CSHARP-1623: add support for $dateToString.

Author: craiggwilson

src/MongoDB.Driver/Linq/Translators/AggregateLanguageTranslator.cs

@@ -366,6 +366,12 @@ private BsonValue TranslateMethodCall(MethodCallExpression node)
                     return result;
                 }
 
+                if (node.Object.Type == typeof(DateTime)
+                    && TryTranslateDateTimeCall(node, out result))
+                {
+                    return result;
+                }
+
                 if (node.Object.Type.GetTypeInfo().IsGenericType
                     && node.Object.Type.GetGenericTypeDefinition() == typeof(HashSet<>)
                     && TryTranslateHashSetMethodCall(node, out result))
@@ -672,6 +678,29 @@ private bool TryTranslateCountResultOperator(PipelineExpression node, out BsonVa
             return false;
         }
 
+        private bool TryTranslateDateTimeCall(MethodCallExpression node, out BsonValue result)
+        {
+            result = null;
+            var field = TranslateValue(node.Object);
+
+            switch (node.Method.Name)
+            {
+                case "ToString":
+                    if (node.Arguments.Count == 1)
+                    {
+                        var format = TranslateValue(node.Arguments[0]);
+                        result = new BsonDocument("$dateToString", new BsonDocument
+                        {
+                            { "format", format },
+                            { "date", field }
+                        });
+                    }
+                    return true;
+            }
+
+            return false;
+        }
+
         private bool TryTranslateDateTimeMemberAccess(MemberExpression node, out BsonValue result)
         {
             result = null;

tests/MongoDB.Driver.Tests/Linq/Translators/AggregateProjectTranslatorTests.cs

@@ -412,6 +412,18 @@ public void Should_translate_condition()
             result.Value.Result.Should().Be("b");
         }
 
+        [SkippableFact]
+        public void Should_translate_dateToString()
+        {
+            RequireServer.Where(minimumVersion: "3.2.0");
+
+            var result = Project(x => new { Result = x.J.ToString("%Y-%m-%d") });
+
+            result.Projection.Should().Be("{ Result: { \"$dateToString\": {format: \"%Y-%m-%d\", date: \"$J\" } }, _id: 0 }");
+
+            result.Value.Result.Should().Be("2012-12-01");
+        }
+
         [Fact]
         public void Should_translate_day_of_month()
         {