SERVER-72463 Use MinKey in multikey path index keys generated for compound wildcard indexes

2 files changed, 21 insertions, 11 deletions

diff --git a/src/mongo/db/index/wildcard_key_generator.cpp b/src/mongo/db/index/wildcard_key_generator.cpp
index 3d963c2946e..4958ed0d5f7 100644
--- a/src/mongo/db/index/wildcard_key_generator.cpp
+++ b/src/mongo/db/index/wildcard_key_generator.cpp
@@ -79,6 +79,14 @@ void appendToKeyString(const std::vector<BSONElement>& elems,
     }
 }
 
+// Append 'MinKey' to 'keyString'. Multikey path keys use 'MinKey' for non-wildcard fields.
+void appendToMultiKeyString(const std::vector<BSONElement>& elems,
+                            KeyString::PooledBuilder* keyString) {
+    for (size_t i = 0; i < elems.size(); i++) {
+        keyString->appendBSONElement(kMinBSONKey.firstElement());
+    }
+}
+
 // We should make a new Ordering for wildcard key generator because the index keys generated for
 // wildcard indexes include a "$_path" field prior to the wildcard field and the Ordering passed in
 // does not account for the "$_path" field.
@@ -226,13 +234,13 @@ void SingleDocumentKeyEncoder::_addMultiKey(const FieldRef& fullPath) {
         KeyString::PooledBuilder keyString(_pooledBufferBuilder, _keyStringVersion, _ordering);
 
         if (!_preElems.empty()) {
-            appendToKeyString(_preElems, _collator, &keyString);
+            appendToMultiKeyString(_preElems, &keyString);
         }
         for (auto elem : BSON("" << 1 << "" << fullPath.dottedField())) {
             keyString.appendBSONElement(elem);
         }
         if (!_postElems.empty()) {
-            appendToKeyString(_postElems, _collator, &keyString);
+            appendToMultiKeyString(_postElems, &keyString);
         }
 
         keyString.appendRecordId(record_id_helpers::reservedIdFor(
diff --git a/src/mongo/db/index/wildcard_key_generator_test.cpp b/src/mongo/db/index/wildcard_key_generator_test.cpp
index 107a6e62525..ed6407eabe2 100644
--- a/src/mongo/db/index/wildcard_key_generator_test.cpp
+++ b/src/mongo/db/index/wildcard_key_generator_test.cpp
@@ -1387,21 +1387,23 @@ TEST_F(WildcardKeyGeneratorCompoundTest, CompoundWildcardIndexShouldBeSparse) {
 }
 
 TEST_F(WildcardKeyGeneratorCompoundTest, CanGenerateKeysForMultikeyFieldCompound) {
-    WildcardKeyGenerator keyGen{fromjson("{'$**': 1, c: 1}"),
-                                fromjson("{c: 0}"),
+    WildcardKeyGenerator keyGen{fromjson("{a: 1, '$**': 1, c: 1}"),
+                                fromjson("{a: 0, c: 0}"),
                                 nullptr,
                                 KeyString::Version::kLatestVersion,
                                 Ordering::make(BSONObj()),
                                 rsKeyFormat};
-    auto inputDoc = fromjson("{a: [], b: [1, {c: [3]}]}");
+    auto inputDoc = fromjson("{a: 1, b: [1, {c: [3]}]}");
 
-    auto expectedKeys = makeKeySet({fromjson("{'': 'a', '': undefined, '': null}"),
-                                    fromjson("{'': 'b', '': 1, '': null}"),
-                                    fromjson("{'': 'b.c', '': 3, '': null}")});
+    auto expectedKeys = makeKeySet({fromjson("{'': 1, '': 'b', '': 1, '': null}"),
+                                    fromjson("{'': 1, '': 'b.c', '': 3, '': null}")});
     auto expectedMultikeyPaths =
-        makeKeySet({fromjson("{'': 1, '': 'a', '': null}"),
-                    fromjson("{'': 1, '': 'b', '': null}"),
-                    fromjson("{'': 1, '': 'b.c', '': null}")},
+        makeKeySet({BSON("" << MINKEY << "" << 1 << ""
+                            << "b"
+                            << "" << MINKEY),
+                    BSON("" << MINKEY << "" << 1 << ""
+                            << "b.c"
+                            << "" << MINKEY)},
                    record_id_helpers::reservedIdFor(
                        record_id_helpers::ReservationId::kWildcardMultikeyMetadataId, rsKeyFormat));