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CSHARP-5785: Optimize LINQ translation for First() and FirstOrDefault() methods with predicates #1817

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Nov 21, 2025
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CSHARP-5785: Optimize LINQ translation for First() and FirstOrDefault() methods with predicates #1817

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CSHARP-5785: Optimize LINQ translation for First() and FirstOrDefault…
adelinowona Nov 18, 2025
d5f7846
fix some more tests
adelinowona Nov 18, 2025
e13fd75
gate $filter limit optimization based on server version
adelinowona Nov 19, 2025
8cc0d37
Restore UTF-8 BOM to edited files
adelinowona Nov 19, 2025
fed12d1
adress review comments
adelinowona Nov 20, 2025
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12 changes: 8 additions & 4 deletions ...essionTranslators/MethodTranslators/FirstOrLastMethodToAggregationExpressionTranslator.cs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
/* Copyright 2010-present MongoDB Inc.
/* 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.
Expand Down Expand Up @@ -95,16 +95,20 @@ public static TranslatedExpression Translate(TranslationContext context, MethodC
var sourceAst = sourceTranslation.Ast;
var itemSerializer = ArraySerializerHelper.GetItemSerializer(sourceTranslation.Serializer);

var isFirstMethod = method.Name.StartsWith("First");
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@rstam rstam Nov 18, 2025

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In general we prefer to test which method it is like this:

var isFirstMethod = method.IsOneOf(__firstMethods);

It is less likely to have false matches than string matching (partial or otherwise).

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@adelinowona adelinowona Nov 19, 2025

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I did it this way since I reasoned that if we passed the initial if statement:
if (method.IsOneOf(__firstOrLastMethods)) then we already know that the method has matched one of the first methods so no need to test the methods again.

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@rstam rstam Nov 19, 2025

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The statement you make is true, but even so we prefer to stick to method.IsOneOf instead of string matching.

The only time we don't use method.IsOneOf is when we are translating a method that might be implemented by classes we don't even know about (e.g. CompareTo).


if (method.IsOneOf(__withPredicateMethods))
{
var predicateLambda = ExpressionHelper.UnquoteLambdaIfQueryableMethod(method, arguments[1]);
var parameterExpression = predicateLambda.Parameters.Single();
var parameterSymbol = context.CreateSymbol(parameterExpression, itemSerializer, isCurrent: false);
var predicateTranslation = ExpressionToAggregationExpressionTranslator.TranslateLambdaBody(context, predicateLambda, parameterSymbol);
var limit = isFirstMethod ? AstExpression.Constant(1) : null;
sourceAst = AstExpression.Filter(
input: sourceAst,
cond: predicateTranslation.Ast,
@as: parameterSymbol.Var.Name);
@as: parameterSymbol.Var.Name,
limit: limit);
}

AstExpression ast;
Expand All @@ -119,11 +123,11 @@ public static TranslatedExpression Translate(TranslationContext context, MethodC
@in: AstExpression.Cond(
@if: AstExpression.Eq(AstExpression.Size(valuesAst), 0),
then: serializedDefaultValue,
@else: method.IsOneOf(__firstMethods) ? AstExpression.First(valuesAst) : AstExpression.Last(valuesAst)));
@else: isFirstMethod ? AstExpression.First(valuesAst) : AstExpression.Last(valuesAst)));
}
else
{
ast = method.Name == "First" ? AstExpression.First(sourceAst) : AstExpression.Last(sourceAst);
ast = isFirstMethod ? AstExpression.First(sourceAst) : AstExpression.Last(sourceAst);
}

return new TranslatedExpression(expression, ast, itemSerializer);
Expand Down
2 changes: 1 addition & 1 deletion tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp3524Tests.cs
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Expand Up @@ -60,7 +60,7 @@ public void SelectMany_should_translate_correctly()
Meta : '$$meta',
Property : {
$arrayElemAt : [
{ $filter : { input : '$Properties', as : 'p', cond : { $eq : ['$$p._id', '$$meta.PropertyId'] } } },
{ $filter : { input : '$Properties', as : 'p', cond : { $eq : ['$$p._id', '$$meta.PropertyId'] }, limit : 1 } },
0
]
}
Expand Down
10 changes: 5 additions & 5 deletions tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp4048Tests.cs
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Expand Up @@ -118,7 +118,7 @@ public void List_get_Item_of_scalar_should_work()
var expectedStages = new[]
{
"{ $group : { _id : '$_id', _elements : { $push: '$X' } } }",
"{ $project : { _id : '$_id' Result : { $arrayElemAt : ['$_elements', 0] } } }",
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@rstam rstam Nov 18, 2025

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How was this test not failing?

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@adelinowona adelinowona Nov 19, 2025

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I have no idea. I just happened to catch it.

"{ $project : { _id : '$_id', Result : { $arrayElemAt : ['$_elements', 0] } } }",
"{ $sort : { _id : 1 } }"
};
AssertStages(stages, expectedStages);
Expand Down Expand Up @@ -1043,7 +1043,7 @@ public void IGrouping_First_with_predicate_of_root_should_work()
var expectedStages = new[]
{
"{ $group : { _id : '$_id', _elements : { $push : '$$ROOT' } } }",
"{ $project : { _id : '$_id', Result : { $arrayElemAt : [{ $filter : { input : '$_elements', as : 'e', cond : { $ne : ['$$e.X', 1] } } }, 0] } } }",
"{ $project : { _id : '$_id', Result : { $arrayElemAt : [{ $filter : { input : '$_elements', as : 'e', cond : { $ne : ['$$e.X', 1] }, limit : 1 } }, 0] } } }",
"{ $sort : { _id : 1 } }"
};
AssertStages(stages, expectedStages);
Expand All @@ -1068,7 +1068,7 @@ public void IGrouping_First_with_predicate_of_scalar_should_work()
var expectedStages = new[]
{
"{ $group : { _id : '$_id', _elements : { $push : '$X' } } }",
"{ $project : { _id : '$_id', Result : { $arrayElemAt : [{ $filter : { input : '$_elements', as : 'e', cond : { $ne : ['$$e', 1] } } }, 0] } } }",
"{ $project : { _id : '$_id', Result : { $arrayElemAt : [{ $filter : { input : '$_elements', as : 'e', cond : { $ne : ['$$e', 1] }, limit : 1 } }, 0] } } }",
"{ $sort : { _id : 1 } }"
};
AssertStages(stages, expectedStages);
Expand Down Expand Up @@ -1143,7 +1143,7 @@ public void IGrouping_FirstOrDefault_with_predicate_of_root_should_work()
var expectedStages = new[]
{
"{ $group : { _id : '$_id', _elements : { $push : '$$ROOT' } } }",
"{ $project : { _id : '$_id', Result : { $let : { vars : { values : { $filter : { input : '$_elements', as : 'e', cond : { $ne : ['$$e.X', 1] } } } }, in : { $cond : { if : { $eq : [{ $size : '$$values' }, 0] }, then : null, else : { $arrayElemAt : ['$$values', 0] } } } } } } }",
"{ $project : { _id : '$_id', Result : { $let : { vars : { values : { $filter : { input : '$_elements', as : 'e', cond : { $ne : ['$$e.X', 1] }, limit : 1 } } }, in : { $cond : { if : { $eq : [{ $size : '$$values' }, 0] }, then : null, else : { $arrayElemAt : ['$$values', 0] } } } } } } }",
"{ $sort : { _id : 1 } }"
};
AssertStages(stages, expectedStages);
Expand All @@ -1169,7 +1169,7 @@ public void IGrouping_FirstOrDefault_with_predicate_of_scalar_should_work()
var expectedStages = new[]
{
"{ $group : { _id : '$_id', _elements : { $push : '$X' } } }",
"{ $project : { _id : '$_id', Result : { $let : { vars : { values : { $filter : { input : '$_elements', as : 'e', cond : { $ne : ['$$e', 1] } } } }, in : { $cond : { if : { $eq : [{ $size : '$$values' }, 0] }, then : 0, else : { $arrayElemAt : ['$$values', 0] } } } } } } }",
"{ $project : { _id : '$_id', Result : { $let : { vars : { values : { $filter : { input : '$_elements', as : 'e', cond : { $ne : ['$$e', 1] }, limit : 1 } } }, in : { $cond : { if : { $eq : [{ $size : '$$values' }, 0] }, then : 0, else : { $arrayElemAt : ['$$values', 0] } } } } } } }",
"{ $sort : { _id : 1 } }"
};
AssertStages(stages, expectedStages);
Expand Down
2 changes: 1 addition & 1 deletion tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp5258Tests.cs
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Expand Up @@ -37,7 +37,7 @@ public void Select_First_with_predicate_should_work()
.Select(_x => _x.List.First(_y => _y > 2));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : '$List', as : 'v__0', cond : { $gt : ['$$v__0', 2] } } }, 0] }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : '$List', as : 'v__0', cond : { $gt : ['$$v__0', 2] }, limit : 1 } }, 0] }, _id : 0 } }");

var results = queryable.ToList();
results.Should().Equal(3, 4);
Expand Down
30 changes: 15 additions & 15 deletions tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp5779Tests.cs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
/* Copyright 2010-present MongoDB Inc.
/* 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.
Expand Down Expand Up @@ -73,7 +73,7 @@ public void DictionaryAsArrayOfArrays_Keys_First_with_predicate_should_work()
.Select(x => x.DictionaryAsArrayOfArrays.Keys.First(k => k == "b"));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : { $map : { input : '$DictionaryAsArrayOfArrays', as : 'kvp', in : { $arrayElemAt : ['$$kvp', 0] } } }, as : 'k', cond : { $eq : ['$$k', 'b'] } } }, 0] }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : { $map : { input : '$DictionaryAsArrayOfArrays', as : 'kvp', in : { $arrayElemAt : ['$$kvp', 0] } } }, as : 'k', cond : { $eq : ['$$k', 'b'] }, limit : 1 } }, 0] }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand All @@ -92,7 +92,7 @@ public void DictionaryAsArrayOfArrays_Values_should_work()
.Select(x => x.DictionaryAsArrayOfArrays.Values);

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $map : { input : '$DictionaryAsArrayOfArrays', as : 'kvp' in : { k : { $arrayElemAt : ['$$kvp', 0] }, v : { $arrayElemAt : ['$$kvp', 1] } } } } , _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $map : { input : '$DictionaryAsArrayOfArrays', as : 'kvp', in : { k : { $arrayElemAt : ['$$kvp', 0] }, v : { $arrayElemAt : ['$$kvp', 1] } } } }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand All @@ -111,7 +111,7 @@ public void DictionaryAsArrayOfArrays_Values_First_with_predicate_should_work()
.Select(x => x.DictionaryAsArrayOfArrays.Values.First(v => v == 2));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $let : { vars : { this : { $arrayElemAt : [{ $filter : { input : { $map : { input : '$DictionaryAsArrayOfArrays', as : 'kvp', in : { k : { $arrayElemAt : ['$$kvp', 0] }, v : { $arrayElemAt : ['$$kvp', 1] } } } }, as : 'v', cond : { $eq : ['$$v.v', 2] } } }, 0] } }, in : '$$this.v' } }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $let : { vars : { this : { $arrayElemAt : [{ $filter : { input : { $map : { input : '$DictionaryAsArrayOfArrays', as : 'kvp', in : { k : { $arrayElemAt : ['$$kvp', 0] }, v : { $arrayElemAt : ['$$kvp', 1] } } } }, as : 'v', cond : { $eq : ['$$v.v', 2] }, limit : 1 } }, 0] } }, in : '$$this.v' } }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand Down Expand Up @@ -149,7 +149,7 @@ public void DictionaryAsArrayOfDocuments_Keys_First_with_predicate_should_work()
.Select(x => x.DictionaryAsArrayOfDocuments.Keys.First(k => k == "b"));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : '$DictionaryAsArrayOfDocuments.k', as : 'k', cond : { $eq : ['$$k', 'b'] } } }, 0] }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : '$DictionaryAsArrayOfDocuments.k', as : 'k', cond : { $eq : ['$$k', 'b'] }, limit : 1 } }, 0] }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand Down Expand Up @@ -187,7 +187,7 @@ public void DictionaryAsArrayOfDocuments_Values_First_with_predicate_should_work
.Select(x => x.DictionaryAsArrayOfDocuments.Values.First(v => v == 2));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $let : { vars : { this : { $arrayElemAt : [{ $filter : { input : '$DictionaryAsArrayOfDocuments', as : 'v', cond : { $eq : ['$$v.v', 2] } } }, 0] } }, in : '$$this.v' } }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $let : { vars : { this : { $arrayElemAt : [{ $filter : { input : '$DictionaryAsArrayOfDocuments', as : 'v', cond : { $eq : ['$$v.v', 2] }, limit : 1 } }, 0] } }, in : '$$this.v' } }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand Down Expand Up @@ -225,7 +225,7 @@ public void DictionaryAsDocument_Keys_First_with_predicate_should_work()
.Select(x => x.DictionaryAsDocument.Keys.First(k => k == "b"));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : { $map : { input : { $objectToArray : '$DictionaryAsDocument' }, as : 'kvp', in : '$$kvp.k' } }, as : 'k', cond : { $eq : ['$$k', 'b'] } } }, 0] }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : { $map : { input : { $objectToArray : '$DictionaryAsDocument' }, as : 'kvp', in : '$$kvp.k' } }, as : 'k', cond : { $eq : ['$$k', 'b'] }, limit : 1 } }, 0] }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand Down Expand Up @@ -263,7 +263,7 @@ public void DictionaryAsDocument_Values_First_with_predicate_should_work()
.Select(x => x.DictionaryAsDocument.Values.First(v => v == 2));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $let : { vars : { this : { $arrayElemAt : [{ $filter : { input : { $objectToArray : '$DictionaryAsDocument' }, as : 'v', cond : { $eq : ['$$v.v', 2] } } }, 0] } }, in : '$$this.v' } }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $let : { vars : { this : { $arrayElemAt : [{ $filter : { input : { $objectToArray : '$DictionaryAsDocument' }, as : 'v', cond : { $eq : ['$$v.v', 2] }, limit : 1 } }, 0] } }, in : '$$this.v' } }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand Down Expand Up @@ -301,7 +301,7 @@ public void IDictionaryAsArrayOfArrays_Keys_First_with_predicate_should_work()
.Select(x => x.IDictionaryAsArrayOfArrays.Keys.First(k => k == "b"));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : { $map : { input : '$IDictionaryAsArrayOfArrays', as : 'kvp', in : { $arrayElemAt : ['$$kvp', 0] } } }, as : 'k', cond : { $eq : ['$$k', 'b'] } } }, 0] }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : { $map : { input : '$IDictionaryAsArrayOfArrays', as : 'kvp', in : { $arrayElemAt : ['$$kvp', 0] } } }, as : 'k', cond : { $eq : ['$$k', 'b'] }, limit : 1 } }, 0] }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand Down Expand Up @@ -339,7 +339,7 @@ public void IDictionaryAsArrayOfArrays_Values_First_with_predicate_should_work()
.Select(x => x.IDictionaryAsArrayOfArrays.Values.First(v => v == 2));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : { $map : { input : '$IDictionaryAsArrayOfArrays', as : 'kvp', in : { $arrayElemAt : ['$$kvp', 1] } } }, as : 'v', cond : { $eq : ['$$v', 2] } } }, 0] }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : { $map : { input : '$IDictionaryAsArrayOfArrays', as : 'kvp', in : { $arrayElemAt : ['$$kvp', 1] } } }, as : 'v', cond : { $eq : ['$$v', 2] }, limit : 1 } }, 0] }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand Down Expand Up @@ -377,7 +377,7 @@ public void IDictionaryAsArrayOfDocuments_Keys_First_with_predicate_should_work(
.Select(x => x.IDictionaryAsArrayOfDocuments.Keys.First(k => k == "b"));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : '$IDictionaryAsArrayOfDocuments.k', as : 'k', cond : { $eq : ['$$k', 'b'] } } }, 0] }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : '$IDictionaryAsArrayOfDocuments.k', as : 'k', cond : { $eq : ['$$k', 'b'] }, limit : 1 } }, 0] }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand Down Expand Up @@ -415,7 +415,7 @@ public void IDictionaryAsArrayOfDocuments_Values_First_with_predicate_should_wor
.Select(x => x.IDictionaryAsArrayOfDocuments.Values.First(v => v == 2));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : '$IDictionaryAsArrayOfDocuments.v', as : 'v', cond : { $eq : ['$$v', 2] } } }, 0] }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : '$IDictionaryAsArrayOfDocuments.v', as : 'v', cond : { $eq : ['$$v', 2] }, limit : 1 } }, 0] }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand Down Expand Up @@ -453,7 +453,7 @@ public void IDictionaryAsDocument_Keys_First_with_predicate_should_work()
.Select(x => x.IDictionaryAsDocument.Keys.First(k => k == "b"));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : { $map : { input : { $objectToArray : '$IDictionaryAsDocument' }, as : 'kvp', in : '$$kvp.k' } }, as : 'k', cond : { $eq : ['$$k', 'b'] } } }, 0] }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : { $map : { input : { $objectToArray : '$IDictionaryAsDocument' }, as : 'kvp', in : '$$kvp.k' } }, as : 'k', cond : { $eq : ['$$k', 'b'] }, limit : 1 } }, 0] }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand Down Expand Up @@ -491,7 +491,7 @@ public void IDictionaryAsDocument_Values_First_with_predicate_should_work()
.Select(x => x.IDictionaryAsDocument.Values.First(v => v == 2));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : { $map : { input : { $objectToArray : '$IDictionaryAsDocument' }, as : 'kvp', in : '$$kvp.v' } }, as : 'v', cond : { $eq : ['$$v', 2] } } }, 0] }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : { $map : { input : { $objectToArray : '$IDictionaryAsDocument' }, as : 'kvp', in : '$$kvp.v' } }, as : 'v', cond : { $eq : ['$$v', 2] }, limit : 1 } }, 0] }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand All @@ -510,7 +510,7 @@ public void DictionaryAsDocumentOfNestedDictionaryAsArrayOfArrays_Values_SelectM
.Select(x => x.DictionaryAsDocumentOfNestedDictionaryAsArrayOfArrays.Values.SelectMany(n => n.Keys));

var stages = Translate(collection, queryable);
AssertStages(stages, "{ $project : { _v : { $reduce : { input : { $map : { input : { $objectToArray : '$DictionaryAsDocumentOfNestedDictionaryAsArrayOfArrays' }, as : 'n', in : { $map : { input : '$$n.v', as : 'kvp', in : { $arrayElemAt : ['$$kvp', 0] } } } } }, initialValue : [], in : { $concatArrays : ['$$value', '$$this'] } } }, _id : 0 } }");
AssertStages(stages, "{ $project : { _v : { $reduce : { input : { $map : { input : { $objectToArray : '$DictionaryAsDocumentOfNestedDictionaryAsArrayOfArrays' }, as : 'n', in : { $map : { input : '$$n.v', as : 'kvp', in : { $arrayElemAt : ['$$kvp', 0] } } } } }, initialValue : [], in : { $concatArrays : ['$$value', '$$this'] } } }, _id : 0 } }");

var results = queryable.ToList();
results.Count.Should().Be(4);
Expand Down
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