CSHARP-4061: Use correct LinqProvider in AggregateFluent ToString.

Author: rstam

src/MongoDB.Driver/AggregateFluent.cs

@@ -270,7 +270,8 @@ public override IAggregateFluent<TNewResult> Unwind<TNewResult>(FieldDefinition<
 
         public override string ToString()
         {
-            return $"aggregate({_pipeline})";
+            var linqProvider = Database.Client.Settings.LinqProvider;
+            return $"aggregate({_pipeline.ToString(linqProvider)})";
         }
 
         protected abstract IAggregateFluent<TNewResult> WithPipeline<TNewResult>(PipelineDefinition<TInput, TNewResult> pipeline);

src/MongoDB.Driver/PipelineDefinition.cs

@@ -100,10 +100,22 @@ public virtual RenderedPipelineDefinition<TOutput> Render(IBsonSerializer<TInput
 
         /// <inheritdoc/>
         public override string ToString()
+        {
+            return ToString(LinqProvider.V2);
+        }
+
+        /// <summary>
+        /// Returns a <see cref="System.String" /> that represents this instance.
+        /// </summary>
+        /// <param name="linqProvider">The LINQ provider.</param>
+        /// <returns>
+        /// A <see cref="System.String" /> that represents this instance.
+        /// </returns>
+        public string ToString(LinqProvider linqProvider)
         {
             var serializerRegistry = BsonSerializer.SerializerRegistry;
             var inputSerializer = serializerRegistry.GetSerializer<TInput>();
-            return ToString(inputSerializer, serializerRegistry);
+            return ToString(inputSerializer, serializerRegistry, linqProvider);
         }
 
         /// <summary>

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp4061Tests.cs

@@ -0,0 +1,42 @@
+/* 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 Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
+{
+    public class CSharp4061Tests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void AggregateFluent_ToString_should_return_expected_result()
+        {
+            var collection = GetCollection<C>();
+            var subject = collection.Aggregate()
+                .Group(x => x.X, g => new { X = g.Key, Count = g.Count() });
+
+            var result = subject.ToString();
+
+            result.Should().Be("aggregate([{ \"$group\" : { \"_id\" : \"$X\", \"__agg0\" : { \"$sum\" : 1 } } }, { \"$project\" : { \"X\" : \"$_id\", \"Count\" : \"$__agg0\", \"_id\" : 0 } }])");
+        }
+
+        private class C
+        {
+            public int Id { get; set; }
+            public int X { get; set; }
+        }
+    }
+}