diff --git a/src/Bundle/ChillMainBundle/Doctrine/ORM/Hydration/FlatHierarchyEntityHydrator.php b/src/Bundle/ChillMainBundle/Doctrine/ORM/Hydration/FlatHierarchyEntityHydrator.php
index d28c7af09..0bd9505a6 100644
--- a/src/Bundle/ChillMainBundle/Doctrine/ORM/Hydration/FlatHierarchyEntityHydrator.php
+++ b/src/Bundle/ChillMainBundle/Doctrine/ORM/Hydration/FlatHierarchyEntityHydrator.php
@@ -11,44 +11,38 @@ final class FlatHierarchyEntityHydrator extends ObjectHydrator
 {
     public const LIST = 'chill_flat_hierarchy_list';
 
-    protected function hydrateAllData(): array
+    protected function hydrateAllData()
     {
-        $generator = $this->buildRecursively(
-            parent::hydrateAllData(),
-            static function($value): iterable {
-                $children = $value->getChildren();
-
-                $children->setInitialized(true);
-
-                return $children;
-            }
-        );
-
-        $result = [];
-
-        foreach ($generator as $entity) {
-            // We cannot expect anything from the object entity
-            // so, we cannot expect it to have a ::getId() method.
-            // So we use spl_object_hash() instead.
-            $result += [spl_object_hash($entity) => $entity];
-        }
-
-        return array_values($result);
+        return array_values(iterator_to_array($this->flatListGenerator($this->buildChildrenHashmap(parent::hydrateAllData()))));
     }
 
-    /**
-     * @param iterable<int, SocialIssue> $iterable
-     * @param callable(SocialIssue): iterable<int, SocialIssue> $childrenAccessor
-     *
-     * @return Generator<int, SocialIssue>
-     */
-    private function buildRecursively(iterable $iterable, callable $childrenAccessor): Generator
+    private function flatListGenerator(array $hashMap, $parent = null): Generator
     {
-        foreach ($iterable as $item)
-        {
-            yield $item;
+        $parent = null === $parent ?
+            null :
+            spl_object_hash($parent);
 
-            yield from $this->buildRecursively($childrenAccessor($item), $childrenAccessor);
+        $hashMap += [$parent => []];
+
+        foreach ($hashMap[$parent] as $node) {
+            yield $node->getId() => $node;
+            yield from $this->flatListGenerator($hashMap, $node);
         }
     }
+
+    private function buildChildrenHashmap(array $nodes): array
+    {
+        $r = [];
+
+        foreach ($nodes as $node) {
+            $parentId = (null !== $parent = $node->getParent())
+                ? spl_object_hash($parent)
+                : null;
+
+            $r[$parentId][] = $node;
+        }
+
+        return $r;
+    }
+
 }