2 files changed, 17 insertions, 0 deletions
diff --git a/src/mongo/db/pipeline/field_path.h b/src/mongo/db/pipeline/field_path.h
index 0f69ebb7043..347b236fb6b 100644
--- a/src/mongo/db/pipeline/field_path.h
+++ b/src/mongo/db/pipeline/field_path.h
@@ -78,6 +78,14 @@ public:
     }
 
     /**
+     * Get the subpath including path elements [0, n].
+     */
+    StringData getSubpath(size_t n) const {
+        invariant(n + 1 < _fieldPathDotPosition.size());
+        return StringData(_fieldPath.c_str(), _fieldPathDotPosition[n + 1]);
+    }
+
+    /**
      * Return the ith field name from this path using zero-based indexes.
      */
     StringData getFieldName(size_t i) const {
diff --git a/src/mongo/db/pipeline/field_path_test.cpp b/src/mongo/db/pipeline/field_path_test.cpp
index 999cb9b8134..b869bc38d27 100644
--- a/src/mongo/db/pipeline/field_path_test.cpp
+++ b/src/mongo/db/pipeline/field_path_test.cpp
@@ -188,5 +188,14 @@ TEST(FieldPathTest, ConstructorAssertsOnDeeplyNestedArrayPath) {
                        AssertionException,
                        ErrorCodes::Overflow);
 }
+
+// Test FieldPath::getSubpath().
+TEST(FieldPathTest, GetSubpath) {
+    FieldPath path = FieldPath("foo.bar.baz");
+    ASSERT_EQUALS("foo", path.getSubpath(0));
+    ASSERT_EQUALS("foo.bar", path.getSubpath(1));
+    ASSERT_EQUALS("foo.bar.baz", path.getSubpath(2));
+}
+
 }  // namespace
 }  // namespace mongo