CSHARP-2804: Support for JavaScript User Defined Functions.

Author: DmitryLukyanov

src/MongoDB.Driver.Core/Core/Misc/Feature.cs

@@ -33,6 +33,7 @@ public class Feature
         private static readonly Feature __aggregateCursorResult = new Feature("AggregateCursorResult", new SemanticVersion(2, 6, 0));
         private static readonly Feature __aggregateExplain = new Feature("AggregateExplain", new SemanticVersion(2, 6, 0));
         private static readonly Feature __aggregateFacetStage = new Feature("AggregateFacetStage", new SemanticVersion(3, 4, 0, "rc0"));
+        private static readonly Feature __aggregateFunction = new Feature("AggregateFunction", new SemanticVersion(4, 3, 4));
         private static readonly Feature __aggregateGraphLookupStage = new Feature("AggregateGraphLookupStage", new SemanticVersion(3, 4, 0, "rc0"));
         private static readonly Feature __aggregateHint = new Feature("AggregateHint", new SemanticVersion(3, 6, 0, "rc0"));
         private static readonly Feature __aggregateLet = new Feature("AggregateLet", new SemanticVersion(3, 6, 0));
@@ -143,6 +144,11 @@ public class Feature
         /// </summary>
         public static Feature AggregateFacetStage => __aggregateFacetStage;
 
+        /// <summary>
+        /// Gets the aggregate $function stage feature.
+        /// </summary>
+        public static Feature AggregateFunction => __aggregateFunction;
+
         /// <summary>
         /// Gets the aggregate $graphLookup stage feature.
         /// </summary>

tests/MongoDB.Driver.Tests/AggregateFluentTests.cs

@@ -196,6 +196,72 @@ public void Count_should_return_the_expected_result(
             result.Should().Be(1);
         }
 
+        [SkippableFact]
+        public void Function_should_return_expected_result()
+        {
+            RequireServer.Check().Supports(Feature.AggregateFunction);
+
+            var client = CreateClient();
+            var databaseName = "test";
+            var collectionName = "collection";
+            DropCollection(client, databaseName, collectionName);
+            var collection = client.GetDatabase(databaseName).GetCollection<BsonDocument>(collectionName);
+            var players = new[]
+            {
+                BsonDocument.Parse("{ _id : 1, name : 'Miss Cheevous',  scores : [ 10, 5, 10 ] }"),
+                BsonDocument.Parse("{ _id : 2, name : 'Miss Ann Thrope', scores : [ 10, 10, 10 ] }"),
+                BsonDocument.Parse("{ _id : 3, name : 'Mrs. Eppie Delta', scores : [ 9, 8, 8 ] }")
+            };
+            collection.InsertMany(players);
+
+            var isFoundFunction = "function(name) { return hex_md5(name) == '15b0a220baa16331e8d80e15367677ad' }";
+            var messageFunction = "function(name, scores) { let total = Array.sum(scores); return `Hello ${ name}. Your total score is ${ total}.` }";
+            var isFoundExpression = new BsonDocument
+            {
+                {
+                    "$function",
+                    new BsonDocument
+                    {
+                        { "body", isFoundFunction },
+                        { "args", new BsonArray { "$name" } },
+                        { "lang", "js" }
+                    }
+                },
+            };
+            var messageExpression = new BsonDocument
+            {
+                {
+                    "$function",
+                    new BsonDocument
+                    {
+                        { "body", messageFunction },
+                        { "args", new BsonArray { "$name", "$scores" } },
+                        { "lang", "js" }
+                    }
+                },
+            };
+            var addFieldsStage = new BsonDocument
+            {
+                {
+                    "$addFields",
+                    new BsonDocument
+                    {
+                        { "isFound", isFoundExpression },
+                        { "message", messageExpression },
+                    }
+                }
+            };
+            var stages = new[] { addFieldsStage };
+            var pipeline = new BsonDocumentStagePipelineDefinition<BsonDocument, BsonDocument>(stages);
+
+            var result = collection.Aggregate(pipeline).ToList();
+
+            result.Count.Should().Be(3);
+            result[0].Should().Be("{ _id : 1, name : 'Miss Cheevous', scores : [ 10, 5, 10 ], isFound : false, message : 'Hello Miss Cheevous. Your total score is 25.' }");
+            result[1].Should().Be("{ _id : 2, name : 'Miss Ann Thrope', scores : [ 10, 10, 10 ], isFound : true, message : 'Hello Miss Ann Thrope. Your total score is 30.' }");
+            result[2].Should().Be("{ _id : 3, name : 'Mrs. Eppie Delta', scores : [ 9, 8, 8 ], isFound : false, message : 'Hello Mrs. Eppie Delta. Your total score is 25.' }");
+        }
+
         [SkippableFact]
         public void Group_with_accumulator_should_return_expected_result()
         {