CSHARP-5847: Support Select/SelectMany/Where index overloads in LINQ provider by adelinowona · Pull Request #1949 · mongodb/mongo-csharp-driver

adelinowona · 2026-04-14T15:56:58Z

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…provider

Copilot

Pull request overview

Adds LINQ3 translation support for the index-taking overloads of Select, SelectMany, and Where by emitting aggregation expressions that use the server-side arrayIndexAs capability, along with integration tests validating the generated stages and results.

Changes:

Translate Select((item, index) => ...) to $map with arrayIndexAs.
Translate Where((item, index) => ...) to $filter with arrayIndexAs.
Translate SelectMany((item, index) => ...) via $map (with arrayIndexAs) + $reduce/$concatArrays, and add coverage/tests plus a new Feature.ArrayIndexAs.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
`src/MongoDB.Driver/Linq/Linq3Implementation/Translators/.../WhereMethodToAggregationExpressionTranslator.cs`	Adds translation for index-based `Where` using `$filter.arrayIndexAs`.
`src/MongoDB.Driver/Linq/Linq3Implementation/Translators/.../SelectMethodToAggregationExpressionTranslator.cs`	Adds translation for index-based `Select` using `$map.arrayIndexAs`.
`src/MongoDB.Driver/Linq/Linq3Implementation/Translators/.../SelectManyMethodToAggregationExpressionTranslator.cs`	Adds translation for index-based `SelectMany` using `$map.arrayIndexAs` and flattens with `$reduce`.
`src/MongoDB.Driver/Linq/Linq3Implementation/SerializerFinders/SerializerFinderVisitMethodCall.cs`	Deduces serializers for the new index-based method overloads (including `int` index).
`src/MongoDB.Driver/Linq/Linq3Implementation/Reflection/EnumerableOrQueryableMethod.cs`	Registers the new method sets for the index-taking overloads.
`src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Visitors/AstNodeVisitor.cs`	Updates AST visiting for `$map` to include `ArrayIndexAs`.
`src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Expressions/AstMapExpression.cs`	Extends `$map` AST node to render `arrayIndexAs`.
`src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Expressions/AstFilterExpression.cs`	Extends `$filter` AST node to render `arrayIndexAs`.
`src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Expressions/AstExpression.cs`	Extends factory methods for `$map`/`$filter` to accept `arrayIndexAs`.
`src/MongoDB.Driver/Core/Misc/Feature.cs`	Introduces `Feature.ArrayIndexAs`.
`tests/.../WhereMethodToAggregationExpressionTranslatorTests.cs`	Adds integration tests for index-based `Where` translation.
`tests/.../SelectMethodToAggregationExpressionTranslatorTests.cs`	Adds integration tests for index-based `Select` translation.
`tests/.../SelectManyMethodToAggregationExpressionTranslatorTests.cs`	New integration tests for index-based `SelectMany` translation.

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Copilot · 2026-04-14T16:02:36Z

+                var ast = AstExpression.Filter(
+                    sourceAst,
+                    predicateTranslation.Ast,
+                    @as: itemSymbol.Var.Name,
+                    arrayIndexAs: indexSymbol.Var.Name);
+
+                var resultSerializer = context.GetSerializer(expression);
+                return new TranslatedExpression(expression, ast, resultSerializer);


The index-based Where translation uses $filter with arrayIndexAs but does not validate Feature.ArrayIndexAs support for the current compatibility level. Please add an explicit feature gate (similar to Feature.FilterLimit usage elsewhere) so that on unsupported servers/compatibility settings the driver fails early with a clear exception instead of sending an invalid aggregation pipeline.

This is intentional. The driver doesn't generally gate LINQ translations on server version — we translate the query and let the server reject it if the feature isn't supported. The tests are gated with RequireServer.Check().Supports(Feature.ArrayIndexAs) to avoid running against incompatible servers.

Copilot · 2026-04-14T16:02:37Z

+                var selectorLambda = ExpressionHelper.UnquoteLambdaIfQueryableMethod(method, arguments[1]);
+                var itemParameter = selectorLambda.Parameters[0];
+                var indexParameter = selectorLambda.Parameters[1];
+                var itemSymbol = context.CreateSymbol(itemParameter, context.GetSerializer(itemParameter));
+                var indexSymbol = context.CreateSymbol(indexParameter, context.GetSerializer(indexParameter));
+                var selectorContext = context.WithSymbols(itemSymbol, indexSymbol);
+                var selectorTranslation = ExpressionToAggregationExpressionTranslator.Translate(selectorContext, selectorLambda.Body);
+                var resultItemSerializer = ArraySerializerHelper.GetItemSerializer(selectorTranslation.Serializer);


In the index overload branch, itemSymbol is created via context.GetSerializer(itemParameter) instead of deriving the selector parameter serializer from sourceTranslation.Serializer (as is done in the non-index SelectMany branch). For consistency and correctness (especially with wrapped/nested sources), consider unwrapping IWrappedValueSerializer if needed and using ArraySerializerHelper.GetItemSerializer(sourceTranslation.Serializer) for the item parameter, and Int32Serializer.Instance for the index parameter.

This is intentional. The serializer finder (SerializerFinderVisitMethodCall) pre-deduces the correct serializer for the item and index parameters before the translator runs.

context.GetSerializer(itemParameter) retrieves that pre-deduced serializer, so the result is the same as manually deriving it from the source serializer. This approach is consistent with how the serializer finder is designed to work.

Copilot · 2026-04-14T16:02:38Z

        public static Feature AggregateUnionWith => __aggregateUnionWith;

+        /// <summary>
+        /// Gets the arrayIndexAs feature for $map, $filter and $reduce.


The XML doc comment for Feature.ArrayIndexAs says it's for "$map, $filter and $reduce", but arrayIndexAs is used by $map and $filter (and is not a parameter of $reduce). Please adjust the comment to avoid misleading readers about server feature coverage.

Suggested change

/// Gets the arrayIndexAs feature for $map, $filter and $reduce.

/// Gets the arrayIndexAs feature for $map and $filter.

The XML comment is correct as written — arrayIndexAs is also available on $reduce. Support for it is tracked separately in another ticket - CSHARP-5973

adelinowona · 2026-04-14T18:47:10Z


-                var ienumerableSerializer = IEnumerableSerializer.Create(itemSerializer);
-                return new TranslatedExpression(expression, ast, ienumerableSerializer);
+                var serializer = NestedAsQueryableSerializer.CreateIEnumerableOrNestedAsQueryableSerializer(expression.Type, itemSerializer);


Changed from IEnumerableSerializer.Create(itemSerializer) to NestedAsQueryableSerializer.CreateIEnumerableOrNestedAsQueryableSerializer(expression.Type, itemSerializer) to correctly handle both IEnumerable and IQueryable source types. This aligns with how Select and the new index overload already create their result serializers.

Shouldn't we use the serializer resolved by SerializerFinder?

Found the answer in tests. Cool!

adelinowona · 2026-04-14T18:52:20Z

 {
    internal sealed class AstFilterExpression : AstExpression
    {
+        private readonly string _arrayIndexAs;


arrayIndexAs is typed as string here (not AstVarExpression) to match the existing convention for AstFilterExpression.As, which is also a string. AstMapExpression uses AstVarExpression for both its As and ArrayIndexAs fields — each class follows its own established pattern. Not sure if I should change it to use AstVarExpression thoughts?

I'm OK with string name.

adelinowona · 2026-04-14T19:28:57Z

        [TestHelpers.MakeLambda((MyModel model) => model.FloatItems.Percentile(new double[] { 0.5 })), typeof(ArraySerializer<float>)],
        [TestHelpers.MakeLambda((MyModel model) => model.FloatItems.Percentile(x => x * 2, new double[] { 0.5 })), typeof(ArraySerializer<float>)],

+        [TestHelpers.MakeLambda((MyModel model) => model.Items.Select(x => x + 1)), typeof(IEnumerableSerializer<int>)],


IQueryable variants for Select/SelectMany/Where (with and without index) are intentionally omitted. The serializer finder currently produces IQueryableSerializer for nested AsQueryable() expressions, but the translators need NestedAsQueryableSerializer. Adding IQueryable test cases here would assert on the wrong serializer type. Tracked in CSHARP-5976.

BorisDog

Looks good. But we need a sign off from someone with a deeper LINQ expertise.

BorisDog · 2026-04-15T22:35:13Z

            AstExpression limit)
        {
-            if (input == _input && cond == _cond)
+            if (input == _input && cond == _cond && limit == _limit)


Good catch!

sanych-sun

LGTM

sanych-sun · 2026-04-16T00:55:01Z

 {
    internal sealed class AstFilterExpression : AstExpression
    {
+        private readonly string _arrayIndexAs;


I'm OK with string name.

adelinowona added 4 commits April 14, 2026 10:53

CSHARP-5847: Support Select/SelectMany/Where index overloads in LINQ …

b681339

…provider

re-order test methods

b34bc17

get serializer info from context serializer map

80d3fd4

feature gate the index overloads tests

2ead7bc

adelinowona requested a review from sanych-sun April 14, 2026 15:56

adelinowona requested a review from a team as a code owner April 14, 2026 15:56

adelinowona added the feature Adds new user-facing functionality. label Apr 14, 2026

Copilot AI review requested due to automatic review settings April 14, 2026 15:56

Copilot started reviewing on behalf of adelinowona April 14, 2026 15:57 View session

Copilot AI reviewed Apr 14, 2026

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adelinowona added 2 commits April 14, 2026 12:46

add check on limit

6ed5238

add comment for unused selectmany methodinfo

aac6a3c

adelinowona commented Apr 14, 2026

View reviewed changes

add serializerFinder unit tests

eb52b60

adelinowona commented Apr 14, 2026

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BorisDog approved these changes Apr 15, 2026

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sanych-sun approved these changes Apr 16, 2026

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adelinowona merged commit c24ee00 into mongodb:main Apr 16, 2026
33 checks passed

	/// Gets the arrayIndexAs feature for $map, $filter and $reduce.
	/// Gets the arrayIndexAs feature for $map and $filter.

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