Work in progress.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/KnownSerializerFinders/KnownSerializerFinderNewExpressionSerializerCreator.cs

@@ -20,6 +20,7 @@
 using System.Reflection;
 using MongoDB.Bson.Serialization;
 using MongoDB.Driver.Linq.Linq3Implementation.Misc;
+using MongoDB.Driver.Linq.Linq3Implementation.Serializers;
 using MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators;
 
 namespace MongoDB.Driver.Linq.Linq3Implementation.KnownSerializerFinders;

src/MongoDB.Driver/Linq/Linq3Implementation/KnownSerializerFinders/KnownSerializerFinderVisitListInit.cs

@@ -0,0 +1,38 @@
+/* Copyright 2010-present MongoDB Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+using System.Linq.Expressions;
+
+namespace MongoDB.Driver.Linq.Linq3Implementation.KnownSerializerFinders;
+
+internal partial class KnownSerializerFinderVisitor
+{
+    protected override Expression VisitListInit(ListInitExpression node)
+    {
+        var newExpression = node.NewExpression;
+        var initializers = node.Initializers;
+
+        DeduceSerialiers();
+        base.VisitListInit(node);
+        DeduceSerialiers();
+
+        return node;
+
+        void DeduceSerialiers()
+        {
+            DeduceSerializers(node, newExpression);
+        }
+    }
+}

src/MongoDB.Driver/Linq/Linq3Implementation/KnownSerializerFinders/KnownSerializerFinderVisitMethodCall.cs

@@ -862,6 +862,10 @@ void DeduceAddMethodSerializers()
             {
                 DeduceReturnsDateTimeSerializer();
             }
+            else
+            {
+                DeduceUnknowableSerializer(node);
+            }
         }
 
         void DeduceAddDaysMethodSerializers()

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationWithLinq2Tests/Translators/AggregateProjectTranslatorTests.cs

@@ -19,6 +19,7 @@
 using System.Linq.Expressions;
 using FluentAssertions;
 using MongoDB.Bson;
+using MongoDB.Bson.Serialization;
 using MongoDB.Driver;
 using MongoDB.Driver.Core.Misc;
 using MongoDB.Driver.Core.TestHelpers.XunitExtensions;
@@ -1756,9 +1757,16 @@ private ProjectedResult<TResult> Project<TResult>(Expression<Func<Root, TResult>
             var query = __collection.AsQueryable().Select(projector);
 
             var provider = (MongoQueryProvider<Root>)query.Provider;
+            var inputSerializer = (IBsonSerializer<Root>)provider.PipelineInputSerializer;
+            var serializerRegistry = provider.Collection.Settings.SerializerRegistry;
             var translationOptions = new ExpressionTranslationOptions { EnableClientSideProjections = false };
-            var executableQuery = ExpressionToExecutableQueryTranslator.Translate<Root, TResult>(provider, query.Expression, translationOptions);
-            var projection = executableQuery.Pipeline.Ast.Stages.First().Render()["$project"].AsBsonDocument;
+            var renderedProjection = LinqProviderAdapter.TranslateExpressionToProjection(
+                projector,
+                inputSerializer,
+                serializerRegistry,
+                translationOptions);
+
+            var projection = renderedProjection.Document;
             var value = query.Take(1).FirstOrDefault();
 
             return new ProjectedResult<TResult>