SERVER-21647 fix $rename to not re-order fields when the destination field is already present

2 files changed, 44 insertions, 4 deletions

diff --git a/src/mongo/db/ops/modifier_rename.cpp b/src/mongo/db/ops/modifier_rename.cpp
index 123b253f227..38555546a70 100644
--- a/src/mongo/db/ops/modifier_rename.cpp
+++ b/src/mongo/db/ops/modifier_rename.cpp
@@ -221,10 +221,8 @@ Status ModifierRename::apply() const {
         (_preparedState->toIdxFound == (_toFieldRef.numParts() - 1));
 
     if (destExists) {
-        removeStatus = _preparedState->toElemFound.remove();
-        if (!removeStatus.isOK()) {
-            return removeStatus;
-        }
+        // Set destination element to the value of the source element.
+        return _preparedState->toElemFound.setValueElement(_preparedState->fromElemFound);
     }
 
     // Creates the final element that's going to be the in 'doc'.
diff --git a/src/mongo/db/ops/modifier_rename_test.cpp b/src/mongo/db/ops/modifier_rename_test.cpp
index db03531d4b4..0958c1581cb 100644
--- a/src/mongo/db/ops/modifier_rename_test.cpp
+++ b/src/mongo/db/ops/modifier_rename_test.cpp
@@ -220,6 +220,48 @@ TEST(SimpleReplace, FromDottedElement) {
     ASSERT_EQUALS(logDoc, logObj);
 }
 
+TEST(SimpleReplace, RenameToExistingFieldDoesNotReorderFields) {
+    Document doc(fromjson("{a: 1, b: 2, c: 3}"));
+    Mod setMod(fromjson("{$rename: {a: 'b'}}"));
+
+    ModifierInterface::ExecInfo execInfo;
+    ASSERT_OK(setMod.prepare(doc.root(), "", &execInfo));
+    ASSERT_EQUALS(execInfo.fieldRef[0]->dottedField(), "a");
+    ASSERT_EQUALS(execInfo.fieldRef[1]->dottedField(), "b");
+    ASSERT_FALSE(execInfo.noOp);
+
+    ASSERT_OK(setMod.apply());
+    ASSERT_FALSE(doc.isInPlaceModeEnabled());
+    ASSERT_EQUALS(doc, fromjson("{b: 1, c: 3}"));
+
+    Document logDoc;
+    LogBuilder logBuilder(logDoc.root());
+    BSONObj logObj = fromjson("{$set: {b: 1}, $unset: {a: true}}");
+    ASSERT_OK(setMod.log(&logBuilder));
+    ASSERT_EQUALS(logDoc, logObj);
+}
+
+TEST(SimpleReplace, RenameToExistingNestedFieldDoesNotReorderFields) {
+    Document doc(fromjson("{a: {b: {c: 1, d: 2}}, b: 3, c: {d: 4}}"));
+    Mod setMod(fromjson("{$rename: {'c.d': 'a.b.c'}}"));
+
+    ModifierInterface::ExecInfo execInfo;
+    ASSERT_OK(setMod.prepare(doc.root(), "", &execInfo));
+    ASSERT_EQUALS(execInfo.fieldRef[0]->dottedField(), "c.d");
+    ASSERT_EQUALS(execInfo.fieldRef[1]->dottedField(), "a.b.c");
+    ASSERT_FALSE(execInfo.noOp);
+
+    ASSERT_OK(setMod.apply());
+    ASSERT_FALSE(doc.isInPlaceModeEnabled());
+    ASSERT_EQUALS(doc, fromjson("{a: {b: {c: 4, d: 2}}, b: 3, c: {}}"));
+
+    Document logDoc;
+    LogBuilder logBuilder(logDoc.root());
+    BSONObj logObj = fromjson("{$set: {'a.b.c': 4}, $unset: {'c.d': true}}");
+    ASSERT_OK(setMod.log(&logBuilder));
+    ASSERT_EQUALS(logDoc, logObj);
+}
+
 TEST(DottedTo, MissingCompleteTo) {
     Document doc(fromjson("{a: 2, b: 1, c: {}}"));
     Mod setMod(fromjson("{$rename: {'a':'c.r.d'}}"));