CSHARP-1373: added $sum expression support.

Author: craiggwilson

Docs/reference/content/reference/driver/expressions.md

@@ -788,6 +788,15 @@ p => p.FavoriteNumbers.Count();
 { $size: '$FavoriteNumbers' }
 ```
 
+#### $sum
+
+```csharp
+p => p.FavoriteNumbers.Sum()
+```
+```json
+{ $sum: '$FavoriteNumbers' }
+```
+
 ### Variable Expressions
 
 See the [MongoDB documentation]({{< docsref "meta/aggregation-quick-reference/#variable-expressions" >}}) for more information on each operator.

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

@@ -948,6 +948,28 @@ public async Task Should_translate_subtract_3_numbers()
             result.Value.Result.Should().Be(-23);
         }
 
+        [Test]
+        [RequiresServer(MinimumVersion = "3.1.7")]
+        public async Task Should_translate_sum()
+        {
+            var result = await Project(x => new { Result = x.M.Sum() });
+
+            result.Projection.Should().Be("{ Result: { \"$sum\": \"$M\" }, _id: 0 }");
+
+            result.Value.Result.Should().Be(11);
+        }
+
+        [Test]
+        [RequiresServer(MinimumVersion = "3.1.7")]
+        public async Task Should_translate_sum_with_selector()
+        {
+            var result = await Project(x => new { Result = x.G.Sum(g => g.E.F) });
+
+            result.Projection.Should().Be("{ Result: { \"$sum\": \"$G.E.F\" }, _id: 0 }");
+
+            result.Value.Result.Should().Be(88);
+        }
+
         [Test]
         public async Task Should_translate_to_lower()
         {

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

@@ -397,7 +397,8 @@ private BsonValue TranslatePipeline(PipelineExpression node)
                 TryTranslateContainsResultOperator(node, out result) ||
                 TryTranslateCountResultOperator(node, out result) ||
                 TryTranslateMaxResultOperator(node, out result) ||
-                TryTranslateMinResultOperator(node, out result))
+                TryTranslateMinResultOperator(node, out result) ||
+                TryTranslateSumResultOperator(node, out result))
             {
                 return result;
             }
@@ -789,6 +790,19 @@ private bool TryTranslateStringMethodCall(MethodCallExpression node, out BsonVal
             return false;
         }
 
+        private bool TryTranslateSumResultOperator(PipelineExpression node, out BsonValue result)
+        {
+            var resultOperator = node.ResultOperator as SumResultOperator;
+            if (resultOperator != null)
+            {
+                result = new BsonDocument("$sum", TranslateValue(node.Source));
+                return true;
+            }
+
+            result = null;
+            return false;
+        }
+
         private static bool TryFindSerializationExpression(MethodCallExpression node, out ISerializationExpression serializationExpression)
         {
             var current = node.Arguments[0];