SERVER-39019 $elemMatch $ne serialization is incorrect, doesn't roundtrip

(cherry picked from commit 3db787da9dc453192adcf7706991d2d09f0ff0c7)

2 files changed, 84 insertions, 0 deletions

diff --git a/src/mongo/db/matcher/expression_array.cpp b/src/mongo/db/matcher/expression_array.cpp
index 86fbc384eae..b6a9734233d 100644
--- a/src/mongo/db/matcher/expression_array.cpp
+++ b/src/mongo/db/matcher/expression_array.cpp
@@ -201,6 +201,40 @@ void ElemMatchValueMatchExpression::debugString(StringBuilder& debug, int level)
 void ElemMatchValueMatchExpression::serialize(BSONObjBuilder* out) const {
     BSONObjBuilder emBob;
 
+    // NotMatchExpression will serialize to a $nor. This serialization is incorrect for
+    // ElemMatchValue however as $nor is a top-level expression and expects that contained
+    // expressions have path information. For this case we will serialize to $not.
+    if (_subs.size() == 1 && _subs[0]->matchType() == MatchType::NOT) {
+        std::vector<MatchExpression*> childList{_subs[0]->getChild(0)};
+
+        MatchExpression* notChildExpr = _subs[0]->getChild(0);
+        if (notChildExpr->matchType() == MatchType::AND) {
+            childList = *notChildExpr->getChildVector();
+        }
+
+        const bool allChildrenAreLeafMatchExpression =
+            std::all_of(childList.begin(), childList.end(), [](MatchExpression* child) {
+                return dynamic_cast<LeafMatchExpression*>(child);
+            });
+
+        if (allChildrenAreLeafMatchExpression) {
+            BSONObjBuilder pathBuilder(out->subobjStart(path()));
+            BSONObjBuilder elemMatchBuilder(pathBuilder.subobjStart("$elemMatch"));
+            BSONObjBuilder notBuilder(elemMatchBuilder.subobjStart("$not"));
+
+            for (auto&& child : childList) {
+                BSONObjBuilder predicate;
+                child->serialize(&predicate);
+                notBuilder.appendElements(predicate.obj().firstElement().embeddedObject());
+            }
+
+            notBuilder.doneFast();
+            elemMatchBuilder.doneFast();
+            pathBuilder.doneFast();
+            return;
+        }
+    }
+
     for (unsigned i = 0; i < _subs.size(); i++) {
         BSONObjBuilder predicate;
         _subs[i]->serialize(&predicate);
diff --git a/src/mongo/db/matcher/expression_serialization_test.cpp b/src/mongo/db/matcher/expression_serialization_test.cpp
index 13a2aaa172d..cac2f5155ba 100644
--- a/src/mongo/db/matcher/expression_serialization_test.cpp
+++ b/src/mongo/db/matcher/expression_serialization_test.cpp
@@ -207,6 +207,56 @@ TEST(SerializeBasic, ExpressionElemMatchValueWithEmptyStringSerializesCorrectly)
     ASSERT_EQ(original.matches(obj), reserialized.matches(obj));
 }
 
+TEST(SerializeBasic, ExpressionElemMatchValueWithNotEqualSerializesCorrectly) {
+    const CollatorInterface* collator = nullptr;
+    Matcher original(fromjson("{x: {$elemMatch: {$ne: 1}}}"), ExtensionsCallbackNoop(), collator);
+    Matcher reserialized(
+        serialize(original.getMatchExpression()), ExtensionsCallbackNoop(), collator);
+    ASSERT_BSONOBJ_EQ(*reserialized.getQuery(), fromjson("{x: {$elemMatch: {$not: {$eq: 1}}}}"));
+    ASSERT_BSONOBJ_EQ(*reserialized.getQuery(), serialize(reserialized.getMatchExpression()));
+
+    BSONObj obj = fromjson("{x: [{a: 1, b: -1}, {a: -1, b: 1}]}");
+    ASSERT_EQ(original.matches(obj), reserialized.matches(obj));
+
+    obj = fromjson("{x: [{a: 1, b: 1}, {a: 0, b: 0}]}");
+    ASSERT_EQ(original.matches(obj), reserialized.matches(obj));
+
+    obj = fromjson("{x: [1, 0]}");
+    ASSERT_EQ(original.matches(obj), reserialized.matches(obj));
+}
+
+TEST(SerializeBasic, ExpressionElemMatchValueWithNotLessThanGreaterThanSerializesCorrectly) {
+    const CollatorInterface* collator = nullptr;
+    Matcher original(fromjson("{x: {$elemMatch: {$not: {$lt: 10, $gt: 5}}}}"),
+                     ExtensionsCallbackNoop(),
+                     collator);
+    Matcher reserialized(
+        serialize(original.getMatchExpression()), ExtensionsCallbackNoop(), collator);
+    ASSERT_BSONOBJ_EQ(*reserialized.getQuery(),
+                      fromjson("{x: {$elemMatch: {$not: {$lt: 10, $gt: 5}}}}"));
+    ASSERT_BSONOBJ_EQ(*reserialized.getQuery(), serialize(reserialized.getMatchExpression()));
+
+    auto obj = fromjson("{x: [5]}");
+    ASSERT_EQ(original.matches(obj), reserialized.matches(obj));
+}
+
+TEST(SerializeBasic, ExpressionElemMatchObjectWithNorSerializesCorrectly) {
+    const CollatorInterface* collator = nullptr;
+    Matcher original(
+        fromjson("{x: {$elemMatch: {$nor: [{y: 2}]}}}"), ExtensionsCallbackNoop(), collator);
+    Matcher reserialized(
+        serialize(original.getMatchExpression()), ExtensionsCallbackNoop(), collator);
+    ASSERT_BSONOBJ_EQ(*reserialized.getQuery(),
+                      fromjson("{x: {$elemMatch: {$nor: [{y: {$eq: 2}}]}}}"));
+    ASSERT_BSONOBJ_EQ(*reserialized.getQuery(), serialize(reserialized.getMatchExpression()));
+
+    BSONObj obj = fromjson("{x: [{y: 100}]}");
+    ASSERT_EQ(original.matches(obj), reserialized.matches(obj));
+
+    obj = fromjson("{x: [{y: 2}]}");
+    ASSERT_EQ(original.matches(obj), reserialized.matches(obj));
+}
+
 TEST(SerializeBasic, ExpressionSizeSerializesCorrectly) {
     const CollatorInterface* collator = nullptr;
     Matcher original(fromjson("{x: {$size: 2}}"), ExtensionsCallbackNoop(), collator);