PR

Author: Oleksandr Poliakov

src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Optimizers/AstSimplifier.cs

@@ -609,7 +609,29 @@ arg is AstBinaryExpression argBinaryExpression &&
                 return AstExpression.Binary(oppositeComparisonOperator, argBinaryExpression.Arg1, argBinaryExpression.Arg2);
             }
 
+            // { $arrayToObject : [[{ k : 'A', v : '$A' }, { k : 'B', v : '$B' }]] } => { A : '$A', B : '$B' }
+            if (node.Operator is AstUnaryOperator.ArrayToObject &&
+                arg is AstComputedArrayExpression computedArrayExpression &&
+                computedArrayExpression.Items.All(
+                    i => i is AstComputedDocumentExpression computedDocumentExpression &&
+                    computedDocumentExpression.Fields.FirstOrDefault(f => f.Path == "k")?.Value is AstConstantExpression &&
+                    computedDocumentExpression.Fields.Any(f => f.Path == "v"))
+               )
+            {
+                var fields = computedArrayExpression.Items.Select(KeyValuePairDocumentToComputedField);
+                return AstExpression.ComputedDocument(fields);
+            }
+
             return node.Update(arg);
+
+            static AstComputedField KeyValuePairDocumentToComputedField(AstExpression expression)
+            {
+                var documentExpression = (AstComputedDocumentExpression)expression;
+                var keyExpression = documentExpression.Fields.First(f => f.Path == "k").Value;
+                var valueExpression = documentExpression.Fields.First(f => f.Path == "v").Value;
+
+                return AstExpression.ComputedField(((AstConstantExpression)keyExpression).Value.AsString, valueExpression);
+            }
         }
     }
 }

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

@@ -45,7 +45,7 @@ public void NewDictionary_with_KeyValuePairs_should_translate()
 
             var stages = Translate(collection, queryable);
 
-            AssertStages(stages, "{ $project : { _v : { $arrayToObject : [[{ k : 'A', v : '$A' }, { k : 'B', v : '$B' }]] }, _id : 0 } }");
+            AssertStages(stages, "{ $project : { _v : { A : '$A', B: '$B' }, _id : 0 } }");
 
             var result = queryable.Single();
             result.Should().Equal(new Dictionary<string, string>{ ["A"] = "a", ["B"] = "b" });