CSHARP-4700: Fix issue with GroupBy followed by OrderBy.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Optimizers/AstGroupingPipelineOptimizer.cs

@@ -16,6 +16,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Linq;
 using MongoDB.Bson;
 using MongoDB.Driver.Linq.Linq3Implementation.Ast.Expressions;
 using MongoDB.Driver.Linq.Linq3Implementation.Ast.Filters;
@@ -169,11 +170,20 @@ static bool StageCanBeOptimized(AstStage stage)
 
                 static bool IsLastStageThatCanBeOptimized(AstStage stage)
                 {
-                    return stage.NodeType switch
+                    return stage switch
                     {
-                        AstNodeType.ProjectStage => true,
+                        AstProjectStage projectStage => !ProjectsRoot(projectStage),
                         _ => false
                     };
+
+                    static bool ProjectsRoot(AstProjectStage projectStage)
+                    {
+                        return projectStage.Specifications.Any(
+                            specification =>
+                                specification is AstProjectStageSetFieldSpecification setFieldSpecification &&
+                                setFieldSpecification.Value is AstVarExpression varExpression &&
+                                varExpression.Name == "ROOT");
+                    }
                 }
             }
         }

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToPipelineTranslators/OrderByMethodToPipelineTranslator.cs

@@ -223,7 +223,7 @@ private static AstStage[] CreateSortStages(string methodName, AggregationExpress
             {
                 var projectStage = AstStage.Project(
                     AstProject.Exclude("_id"),
-                    AstProject.Set("_document", AstExpression.FieldPath("$$ROOT")),
+                    AstProject.Set("_document", AstExpression.Var("ROOT")),
                     AstProject.Set("_key1", keySelectorTranslation.Ast));
                 var sortStage = AstStage.Sort(AstSort.Field("_key1", sortOrder));
                 var replaceRootStage = AstStage.ReplaceRoot(AstExpression.FieldPath("$_document"));

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp4700Tests.cs

@@ -0,0 +1,88 @@
+/* Copyright 2010-present MongoDB Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using System.Linq;
+using FluentAssertions;
+using MongoDB.Driver.Linq;
+using MongoDB.TestHelpers.XunitExtensions;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Jira
+{
+    public class CSharp4700Tests : Linq3IntegrationTest
+    {
+        [Theory]
+        [ParameterAttributeData]
+        public void OrderBy_Count_should_work(
+            [Values(LinqProvider.V2, LinqProvider.V3)] LinqProvider linqProvider)
+        {
+            var collection = GetCollection(linqProvider);
+
+            var queryable = collection.AsQueryable()
+                .GroupBy(x => x.Name)
+                .OrderBy(x => x.Count());
+
+            var stages = Translate(collection, queryable);
+            var results = queryable.ToList();
+
+            if (linqProvider == LinqProvider.V2)
+            {
+                AssertStages(
+                    stages,
+                    "{ $group : { _id : '$Name', __agg0 : { $sum : 1 } } }",
+                    "{ $sort : { __agg0 : 1 } }");
+
+                results.Should().HaveCount(2);
+                results[0].Key.Should().Be("Jane");
+                results[0].Count().Should().Be(0); // this result is incorrect in LINQ2
+                results[1].Key.Should().Be("John");
+                results[1].Count().Should().Be(0); // this result is incorrect in LINQ2
+            }
+            else
+            {
+                AssertStages(
+                    stages,
+                    "{ $group : { _id : '$Name', _elements : { $push : '$$ROOT' } } }",
+                    "{ $project : { _id : 0, _document : '$$ROOT', _key1 : { $size : '$_elements' } } }",
+                    "{ $sort : { _key1 : 1 } }",
+                    "{ $replaceRoot : { newRoot : '$_document' } }");
+
+                results.Should().HaveCount(2);
+                results[0].Key.Should().Be("Jane");
+                results[0].Count().Should().Be(1);
+                results[1].Key.Should().Be("John");
+                results[1].Count().Should().Be(2);
+            }
+        }
+
+        private IMongoCollection<C> GetCollection(LinqProvider linqProvider)
+        {
+            var collection = GetCollection<C>("test", linqProvider);
+            CreateCollection(
+                collection,
+                new C { Id = 1, Name = "John" },
+                new C { Id = 2, Name = "John" },
+                new C { Id = 6, Name = "Jane" });
+            return collection;
+        }
+
+        private class C
+        {
+            public int Id { get; set; }
+            public string Name { get; set; }
+        }
+
+    }
+}