[4.x] Update child URIs when parent slug changes (#9454)

Co-authored-by: Jason Varga <jason@pixelfear.com>

src/Entries/Entry.php

@@ -390,6 +390,7 @@ public function save()
 
         if ($this->isDirty('slug')) {
             optional(Collection::findByMount($this))->updateEntryUris();
+            $this->updateChildPageUris();
         }
 
         foreach ($afterSaveCallbacks as $callback) {
@@ -419,6 +420,28 @@ public function save()
         return true;
     }
 
+    private function updateChildPageUris()
+    {
+        $collection = $this->collection();
+
+        // If it's orderable (single depth structure), there are no children to update.
+        // If the collection has no route, there are no uris to update.
+        // If there's no page, there are no children to update.
+        if (
+            $collection->orderable()
+            || ! $this->route()
+            || ! ($page = $this->page())
+        ) {
+            return;
+        }
+
+        if (empty($ids = $page->flattenedPages()->pluck('id'))) {
+            return;
+        }
+
+        $collection->updateEntryUris($ids);
+    }
+
     public function taxonomize()
     {
         Facades\Entry::taxonomize($this);