SERVER-42835 Fix inconsistent handling of arrays as $group keys

1 files changed, 5 insertions, 6 deletions

diff --git a/src/mongo/db/pipeline/document_source_group.cpp b/src/mongo/db/pipeline/document_source_group.cpp
index 6ec80d8bbe5..2f7f32aa844 100644
--- a/src/mongo/db/pipeline/document_source_group.cpp
+++ b/src/mongo/db/pipeline/document_source_group.cpp
@@ -356,8 +356,11 @@ namespace {
 intrusive_ptr<Expression> parseIdExpression(const intrusive_ptr<ExpressionContext>& expCtx,
                                             BSONElement groupField,
                                             const VariablesParseState& vps) {
-    if (groupField.type() == Object && !groupField.Obj().isEmpty()) {
+    if (groupField.type() == Object) {
         // {_id: {}} is treated as grouping on a constant, not an expression
+        if (groupField.Obj().isEmpty()) {
+            return ExpressionConstant::create(expCtx, Value(groupField));
+        }
 
         const BSONObj idKeyObj = groupField.Obj();
         if (idKeyObj.firstElementFieldName()[0] == '$') {
@@ -371,12 +374,8 @@ intrusive_ptr<Expression> parseIdExpression(const intrusive_ptr<ExpressionContex
             }
             return ExpressionObject::parse(expCtx, idKeyObj, vps);
         }
-    } else if (groupField.type() == String && groupField.valuestr()[0] == '$') {
-        // grouping on a field path.
-        return ExpressionFieldPath::parse(expCtx, groupField.str(), vps);
     } else {
-        // constant id - single group
-        return ExpressionConstant::create(expCtx, Value(groupField));
+        return Expression::parseOperand(expCtx, groupField, vps);
     }
 }