CSHARP-1433: added support for includeArrayIndex in the $unwind stage.

Author: craiggwilson

src/MongoDB.Driver.Tests/IAggregateFluentExtensionsTests.cs

@@ -231,6 +231,32 @@ public void Unwind_with_options_with_preserveNullAndEmptyArrays_set()
             AssertLast(subject, expectedUnwind);
         }
 
+        [Test]
+        public void Unwind_with_options_with_includeArrayIndex_set()
+        {
+            var subject = CreateSubject()
+                .Unwind("Age", new AggregateUnwindOptions<BsonDocument> { IncludeArrayIndex = "AgeIndex" });
+
+            var expectedUnwind = BsonDocument.Parse("{$unwind: { path: '$Age', includeArrayIndex: 'AgeIndex' } }");
+
+            AssertLast(subject, expectedUnwind);
+        }
+
+        [Test]
+        public void Unwind_with_options_with_includeArrayIndex_set_and_preserveNullAndEmptyArrays_set()
+        {
+            var subject = CreateSubject()
+                .Unwind("Age", new AggregateUnwindOptions<BsonDocument>
+                {
+                    IncludeArrayIndex = "AgeIndex",
+                    PreserveNullAndEmptyArrays = true
+                });
+
+            var expectedUnwind = BsonDocument.Parse("{$unwind: { path: '$Age', preserveNullAndEmptyArrays: true, includeArrayIndex: 'AgeIndex' } }");
+
+            AssertLast(subject, expectedUnwind);
+        }
+
         private void AssertLast<TDocument>(IAggregateFluent<TDocument> fluent, BsonDocument expectedLast)
         {
             var pipeline = new PipelineStagePipelineDefinition<Person, TDocument>(fluent.Stages);

src/MongoDB.Driver/AggregateFluent.cs

@@ -204,20 +204,29 @@ public override IAggregateFluent<TNewResult> Unwind<TNewResult>(FieldDefinition<
                 operatorName,
                 (s, sr) =>
                 {
+                    var newResultSerializer = options.ResultSerializer ?? (s as IBsonSerializer<TNewResult>) ?? sr.GetSerializer<TNewResult>();
+
                     var fieldName = "$" + field.Render(s, sr).FieldName;
+                    string includeArrayIndexFieldName = null;
+                    if (options.IncludeArrayIndex != null)
+                    {
+                        includeArrayIndexFieldName = options.IncludeArrayIndex.Render(newResultSerializer, sr).FieldName;
+                    }
+
                     BsonValue value = fieldName;
-                    if (options.PreserveNullAndEmptyArrays.HasValue)
+                    if (options.PreserveNullAndEmptyArrays.HasValue || includeArrayIndexFieldName != null)
                     {
                         value = new BsonDocument
                         {
                             { "path", fieldName },
-                            { "preserveNullAndEmptyArrays", options.PreserveNullAndEmptyArrays, options.PreserveNullAndEmptyArrays.HasValue }
+                            { "preserveNullAndEmptyArrays", options.PreserveNullAndEmptyArrays, options.PreserveNullAndEmptyArrays.HasValue },
+                            { "includeArrayIndex", includeArrayIndexFieldName, includeArrayIndexFieldName != null }
                         };
                     }
                     return new RenderedPipelineStageDefinition<TNewResult>(
                         operatorName,
                         new BsonDocument(operatorName, value),
-                        options.ResultSerializer ?? (s as IBsonSerializer<TNewResult>) ?? sr.GetSerializer<TNewResult>());
+                        newResultSerializer);
                 });
 
             return AppendStage<TNewResult>(stage);

src/MongoDB.Driver/AggregateUnwindOptions.cs

@@ -22,9 +22,19 @@ namespace MongoDB.Driver
     /// </summary>
     public class AggregateUnwindOptions<TResult>
     {
+        private FieldDefinition<TResult> _includeArrayIndex;
         private bool? _preserveNullAndEmptyArrays;
         private IBsonSerializer<TResult> _resultSerializer;
 
+        /// <summary>
+        /// Gets or sets the field with which to include the array index.
+        /// </summary>
+        public FieldDefinition<TResult> IncludeArrayIndex
+        {
+            get { return _includeArrayIndex; }
+            set { _includeArrayIndex = value; }
+        }
+
         /// <summary>
         /// Gets or sets whether to preserve null and empty arrays.
         /// </summary>