SERVER-45177 Avoid using getChildVector() to traverse MatchExpression

2 files changed, 24 insertions, 2 deletions

diff --git a/jstests/core/mr_not_equals_predicate.js b/jstests/core/mr_not_equals_predicate.js
new file mode 100644
index 00000000000..5e5426732ff
--- /dev/null
+++ b/jstests/core/mr_not_equals_predicate.js
@@ -0,0 +1,22 @@
+// Tests that a mapReduce command can succeed when given a not equals predicate. This test was
+// designed to reproduce SERVER-45177.
+// @tags: [
+//   # mapReduce does not support afterClusterTime.
+//   does_not_support_causal_consistency,
+//   does_not_support_stepdowns,
+// ]
+(function() {
+"use strict";
+const coll = db.mr_ne_query;
+assert.commandWorked(coll.insert([{key: 'one', val: 1}, {key: 'two', val: 2}]));
+assert.doesNotThrow(() => coll.find({key: {$ne: 'one'}}));
+const mapper = function() {
+    emit(this._id, this.val);
+};
+const reducer = function(k, v) {
+    return Array.sum(v);
+};
+assert.doesNotThrow(() => coll.mapReduce(mapper, reducer, {out: {inline: 1}, query: {}}));
+assert.doesNotThrow(
+    () => coll.mapReduce(mapper, reducer, {out: {inline: 1}, query: {key: {$ne: 'one'}}}));
+}());
diff --git a/src/mongo/db/commands/mr.cpp b/src/mongo/db/commands/mr.cpp
index 5edc6a426aa..f11ed660b9f 100644
--- a/src/mongo/db/commands/mr.cpp
+++ b/src/mongo/db/commands/mr.cpp
@@ -236,8 +236,8 @@ bool queryContainsMatchType(MatchExpression* root, MatchExpression::MatchType ma
     if (root->matchType() == match) {
         return false;
     } else if (root->numChildren() > 0) {
-        for (auto&& child : *(root->getChildVector())) {
-            if (!queryContainsMatchType(child, match)) {
+        for (size_t i = 0; i < root->numChildren(); i++) {
+            if (!queryContainsMatchType(root->getChild(i), match)) {
                 return false;
             }
         }