Added a simple system for treeViews.

src/Repository/StructuralDBElementRepository.php

@@ -0,0 +1,52 @@
+<?php
+/**
+ *
+ * part-db version 0.1
+ * Copyright (C) 2005 Christoph Lechner
+ * http://www.cl-projects.de/
+ *
+ * part-db version 0.2+
+ * Copyright (C) 2009 K. Jacobs and others (see authors.php)
+ * http://code.google.com/p/part-db/
+ *
+ * Part-DB Version 0.4+
+ * Copyright (C) 2016 - 2019 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 General Public License
+ * as published by the Free Software Foundation; either version 2
+ * 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
+ *
+ */
+
+namespace App\Repository;
+
+
+use App\Entity\StructuralDBElement;
+use Doctrine\Common\Collections\Criteria;
+use Doctrine\ORM\EntityRepository;
+
+class StructuralDBElementRepository extends EntityRepository
+{
+
+    /**
+     * Finds all nodes without a parent node. They are our root nodes.
+     *
+     * @return StructuralDBElement[]
+     */
+    public function findRootNodes() : array
+    {
+        return $this->findBy(['parent' => null]);
+    }
+
+}