Do not cache entities directly in NodesListBuilder but cache only the IDs instead

Otherwise the doctrine proxies break, and we get issues with loading the preview_images in structural Elements.

src/Repository/AttachmentContainingDBElementRepository.php

@@ -0,0 +1,72 @@
+<?php
+/*
+ * This file is part of Part-DB (https://github.com/Part-DB/Part-DB-symfony).
+ *
+ *  Copyright (C) 2019 - 2023 Jan Böhmer (https://github.com/jbtronics)
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Affero General Public License as published
+ *  by the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Affero General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Affero General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+declare(strict_types=1);
+
+
+namespace App\Repository;
+
+use App\Entity\Attachments\AttachmentContainingDBElement;
+use Doctrine\ORM\Mapping\ClassMetadataInfo;
+
+/**
+ * @template TEntityClass of AttachmentContainingDBElement
+ * @extends NamedDBElementRepository<TEntityClass>
+ */
+class AttachmentContainingDBElementRepository extends NamedDBElementRepository
+{
+    /**
+     * @var array This array is used to cache the results of getElementsAndPreviewAttachmentByIDs function.
+     */
+    private array $elementsAndPreviewAttachmentCache = [];
+
+    /**
+     * Similar to the findByIDInMatchingOrder function, but it also hints to doctrine that the master picture attachment should be fetched eagerly.
+     * @param  array  $ids
+     * @return array
+     */
+    public function getElementsAndPreviewAttachmentByIDs(array $ids): array
+    {
+        //Convert the ids to a string
+        $cache_key = implode(',', $ids);
+
+        //Check if the result is already cached
+        if (isset($this->elementsAndPreviewAttachmentCache[$cache_key])) {
+            return $this->elementsAndPreviewAttachmentCache[$cache_key];
+        }
+
+        $qb = $this->createQueryBuilder('element');
+        $q = $qb->select('element')
+            ->where('element.id IN (?1)')
+            ->setParameter(1, $ids)
+            ->getQuery();
+
+        $q->setFetchMode($this->getEntityName(), 'master_picture_attachment', ClassMetadataInfo::FETCH_EAGER);
+
+        $result = $q->getResult();
+        $result = array_combine($ids, $result);
+        $result = array_map(fn ($id) => $result[$id], $ids);
+
+        //Cache the result
+        $this->elementsAndPreviewAttachmentCache[$cache_key] = $result;
+
+        return $result;
+    }
+}