add set root ids

backend/app/Element/ElementRepository.php

@@ -10,6 +10,8 @@ interface ElementRepository
 
     public function find(int $id): ?Element;
 
+    public function findRootBySet(DomainSet $set): ?Element;
+
     /**
      * @return Element[]
      */

backend/app/Element/EloquentElementRepository.php

@@ -35,6 +35,16 @@ class EloquentElementRepository implements ElementRepository
         return $model === null ? null : $this->toDomain($model);
     }
 
+    public function findRootBySet(DomainSet $set): ?Element
+    {
+        $model = ElementModel::where('set_id', $set->getId())
+            ->whereNull('parent_element_id')
+            ->orderBy('id')
+            ->first();
+
+        return $model === null ? null : $this->toDomain($model);
+    }
+
     /**
      * @return Element[]
      */