SERVER-43669 Serialize "sortKey" as BSONArray

1 files changed, 8 insertions, 4 deletions

diff --git a/src/mongo/s/query/async_results_merger.cpp b/src/mongo/s/query/async_results_merger.cpp
index c94e309891c..bff8b0902aa 100644
--- a/src/mongo/s/query/async_results_merger.cpp
+++ b/src/mongo/s/query/async_results_merger.cpp
@@ -64,7 +64,11 @@ BSONObj extractSortKey(BSONObj obj, bool compareWholeSortKey) {
     if (compareWholeSortKey) {
         return key.wrap();
     }
-    invariant(key.type() == BSONType::Object);
+    // TODO (SERVER-43361): We expect the sort key to be an array, but if the sort key originated
+    // from a 4.2 mongod, it will be a document, instead. Either way, 'isABSONObj()' will return
+    // true, and 'compareSortKeys()' will behave the same way. After branching for 4.5, we can
+    // tighten this invariant to specifically require a 'BSONArray' type.
+    invariant(key.isABSONObj());
     return key.Obj();
 }
 
@@ -75,8 +79,8 @@ BSONObj extractSortKey(BSONObj obj, bool compareWholeSortKey) {
 int compareSortKeys(BSONObj leftSortKey, BSONObj rightSortKey, BSONObj sortKeyPattern) {
     // This does not need to sort with a collator, since mongod has already mapped strings to their
     // ICU comparison keys as part of the $sortKey meta projection.
-    const bool considerFieldName = false;
-    return leftSortKey.woCompare(rightSortKey, sortKeyPattern, considerFieldName);
+    const BSONObj::ComparisonRulesSet rules = 0;  // 'considerFieldNames' flag is not set.
+    return leftSortKey.woCompare(rightSortKey, sortKeyPattern, rules);
 }
 
 }  // namespace
@@ -679,7 +683,7 @@ bool AsyncResultsMerger::_addBatchToBuffer(WithLock lk,
                            str::stream() << "Missing field '" << AsyncResultsMerger::kSortKeyField
                                          << "' in document: " << obj);
                 return false;
-            } else if (!_params.getCompareWholeSortKey() && key.type() != BSONType::Object) {
+            } else if (!_params.getCompareWholeSortKey() && !key.isABSONObj()) {
                 remote.status =
                     Status(ErrorCodes::InternalError,
                            str::stream() << "Field '" << AsyncResultsMerger::kSortKeyField