Changed DB structure the way, many more elements can have attachments

Also every Element has now a preview attachment field. For the user this will be used as profile picture.

src/Entity/Attachments/CurrencyAttachment.php

@@ -0,0 +1,67 @@
+<?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\Entity\Attachments;
+
+use App\Entity\Devices\Device;
+use App\Entity\Parts\Manufacturer;
+use App\Entity\Parts\MeasurementUnit;
+use App\Entity\Parts\Part;
+use App\Entity\Parts\Storelocation;
+use App\Entity\Parts\Supplier;
+use App\Entity\PriceInformations\Currency;
+use Doctrine\ORM\Mapping as ORM;
+
+/**
+ * A attachment attached to a currency element.
+ * @package App\Entity
+ * @ORM\Entity()
+ */
+class CurrencyAttachment extends Attachment
+{
+
+    /**
+     * @var Currency The element this attachment is associated with.
+     * @ORM\ManyToOne(targetEntity="App\Entity\PriceInformations\Currency", inversedBy="attachments")
+     * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
+     */
+    protected $element;
+
+    public function setElement(AttachmentContainingDBElement $element): Attachment
+    {
+        if (!$element instanceof Currency) {
+            throw new \InvalidArgumentException('The element associated with a CurrencyAttachment must be a Currency!');
+        }
+
+        $this->element = $element;
+        return $this;
+    }
+}