+            AssertSql(
+                @"SELECT CASE
+    WHEN [c].[CustomerID] = N'1' THEN N'01'
+    WHEN [c].[CustomerID] = N'2' THEN N'02'
+    WHEN [c].[CustomerID] = N'3' THEN N'03'
+    WHEN [c].[CustomerID] = N'4' THEN N'04'
+    WHEN [c].[CustomerID] = N'5' THEN N'05'
+    WHEN [c].[CustomerID] = N'6' THEN N'06'
+    WHEN [c].[CustomerID] = N'7' THEN N'07'
+    WHEN [c].[CustomerID] = N'8' THEN N'08'
+    WHEN [c].[CustomerID] = N'9' THEN N'09'
+    WHEN [c].[CustomerID] = N'10' THEN N'10'
+    WHEN [c].[CustomerID] = N'11' THEN N'11'
+    ELSE NULL
+END
+FROM [Customers] AS [c]");
+        }
     }
 }