CSHARP-5588: Unable to select KeyValuePair Key when using camelCase convention

Author: Oleksandr Poliakov

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

@@ -16,17 +16,27 @@
 using System.Collections.Generic;
 using System.Linq;
 using FluentAssertions;
+using MongoDB.Bson.Serialization.Conventions;
 using MongoDB.Driver.TestHelpers;
 using Xunit;
 
 namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators;
 
-#if NET6_0_OR_GREATER || NETCOREAPP3_1_OR_GREATER
 public class NewKeyValuePairExpressionToAggregationExpressionTranslatorTests : LinqIntegrationTest<NewKeyValuePairExpressionToAggregationExpressionTranslatorTests.ClassFixture>
 {
+    private readonly bool _useCamelCase = false;
+
     public NewKeyValuePairExpressionToAggregationExpressionTranslatorTests(ClassFixture fixture)
         : base(fixture)
     {
+        // TODO: uncomment the code to demonstrate the fix, revert changes in the test class before merging into the main
+        // _useCamelCase = true;
+
+        if (_useCamelCase)
+        {
+            var defaultPack = new ConventionPack { new CamelCaseElementNameConvention() };
+            ConventionRegistry.Register("camelCase", defaultPack, c => true);
+        }
     }
 
     [Fact]
@@ -38,13 +48,21 @@ public void NewKeyValuePair_should_translate()
             .Select(d => new KeyValuePair<string,int>("X", d.X));
 
         var stages = Translate(collection, queryable);
-        AssertStages(stages, "{ $project : { Key : 'X', Value : '$X', _id : 0 } }");
+        if (_useCamelCase)
+        {
+            AssertStages(stages, "{ $project : { key : 'X', value : '$X', _id : 0 } }");
+        }
+        else
+        {
+            AssertStages(stages, "{ $project : { Key : 'X', Value : '$X', _id : 0 } }");
+        }
 
         var result = queryable.Single();
         result.Key.Should().Be("X");
         result.Value.Should().Be(42);
     }
 
+#if NET6_0_OR_GREATER || NETCOREAPP3_1_OR_GREATER
     [Fact]
     public void KeyValuePair_Create_should_translate()
     {
@@ -54,12 +72,20 @@ public void KeyValuePair_Create_should_translate()
             .Select(d => KeyValuePair.Create("X", d.X));
 
         var stages = Translate(collection, queryable);
-        AssertStages(stages, "{ $project : { Key : 'X', Value : '$X', _id : 0 } }");
+        if (_useCamelCase)
+        {
+            AssertStages(stages, "{ $project : { key : 'X', value : '$X', _id : 0 } }");
+        }
+        else
+        {
+            AssertStages(stages, "{ $project : { Key : 'X', Value : '$X', _id : 0 } }");
+        }
 
         var result = queryable.Single();
         result.Key.Should().Be("X");
         result.Value.Should().Be(42);
     }
+#endif
 
     public class C
     {
@@ -75,4 +101,3 @@ public sealed class ClassFixture : MongoCollectionFixture<C>
     }
 }
 
-#endif