CSHARP-3075: Collection Aggregation Merge Default Value Throws an Exception

Author: MikalaiMazurenka

src/MongoDB.Driver/MongoCollectionImpl.cs

@@ -806,19 +806,34 @@ private FindOperation<TResult> CreateAggregateToCollectionFindOperation<TResult>
                     break;
                 case "$merge":
                     {
-                        var mergeArguments = outStage[0].AsBsonDocument;
+                        var mergeArguments = outStage[0];
                         DatabaseNamespace outputDatabaseNamespace;
                         string outputCollectionName;
-                        var into = mergeArguments["into"];
-                        if (into.IsString)
+                        if (mergeArguments.IsString)
                         {
                             outputDatabaseNamespace = _collectionNamespace.DatabaseNamespace;
-                            outputCollectionName = into.AsString;
+                            outputCollectionName = mergeArguments.AsString;
                         }
                         else
                         {
-                            outputDatabaseNamespace = new DatabaseNamespace(into["db"].AsString);
-                            outputCollectionName = into["coll"].AsString;
+                            var into = mergeArguments.AsBsonDocument["into"];
+                            if (into.IsString)
+                            {
+                                outputDatabaseNamespace = _collectionNamespace.DatabaseNamespace;
+                                outputCollectionName = into.AsString;
+                            }
+                            else
+                            {
+                                if (into.AsBsonDocument.Contains("db"))
+                                {
+                                    outputDatabaseNamespace = new DatabaseNamespace(into["db"].AsString);
+                                }
+                                else
+                                {
+                                    outputDatabaseNamespace = _collectionNamespace.DatabaseNamespace;
+                                }
+                                outputCollectionName = into["coll"].AsString;
+                            }
                         }
                         outputCollectionNamespace = new CollectionNamespace(outputDatabaseNamespace, outputCollectionName);
                     }

tests/MongoDB.Driver.Tests/MongoCollectionImplTests.cs

@@ -258,6 +258,62 @@ public void Aggregate_should_execute_an_AggregateToCollectionOperation_and_a_Fin
             findOperation.Sort.Should().BeNull();
         }
 
+        [Theory]
+        [InlineData("{ $merge : \"outputcollection\" }", null, "outputcollection", false)]
+        [InlineData("{ $merge : \"outputcollection\" }", null, "outputcollection", true)]
+        [InlineData("{ $merge : { into : \"outputcollection\" } }", null, "outputcollection", false)]
+        [InlineData("{ $merge : { into : \"outputcollection\" } }", null, "outputcollection", true)]
+        [InlineData("{ $merge : { into : { coll : \"outputcollection\" } } }", null, "outputcollection", false)]
+        [InlineData("{ $merge : { into : { coll : \"outputcollection\" } } }", null, "outputcollection", true)]
+        [InlineData("{ $merge : { into : { db: \"outputdatabase\", coll : \"outputcollection\" } } }", "outputdatabase", "outputcollection", false)]
+        [InlineData("{ $merge : { into : { db: \"outputdatabase\", coll : \"outputcollection\" } } }", "outputdatabase", "outputcollection", true)]
+        public void Aggregate_should_recognize_merge_collection_argument(
+            string stageDefinitionString,
+            string expectedDatabaseName,
+            string expectedCollectionName,
+            bool async)
+        {
+            var subject = CreateSubject<BsonDocument>();
+            var stageDefinition = BsonDocument.Parse(stageDefinitionString);
+            var expectedCollectionNamespace = new CollectionNamespace(
+                expectedDatabaseName ?? subject.CollectionNamespace.DatabaseNamespace.DatabaseName,
+                expectedCollectionName);
+
+            var pipeline = new EmptyPipelineDefinition<BsonDocument>()
+                .AppendStage<BsonDocument, BsonDocument, BsonDocument>(stageDefinition);
+            var expectedPipeline = new List<BsonDocument>(RenderPipeline(subject, pipeline).Documents);
+
+            IAsyncCursor<BsonDocument> result;
+            if (async)
+            {
+                result = subject.AggregateAsync(pipeline).GetAwaiter().GetResult();
+            }
+            else
+            {
+                result = subject.Aggregate(pipeline);
+            }
+            var aggregateCall = _operationExecutor.GetWriteCall<BsonDocument>();
+
+            var aggregateOperation = aggregateCall.Operation.Should().BeOfType<AggregateToCollectionOperation>().Subject;
+            aggregateOperation.CollectionNamespace.Should().Be(subject.CollectionNamespace);
+            aggregateOperation.Pipeline.Should().Equal(expectedPipeline);
+
+            var mockCursor = new Mock<IAsyncCursor<BsonDocument>>();
+            _operationExecutor.EnqueueResult(mockCursor.Object);
+            if (async)
+            {
+                result.MoveNextAsync().GetAwaiter().GetResult();
+            }
+            else
+            {
+                result.MoveNext();
+            }
+            var findCall = _operationExecutor.GetReadCall<IAsyncCursor<BsonDocument>>();
+
+            var findOperation = findCall.Operation.Should().BeOfType<FindOperation<BsonDocument>>().Subject;
+            findOperation.CollectionNamespace.Should().Be(expectedCollectionNamespace);
+        }
+
         [Theory]
         [ParameterAttributeData]
         public void AggregateToCollection_should_execute_an_AggregateToCollectionOperation(