add child creation

backend/app/Element/UseCases/CreateChildElement/CreateChildElement.php

@@ -0,0 +1,50 @@
+<?php
+
+namespace App\Element\UseCases\CreateChildElement;
+
+use App\Element\CreateElementDto;
+use App\Element\Element;
+use App\Element\ElementRepository;
+use App\Exceptions\BadRequestException;
+use App\Exceptions\NotFoundException;
+
+class CreateChildElement
+{
+    public function __construct(private ElementRepository $elementRepository)
+    {
+    }
+
+    /**
+     * @throws BadRequestException
+     * @throws NotFoundException
+     */
+    public function execute(CreateChildElementRequest $request): Element
+    {
+        if ($request->parentElementId === null) {
+            throw new BadRequestException('parentElementId is required');
+        }
+
+        if ($request->title === null || $request->title === '') {
+            throw new BadRequestException('title is required');
+        }
+
+        $parentElement = $this->elementRepository->find(
+            $request->parentElementId
+        );
+        if ($parentElement === null) {
+            throw new NotFoundException('Parent element not found');
+        }
+
+        return $this->elementRepository->create(new CreateElementDto(
+            set: $parentElement->getSet(),
+            title: $request->title,
+            description: '',
+            iconImageUrl: null,
+            richText: '',
+            shortPdfPath: null,
+            longPdfPath: null,
+            youtubeUrl: null,
+            parentElement: $parentElement,
+        ));
+    }
+}

backend/app/Element/UseCases/CreateChildElement/CreateChildElementRequest.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace App\Element\UseCases\CreateChildElement;
+
+class CreateChildElementRequest
+{
+    public function __construct(
+        public ?int $parentElementId,
+        public ?string $title,
+    ) {
+    }
+}