CSHARP-5540: Fix exception when using AsQueryable().Last()

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToExecutableQueryTranslators/LastMethodToExecutableQueryTranslator.cs

@@ -75,10 +75,11 @@ public static ExecutableQuery<TDocument, TOutput> Translate<TDocument>(MongoQuer
                         pipeline.OutputSerializer);
                 }
 
-                pipeline = pipeline.AddStage(
+                pipeline = pipeline.AddStages(
                     AstStage.Group(
                         id: BsonNull.Value,
                         fields: AstExpression.AccumulatorField("_last", AstUnaryAccumulatorOperator.Last, AstExpression.RootVar)),
+                    AstStage.ReplaceRoot(AstExpression.GetField(AstExpression.RootVar, "_last")),
                     pipeline.OutputSerializer);
 
                 var finalizer = method.Name == "LastOrDefault" ? __singleOrDefaultFinalizer : __singleFinalizer;

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Translators/ExpressionToExecutableQueryTranslators/LastMethodToExecutableQueryTranslatorTests.cs

@@ -0,0 +1,199 @@
+/* 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;
+using System.Collections.Generic;
+using System.Linq;
+using FluentAssertions;
+using MongoDB.Driver.Linq;
+using MongoDB.Driver.TestHelpers;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Translators.ExpressionToExecutableQueryTranslators
+{
+    public class LastMethodToExecutableQueryTranslator : LinqIntegrationTest<LastMethodToExecutableQueryTranslator.ClassFixture>
+    {
+        public LastMethodToExecutableQueryTranslator(ClassFixture fixture)
+            : base(fixture)
+        {
+        }
+
+        [Fact]
+        public void Last_should_work()
+        {
+            var collection = Fixture.Collection;
+            var queryable = collection.AsQueryable().OrderBy(t => t.Id);
+
+            var result = queryable.Last();
+            var stages = queryable.GetMongoQueryProvider().LoggedStages;
+
+            AssertStages(
+                stages,
+                "{ $sort : { _id : 1 } }",
+                "{ $group : { _id : null, _last : { $last : '$$ROOT' } } }",
+                "{ $replaceRoot : { newRoot : '$_last' } }");
+
+            result.Id.Should().Be(3);
+        }
+
+        [Fact]
+        public void Last_with_no_matching_documents_should_throw()
+        {
+            var collection = Fixture.Collection;
+            var queryable = collection.AsQueryable().Where(t => t.Id > 5);
+
+            var exception = Record.Exception(() => queryable.Last());
+            var stages = queryable.GetMongoQueryProvider().LoggedStages;
+
+            AssertStages(
+                stages,
+                "{ $match : { _id : { $gt : 5 } }}",
+                "{ $group : { _id : null, _last : { $last : '$$ROOT' } } }",
+                "{ $replaceRoot : { newRoot : '$_last' } }");
+
+            exception.Should().BeOfType<InvalidOperationException>();
+            exception.Message.Should().Contain("Sequence contains no elements");
+        }
+
+        [Fact]
+        public void LastOrDefault_should_work()
+        {
+            var collection = Fixture.Collection;
+            var queryable = collection.AsQueryable().OrderBy(t => t.Id);
+
+            var result = queryable.LastOrDefault();
+            var stages = queryable.GetMongoQueryProvider().LoggedStages;
+
+            AssertStages(
+                stages,
+                "{ $sort : { _id : 1 } }",
+                "{ $group : { _id : null, _last : { $last : '$$ROOT' } } }",
+                "{ $replaceRoot : { newRoot : '$_last' } }");
+
+            result.Should().NotBeNull();
+            result.Id.Should().Be(3);
+        }
+
+        [Fact]
+        public void LastOrDefault_with_no_matching_documents_should_work()
+        {
+            var collection = Fixture.Collection;
+            var queryable = collection.AsQueryable().Where(t => t.Id > 5);
+
+            var result = queryable.LastOrDefault();
+            var stages = queryable.GetMongoQueryProvider().LoggedStages;
+
+            AssertStages(
+                stages,
+                "{ $match : { _id : { $gt : 5 } }}",
+                "{ $group : { _id : null, _last : { $last : '$$ROOT' } } }",
+                "{ $replaceRoot : { newRoot : '$_last' } }");
+
+            result.Should().BeNull();
+        }
+
+        [Fact]
+        public void LastOrDefaultWithPredicate_should_work()
+        {
+            var collection = Fixture.Collection;
+            var queryable = collection.AsQueryable().OrderBy(t => t.Id);
+
+            var result = queryable.LastOrDefault(t => t.Id > 1);
+            var stages = queryable.GetMongoQueryProvider().LoggedStages;
+
+            AssertStages(
+                stages,
+                "{ $sort : { _id : 1 } }",
+                "{ $match : { _id : { $gt : 1 } }}",
+                "{ $group : { _id : null, _last : { $last : '$$ROOT' } } }",
+                "{ $replaceRoot : { newRoot : '$_last' } }");
+
+            result.Should().NotBeNull();
+            result.Id.Should().Be(3);
+        }
+
+        [Fact]
+        public void LastOrDefaultWithPredicate_with_no_matching_documents_should_work()
+        {
+            var collection = Fixture.Collection;
+            var queryable = collection.AsQueryable();
+
+            var result = queryable.LastOrDefault(t => t.Id > 4);
+            var stages = queryable.GetMongoQueryProvider().LoggedStages;
+
+            AssertStages(
+                stages,
+                "{ $match : { _id : { $gt : 4 } }}",
+                "{ $group : { _id : null, _last : { $last : '$$ROOT' } } }",
+                "{ $replaceRoot : { newRoot : '$_last' } }");
+
+            result.Should().BeNull();
+        }
+
+        [Fact]
+        public void LastWithPredicate_should_work()
+        {
+            var collection = Fixture.Collection;
+            var queryable = collection.AsQueryable().OrderBy(t => t.Id);
+
+            var result = queryable.Last(t => t.Id > 1);
+            var stages = queryable.GetMongoQueryProvider().LoggedStages;
+
+            AssertStages(
+                stages,
+                "{ $sort : { _id : 1 } }",
+                "{ $match : { _id : { $gt : 1 } }}",
+                "{ $group : { _id : null, _last : { $last : '$$ROOT' } } }",
+                "{ $replaceRoot : { newRoot : '$_last' } }");
+
+            result.Id.Should().Be(3);
+        }
+
+        [Fact]
+        public void LastWithPredicate_with_no_matching_documents_should_throw()
+        {
+            var collection = Fixture.Collection;
+            var queryable = collection.AsQueryable();
+
+            var exception = Record.Exception(() => queryable.Last(t => t.Id > 4));
+            var stages = queryable.GetMongoQueryProvider().LoggedStages;
+
+            AssertStages(
+                stages,
+                "{ $match : { _id : { $gt : 4 } }}",
+                "{ $group : { _id : null, _last : { $last : '$$ROOT' } } }",
+                "{ $replaceRoot : { newRoot : '$_last' } }");
+
+            exception.Should().BeOfType<InvalidOperationException>();
+            exception.Message.Should().Contain("Sequence contains no elements");
+        }
+
+        public class TestClass
+        {
+            public int Id { get; set; }
+            public string StringProperty { get; set; }
+        }
+
+        public sealed class ClassFixture : MongoCollectionFixture<TestClass>
+        {
+            protected override IEnumerable<TestClass> InitialData { get; } =
+            [
+                new TestClass { Id = 1, StringProperty = "A" },
+                new TestClass { Id = 2, StringProperty = "B" },
+                new TestClass { Id = 3, StringProperty = "C" }
+            ];
+        }
+    }
+}