3 files changed, 29 insertions, 2 deletions
diff --git a/src/mongo/db/pipeline/document_source_merge.cpp b/src/mongo/db/pipeline/document_source_merge.cpp
index 2d446b18166..dbe77e843f7 100644
--- a/src/mongo/db/pipeline/document_source_merge.cpp
+++ b/src/mongo/db/pipeline/document_source_merge.cpp
@@ -60,10 +60,11 @@ using WhenNotMatched = MergeStrategyDescriptor::WhenNotMatched;
 using BatchTransform = std::function<void(DocumentSourceMerge::BatchedObjects&)>;
 
 constexpr auto kStageName = DocumentSourceMerge::kStageName;
-constexpr auto kDefaultWhenMatched = WhenMatched::kReplaceWithNew;
+constexpr auto kDefaultWhenMatched = WhenMatched::kMerge;
 constexpr auto kDefaultWhenNotMatched = WhenNotMatched::kInsert;
 constexpr auto kReplaceWithNewInsertMode =
     MergeMode{WhenMatched::kReplaceWithNew, WhenNotMatched::kInsert};
+constexpr auto kMergeInsertMode = MergeMode{WhenMatched::kMerge, WhenNotMatched::kInsert};
 constexpr auto kFailInsertMode = MergeMode{WhenMatched::kFail, WhenNotMatched::kInsert};
 
 /**
@@ -99,6 +100,19 @@ MergeStrategy makeInsertStrategy() {
 }
 
 /**
+ * Creates a batched objects transformation function which wraps each element of the 'batch.objects'
+ * array into the given 'updateOp' operator.
+ */
+BatchTransform makeUpdateTransform(const std::string& updateOp) {
+    return [updateOp](auto& batch) {
+        std::transform(batch.objects.begin(),
+                       batch.objects.end(),
+                       batch.objects.begin(),
+                       [updateOp](const auto& obj) { return BSON(updateOp << obj); });
+    };
+}
+
+/**
  * Returns a map that contains descriptors for all supported merge strategies for the $merge stage.
  * Each descriptor is constant and stateless and thus, can be shared by all $merge stages. A
  * descriptor is accessed using a pair of whenMatched/whenNotMatched merge modes, which defines the
@@ -118,6 +132,10 @@ const MergeStrategyDescriptorsMap& getDescriptors() {
          {kReplaceWithNewInsertMode,
           {ActionType::insert, ActionType::update},
           makeUpdateStrategy(true, {})}},
+        {kMergeInsertMode,
+         {kMergeInsertMode,
+          {ActionType::insert, ActionType::update},
+          makeUpdateStrategy(true, makeUpdateTransform("$set"))}},
         {kFailInsertMode, {kFailInsertMode, {ActionType::insert}, makeInsertStrategy()}}};
     return mergeStrategyDescriptors;
 }
diff --git a/src/mongo/db/pipeline/document_source_merge.idl b/src/mongo/db/pipeline/document_source_merge.idl
index 312d53ae56f..0ffaa2d877b 100644
--- a/src/mongo/db/pipeline/document_source_merge.idl
+++ b/src/mongo/db/pipeline/document_source_merge.idl
@@ -44,6 +44,7 @@ enums:
         type: string
         values:
             kFail: "fail"
+            kMerge: "merge"
             kReplaceWithNew: "replaceWithNew"
 
     MergeWhenNotMatchedMode:
diff --git a/src/mongo/db/pipeline/document_source_merge_test.cpp b/src/mongo/db/pipeline/document_source_merge_test.cpp
index bb819de73cb..8970cc33a5b 100644
--- a/src/mongo/db/pipeline/document_source_merge_test.cpp
+++ b/src/mongo/db/pipeline/document_source_merge_test.cpp
@@ -47,7 +47,7 @@ constexpr StringData kWhenNotMatchedModeFieldName =
 constexpr StringData kIntoFieldName = DocumentSourceMergeSpec::kTargetNssFieldName;
 constexpr StringData kOnFieldName = DocumentSourceMergeSpec::kOnFieldName;
 const StringData kDefaultWhenMatchedMode =
-    MergeWhenMatchedMode_serializer(MergeWhenMatchedModeEnum::kReplaceWithNew);
+    MergeWhenMatchedMode_serializer(MergeWhenMatchedModeEnum::kMerge);
 const StringData kDefaultWhenNotMatchedMode =
     MergeWhenNotMatchedMode_serializer(MergeWhenNotMatchedModeEnum::kInsert);
 
@@ -602,6 +602,14 @@ TEST_F(DocumentSourceMergeTest, CorrectlyHandlesWhenMatchedAndWhenNotMatchedMode
                                  << "merge"
                                  << "whenNotMatched"
                                  << "insert"));
+    ASSERT(createMergeStage(spec));
+
+    spec = BSON("$merge" << BSON("into"
+                                 << "target_collection"
+                                 << "whenMatched"
+                                 << "keepExisting"
+                                 << "whenNotMatched"
+                                 << "insert"));
     ASSERT_THROWS_CODE(createMergeStage(spec), DBException, ErrorCodes::BadValue);
 
     spec = BSON("$merge" << BSON("into"