CSHARP-3535: Regression in support for $out to S3

Author: rstam

src/MongoDB.Driver.Core/Core/Operations/AggregateToCollectionOperation.cs

@@ -296,17 +296,21 @@ private IReadOnlyList<BsonDocument> SimplifyOutStageIfOutputDatabaseIsSameAsInpu
             var lastStageName = lastStage.GetElement(0).Name;
             if (lastStageName == "$out" && lastStage["$out"] is BsonDocument outDocument)
             {
-                var outputDatabaseName = outDocument["db"].AsString;
-                if (outputDatabaseName == _databaseNamespace.DatabaseName)
+                if (outDocument.TryGetValue("db", out var db) && db.IsString &&
+                    outDocument.TryGetValue("coll", out var coll) && coll.IsString)
                 {
-                    var outputCollectionName = outDocument["coll"].AsString;
-                    var simplifiedOutStage = lastStage.Clone().AsBsonDocument;
-                    simplifiedOutStage["$out"] = outputCollectionName;
+                    var outputDatabaseName = db.AsString;
+                    if (outputDatabaseName == _databaseNamespace.DatabaseName)
+                    {
+                        var outputCollectionName = coll.AsString;
+                        var simplifiedOutStage = lastStage.Clone().AsBsonDocument;
+                        simplifiedOutStage["$out"] = outputCollectionName;
 
-                    var modifiedPipeline = new List<BsonDocument>(pipeline);
-                    modifiedPipeline[modifiedPipeline.Count - 1] = simplifiedOutStage;
+                        var modifiedPipeline = new List<BsonDocument>(pipeline);
+                        modifiedPipeline[modifiedPipeline.Count - 1] = simplifiedOutStage;
 
-                    return modifiedPipeline;
+                        return modifiedPipeline;
+                    }
                 }
             }
 

tests/MongoDB.Driver.Core.Tests/Core/Operations/AggregateToCollectionOperationTests.cs

@@ -18,11 +18,13 @@
 using System.Linq;
 using FluentAssertions;
 using MongoDB.Bson;
+using MongoDB.Bson.Serialization;
 using MongoDB.Bson.TestHelpers.XunitExtensions;
 using MongoDB.Driver.Core.Clusters;
 using MongoDB.Driver.Core.Misc;
 using MongoDB.Driver.Core.TestHelpers;
 using MongoDB.Driver.Core.TestHelpers.XunitExtensions;
+using MongoDB.Driver.Core.WireProtocol.Messages.Encoders;
 using Xunit;
 
 namespace MongoDB.Driver.Core.Operations
@@ -70,6 +72,22 @@ public void Constructor_with_collectionNamespace_should_create_a_valid_instance(
             subject.WriteConcern.Should().BeNull();
         }
 
+        [Theory]
+        [InlineData("{ $out : 'collection' }", "{ $out : 'collection' }")]
+        [InlineData("{ $out : { db : 'database', coll : 'collection' } }", "{ $out : 'collection' }")]
+        [InlineData("{ $out : { db : 'differentdatabase', coll : 'collection' } }", "{ $out : { db : 'differentdatabase', coll : 'collection' } }")]
+        [InlineData("{ $out : { s3 : { } } }", "{ $out : { s3 : { } } }")]
+        public void Constructor_should_simplify_out_stage_when_possible(string outStageJson, string expectedOutStageJson)
+        {
+            var databaseNamespace = new DatabaseNamespace("database");
+            var pipeline = new[] { BsonDocument.Parse(outStageJson) };
+            var messageEncoderSettings = new MessageEncoderSettings();
+
+            var subject = new AggregateToCollectionOperation(databaseNamespace, pipeline, messageEncoderSettings);
+
+            subject.Pipeline.Last().Should().Be(BsonDocument.Parse(expectedOutStageJson));
+        }
+
         [Fact]
         public void Constructor_should_throw_when_databaseNamespace_is_null()
         {