1 files changed, 15 insertions, 5 deletions
diff --git a/src/mongo/db/ops/update_driver.cpp b/src/mongo/db/ops/update_driver.cpp
index a99376174a8..94292e24874 100644
--- a/src/mongo/db/ops/update_driver.cpp
+++ b/src/mongo/db/ops/update_driver.cpp
@@ -288,11 +288,21 @@ Status UpdateDriver::update(StringData matchedField,
             // is not a no-op and it is in a valid context -- then we switch back to a
             // non-in-place mode.
             //
-            // TODO: make mightBeIndexed and fieldRef like each other.
-            if (!_affectIndices && !execInfo.noOp && _indexedFields &&
-                _indexedFields->mightBeIndexed(execInfo.fieldRef[i]->dottedField())) {
-                _affectIndices = true;
-                doc->disableInPlaceUpdates();
+            // To determine if indexes are affected: If we did not create a new element in an array,
+            // check whether the full path affects indexes. If we did create a new element in an
+            // array, check whether the array itself might affect any indexes. This is necessary
+            // because if there is an index {"a.b": 1}, and we set "a.1.c" and implicitly create an
+            // array element in "a", then we may need to add a null key to the index {"a.b": 1},
+            // even though "a.1.c" does not appear to affect the index.
+            if (!_affectIndices && !execInfo.noOp && _indexedFields) {
+                auto pathLengthForIndexCheck = execInfo.indexOfArrayWithNewElement[i]
+                    ? *execInfo.indexOfArrayWithNewElement[i] + 1
+                    : execInfo.fieldRef[i]->numParts();
+                if (_indexedFields->mightBeIndexed(
+                        execInfo.fieldRef[i]->dottedSubstring(0, pathLengthForIndexCheck))) {
+                    _affectIndices = true;
+                    doc->disableInPlaceUpdates();
+                }
             }
         }