add child element ordering

backend/app/Element/EloquentElementRepository.php

@@ -5,6 +5,7 @@ namespace App\Element;
 use App\Set\Set as DomainSet;
 use App\Set\SetRepository;
 use DomainException;
+use Illuminate\Support\Facades\DB;
 
 class EloquentElementRepository implements ElementRepository
 {
@@ -14,6 +15,7 @@ class EloquentElementRepository implements ElementRepository
 
     public function create(CreateElementDto $dto): Element
     {
+        $parentElementId = $dto->parentElement?->getId();
         $model = ElementModel::create([
             'set_id' => $dto->set->getId(),
             'title' => $dto->title,
@@ -23,7 +25,8 @@ class EloquentElementRepository implements ElementRepository
             'short_pdf_path' => $dto->shortPdfPath,
             'long_pdf_path' => $dto->longPdfPath,
             'youtube_url' => $dto->youtubeUrl,
-            'parent_element_id' => $dto->parentElement?->getId(),
+            'parent_element_id' => $parentElementId,
+            'sort_order' => $this->nextSortOrder($dto->set, $parentElementId),
         ]);
 
         return new Element(
@@ -86,6 +89,7 @@ class EloquentElementRepository implements ElementRepository
     {
         $model = ElementModel::where('set_id', $set->getId())
             ->whereNull('parent_element_id')
+            ->orderBy('sort_order')
             ->orderBy('id')
             ->first();
 
@@ -117,6 +121,7 @@ class EloquentElementRepository implements ElementRepository
             'parent_element_id',
             $parentElement->getId(),
         )
+            ->orderBy('sort_order')
             ->orderBy('id')
             ->get();
         $elements = [];
@@ -127,6 +132,46 @@ class EloquentElementRepository implements ElementRepository
         return $elements;
     }
 
+    /**
+     * @param int[] $childElementIds
+     * @return Element[]
+     */
+    public function reorderChildren(
+        Element $parentElement,
+        array $childElementIds,
+    ): array {
+        DB::transaction(function () use ($parentElement, $childElementIds) {
+            $sortOrder = 1;
+            foreach ($childElementIds as $childElementId) {
+                ElementModel::where('id', $childElementId)
+                    ->where('parent_element_id', $parentElement->getId())
+                    ->update(['sort_order' => $sortOrder]);
+                $sortOrder++;
+            }
+        });
+
+        return $this->findByParentElement($parentElement);
+    }
+
+    private function nextSortOrder(
+        DomainSet $set,
+        ?int $parentElementId,
+    ): int {
+        $query = ElementModel::where('set_id', $set->getId());
+        if ($parentElementId === null) {
+            $query->whereNull('parent_element_id');
+        } else {
+            $query->where('parent_element_id', $parentElementId);
+        }
+
+        $currentMaxSortOrder = $query->max('sort_order');
+        if ($currentMaxSortOrder === null) {
+            return 1;
+        }
+
+        return (int) $currentMaxSortOrder + 1;
+    }
+
     private function toDomain(ElementModel $model): Element
     {
         $set = $this->setRepo->find($model->set_id);