add element persistence

backend/app/Element/CreateElementDto.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace App\Element;
+
+use App\Set\Set;
+
+final readonly class CreateElementDto
+{
+    public function __construct(
+        public Set $set,
+        public string $name,
+        public string $kind,
+        public ?Element $parentElement,
+    ) {}
+}

backend/app/Element/ElementModel.php

@@ -0,0 +1,47 @@
+<?php
+
+namespace App\Element;
+
+use Illuminate\Database\Eloquent\Attributes\Fillable;
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Model;
+
+/**
+ * @property int $id
+ * @property int $set_id
+ * @property string $name
+ * @property string $kind
+ * @property int|null $parent_element_id
+ * @property int $position
+ *
+ * @method static Builder<static>|ElementModel newModelQuery()
+ * @method static Builder<static>|ElementModel newQuery()
+ * @method static Builder<static>|ElementModel query()
+ *
+ * @mixin \Eloquent
+ */
+#[Fillable([
+    'set_id',
+    'name',
+    'kind',
+    'parent_element_id',
+    'position',
+])]
+class ElementModel extends Model
+{
+    protected $table = 'elements';
+
+    public $timestamps = false;
+
+    /**
+     * @return array<string, string>
+     */
+    protected function casts(): array
+    {
+        return [
+            'set_id' => 'integer',
+            'parent_element_id' => 'integer',
+            'position' => 'integer',
+        ];
+    }
+}

backend/app/Element/ElementRepository.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace App\Element;
+
+interface ElementRepository
+{
+    public function create(CreateElementDto $dto): Element;
+
+    public function find(int $id): ?Element;
+}

backend/app/Element/EloquentElementRepository.php

@@ -0,0 +1,98 @@
+<?php
+
+namespace App\Element;
+
+use App\Set\Set;
+use App\Set\SetRepository;
+use RuntimeException;
+
+class EloquentElementRepository implements ElementRepository
+{
+    public function __construct(
+        private SetRepository $setRepository,
+    ) {}
+
+    public function create(CreateElementDto $dto): Element
+    {
+        $position = $this->nextPosition(
+            $dto->set,
+            $dto->parentElement,
+        );
+        $model = ElementModel::create([
+            'set_id' => $dto->set->getId(),
+            'name' => $dto->name,
+            'kind' => $dto->kind,
+            'parent_element_id' => $dto->parentElement?->getId(),
+            'position' => $position,
+        ]);
+
+        return new Element(
+            id: $model->id,
+            name: $model->name,
+            kind: $model->kind,
+            set: $dto->set,
+            parentElement: $dto->parentElement,
+            position: $model->position,
+        );
+    }
+
+    public function find(int $id): ?Element
+    {
+        $model = ElementModel::find($id);
+
+        return $model === null ? null : $this->toDomain($model);
+    }
+
+    private function nextPosition(
+        Set $set,
+        ?Element $parentElement,
+    ): int {
+        $query = ElementModel::query()
+            ->where('set_id', $set->getId());
+        if ($parentElement === null) {
+            $query->whereNull('parent_element_id');
+        } else {
+            $query->where('parent_element_id', $parentElement->getId());
+        }
+
+        $currentMaximum = $query->max('position');
+        if ($currentMaximum === null) {
+            return 1;
+        }
+
+        return (int) $currentMaximum + 1;
+    }
+
+    private function toDomain(ElementModel $model): Element
+    {
+        $set = $this->findSet($model->set_id);
+
+        $parentElement = null;
+        if ($model->parent_element_id !== null) {
+            $parentElement = $this->find($model->parent_element_id);
+            if ($parentElement === null) {
+                throw new RuntimeException('element parent not found');
+            }
+        }
+
+        return new Element(
+            id: $model->id,
+            name: $model->name,
+            kind: $model->kind,
+            set: $set,
+            parentElement: $parentElement,
+            position: $model->position,
+        );
+    }
+
+    private function findSet(int $id): Set
+    {
+        foreach ($this->setRepository->all() as $set) {
+            if ($set->getId() === $id) {
+                return $set;
+            }
+        }
+
+        throw new RuntimeException('element set not found');
+    }
+}

backend/app/Providers/AppServiceProvider.php

@@ -16,6 +16,8 @@ use App\Email\Emailer;
 use App\Email\EmailFactory;
 use App\Email\LaravelEmailer;
 use App\Email\LaravelEmailFactory;
+use App\Element\ElementRepository;
+use App\Element\EloquentElementRepository;
 use App\Set\EloquentSetRepository;
 use App\Set\SetRepository;
 use App\User\EloquentUserRepository;
@@ -51,6 +53,10 @@ class AppServiceProvider extends ServiceProvider
             SetRepository::class,
             EloquentSetRepository::class,
         );
+        $this->app->bind(
+            ElementRepository::class,
+            EloquentElementRepository::class,
+        );
         $this->app->bind(PasswordHasher::class, BcryptPasswordHasher::class);
         $this->app->bind(TokenGenerator::class, RandomTokenGenerator::class);
         $this->app->bind(Clock::class, SystemClock::class);

backend/database/migrations/2026_08_08_000000_create_elements_table.php

@@ -0,0 +1,35 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('elements', function (Blueprint $table): void {
+            $table->id();
+            $table->foreignId('set_id')
+                ->constrained('sets')
+                ->restrictOnDelete();
+            $table->string('name');
+            $table->string('kind');
+            $table->foreignId('parent_element_id')
+                ->nullable()
+                ->constrained('elements')
+                ->restrictOnDelete();
+            $table->unsignedInteger('position');
+            $table->index([
+                'set_id',
+                'parent_element_id',
+                'position',
+            ]);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('elements');
+    }
+};

backend/tests/Fakes/FakeElementRepository.php

@@ -0,0 +1,78 @@
+<?php
+
+namespace Tests\Fakes;
+
+use App\Element\CreateElementDto;
+use App\Element\Element;
+use App\Element\ElementRepository;
+
+class FakeElementRepository implements ElementRepository
+{
+    /**
+     * @var array<int, Element>
+     */
+    private array $elements = [];
+
+    public function create(CreateElementDto $dto): Element
+    {
+        $id = count($this->elements) + 1;
+        $element = new Element(
+            id: $id,
+            name: $dto->name,
+            kind: $dto->kind,
+            set: $dto->set,
+            parentElement: $dto->parentElement,
+            position: $this->nextPosition($dto),
+        );
+        $this->elements[$id] = $element;
+
+        return $this->copy($element);
+    }
+
+    public function find(int $id): ?Element
+    {
+        $element = $this->elements[$id] ?? null;
+
+        return $element === null ? null : $this->copy($element);
+    }
+
+    private function nextPosition(CreateElementDto $dto): int
+    {
+        $requestedParentId = $dto->parentElement?->getId();
+        $maximumPosition = 0;
+
+        foreach ($this->elements as $element) {
+            if ($element->getSet()->getId() !== $dto->set->getId()) {
+                continue;
+            }
+
+            $elementParentId = $element->getParentElement()?->getId();
+            if ($elementParentId !== $requestedParentId) {
+                continue;
+            }
+
+            $maximumPosition = max(
+                $maximumPosition,
+                $element->getPosition(),
+            );
+        }
+
+        return $maximumPosition + 1;
+    }
+
+    private function copy(Element $element): Element
+    {
+        $parentElement = $element->getParentElement();
+
+        return new Element(
+            id: $element->getId(),
+            name: $element->getName(),
+            kind: $element->getKind(),
+            set: $element->getSet(),
+            parentElement: $parentElement === null
+                ? null
+                : $this->copy($parentElement),
+            position: $element->getPosition(),
+        );
+    }
+}