From 0289f65aedfa978d68832c88a99c3cb85bd16073 Mon Sep 17 00:00:00 2001
From: Adithi Raghavan <adithi.raghavan@mongodb.com>
Date: Wed, 13 May 2026 09:46:19 -0400
Subject: [PATCH] SERVER-125724 Error when only combination.expression
 specified (#53590)

GitOrigin-RevId: 0d956accb2cb14693dd4a11a50ef68f4992021fd
---
 .../document_source_score_fusion_test.cpp     | 66 +++++++++++++++++++
 .../score_fusion_pipeline_builder.cpp         | 19 +++---
 2 files changed, 76 insertions(+), 9 deletions(-)

diff --git a/src/mongo/db/pipeline/document_source_score_fusion_test.cpp b/src/mongo/db/pipeline/document_source_score_fusion_test.cpp
index 6a230fa36ae..764993131e6 100644
--- a/src/mongo/db/pipeline/document_source_score_fusion_test.cpp
+++ b/src/mongo/db/pipeline/document_source_score_fusion_test.cpp
@@ -8902,5 +8902,71 @@ TEST_F(DocumentSourceScoreFusionTest, InternalFieldBehaviorThroughGroupAndReshap
     ASSERT_TRUE(results[0]["__hs_name1_score"_sd].missing());
     ASSERT_TRUE(results[1]["__hs_name1_score"_sd].missing());
 }
+
+TEST_F(DocumentSourceScoreFusionTest,
+       ErrorsIfNoCombinationMethodButCombinationExpressionSpecified) {
+    auto spec = fromjson(R"({
+         $scoreFusion: {
+             input: {
+                 pipelines: {
+                     name1: [
+                         {
+                             $search: {
+                                 index: "search_index",
+                                 text: {
+                                     query: "mystery",
+                                     path: "genres"
+                                 }
+                             }
+                         },
+                         { $match : { author : "dave" } }
+                     ]
+                 },
+                 normalization: "none"
+             },
+             combination: {
+                 expression: {$sum: ["$$name1", 5.0]}
+             }
+         }
+     })");
+
+    ASSERT_THROWS_CODE(DocumentSourceScoreFusion::createFromBson(spec.firstElement(), getExpCtx()),
+                       AssertionException,
+                       10017300);
+}
+
+TEST_F(DocumentSourceScoreFusionTest,
+       ErrorsIfCombinationMethodAvgButCombinationExpressionSpecified) {
+    auto spec = fromjson(R"({
+         $scoreFusion: {
+             input: {
+                 pipelines: {
+                     name1: [
+                         {
+                             $search: {
+                                 index: "search_index",
+                                 text: {
+                                     query: "mystery",
+                                     path: "genres"
+                                 }
+                             }
+                         },
+                         { $match : { author : "dave" } }
+                     ]
+                 },
+                 normalization: "none"
+             },
+             combination: {
+                 method: "avg",
+                 expression: {$sum: ["$$name1", 5.0]}
+             }
+         }
+     })");
+
+    ASSERT_THROWS_CODE(DocumentSourceScoreFusion::createFromBson(spec.firstElement(), getExpCtx()),
+                       AssertionException,
+                       10017300);
+}
+
 }  // namespace
 }  // namespace mongo
diff --git a/src/mongo/db/pipeline/score_fusion_pipeline_builder.cpp b/src/mongo/db/pipeline/score_fusion_pipeline_builder.cpp
index b138cfd913f..5a087ec9574 100644
--- a/src/mongo/db/pipeline/score_fusion_pipeline_builder.cpp
+++ b/src/mongo/db/pipeline/score_fusion_pipeline_builder.cpp
@@ -41,6 +41,8 @@
 #include "mongo/db/pipeline/expression_context.h"
 #include "mongo/db/pipeline/pipeline.h"
 
+#include <string>
+
 #include <boost/smart_ptr/intrusive_ptr.hpp>
 
 namespace mongo {
@@ -59,16 +61,15 @@ public:
         // The default combination method is avg if no combination method is specified.
         ScoreFusionCombinationMethodEnum combinationMethod = ScoreFusionCombinationMethodEnum::kAvg;
         boost::optional<IDLAnyType> combinationExpression = boost::none;
-        if (combination.has_value() && combination->getMethod().has_value()) {
-            combinationMethod = combination->getMethod().get();
-            uassert(10017300,
-                    "combination.expression should only be specified when combination.method "
-                    "has the value \"expression\"",
-                    (combinationMethod != ScoreFusionCombinationMethodEnum::kExpression &&
-                     !combination->getExpression().has_value()) ||
-                        (combinationMethod == ScoreFusionCombinationMethodEnum::kExpression &&
-                         combination->getExpression().has_value()));
+        if (combination.has_value()) {
+            combinationMethod =
+                combination->getMethod().value_or(ScoreFusionCombinationMethodEnum::kAvg);
             combinationExpression = combination->getExpression();
+            uassert(10017300,
+                    "combination.expression must be specified if and only if combination.method "
+                    "is \"expression\"",
+                    combinationExpression.has_value() ==
+                        (combinationMethod == ScoreFusionCombinationMethodEnum::kExpression));
             uassert(10017301,
                     "both combination.expression and combination.weights cannot be specified",
                     !(combination->getWeights().has_value() && combinationExpression.has_value()));