CSHARP-5588: Unable to select KeyValuePair Key when using camelCase convention (#1724)

Author: sanych-sun

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/NewKeyValuePairExpressionToAggregationExpressionTranslator.cs

@@ -43,24 +43,32 @@ public static TranslatedExpression Translate(
             var keyTranslation = ExpressionToAggregationExpressionTranslator.Translate(context, keyExpression);
             var valueTranslation = ExpressionToAggregationExpressionTranslator.Translate(context, valueExpression);
 
+            var serializer = CreateResultSerializer(expression.Type, keyTranslation.Serializer, valueTranslation.Serializer, out var keyElementName, out var valueElementName);
             var ast = AstExpression.ComputedDocument([
-                AstExpression.ComputedField("Key", keyTranslation.Ast),
-                AstExpression.ComputedField("Value", valueTranslation.Ast)
+                AstExpression.ComputedField(keyElementName, keyTranslation.Ast),
+                AstExpression.ComputedField(valueElementName, valueTranslation.Ast)
             ]);
 
-            var serializer = CreateResultSerializer(expression.Type, keyTranslation.Serializer, valueTranslation.Serializer);
-
             return new TranslatedExpression(expression, ast, serializer);
         }
 
-        private static IBsonSerializer CreateResultSerializer(Type resultType, IBsonSerializer keySerializer, IBsonSerializer valueSerializer)
+        private static IBsonSerializer CreateResultSerializer(
+            Type resultType,
+            IBsonSerializer keySerializer,
+            IBsonSerializer valueSerializer,
+            out string keyElementName,
+            out string valueElementName)
         {
             var constructorInfo = resultType.GetConstructor([keySerializer.ValueType, valueSerializer.ValueType]);
             var classMap = new BsonClassMap(resultType);
             classMap.MapConstructor(constructorInfo);
             classMap.AutoMap();
-            classMap.GetMemberMap("Key").SetSerializer(keySerializer);
-            classMap.GetMemberMap("Value").SetSerializer(valueSerializer);
+            var keyMemberMap = classMap.GetMemberMap("Key");
+            keyElementName = keyMemberMap.ElementName;
+            keyMemberMap.SetSerializer(keySerializer);
+            var valueMemberMap = classMap.GetMemberMap("Value");
+            valueElementName = valueMemberMap.ElementName;
+            valueMemberMap.SetSerializer(valueSerializer);
             classMap.Freeze();
 
             // have to use BsonClassMapSerializer here to mimic the MemberInitExpressionToAggregationExpressionTranslator to avoid risking a behavioral breaking change

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/NewKeyValuePairExpressionToAggregationExpressionTranslatorTests.cs

@@ -21,7 +21,6 @@
 
 namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators;
 
-#if NET6_0_OR_GREATER || NETCOREAPP3_1_OR_GREATER
 public class NewKeyValuePairExpressionToAggregationExpressionTranslatorTests : LinqIntegrationTest<NewKeyValuePairExpressionToAggregationExpressionTranslatorTests.ClassFixture>
 {
     public NewKeyValuePairExpressionToAggregationExpressionTranslatorTests(ClassFixture fixture)
@@ -45,6 +44,7 @@ public void NewKeyValuePair_should_translate()
         result.Value.Should().Be(42);
     }
 
+#if NET6_0_OR_GREATER || NETCOREAPP3_1_OR_GREATER
     [Fact]
     public void KeyValuePair_Create_should_translate()
     {
@@ -60,6 +60,7 @@ public void KeyValuePair_Create_should_translate()
         result.Key.Should().Be("X");
         result.Value.Should().Be(42);
     }
+#endif
 
     public class C
     {
@@ -74,5 +75,3 @@ public sealed class ClassFixture : MongoCollectionFixture<C>
         ];
     }
 }
-
-#endif