Part-DB.Part-DB-server/src/Entity/Attachments/PartAttachment.php

<?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\Parts\Part;
use Doctrine\ORM\Mapping as ORM;

/**
 * Class PartAttachment
 * @package App\Entity
 * @ORM\Entity()
 */
class PartAttachment extends Attachment
{

    /**
     *
     * @ORM\ManyToOne(targetEntity="App\Entity\Parts\Part", inversedBy="attachments")
     * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
     */
    protected $element;

    public function setElement(AttachmentContainingDBElement $element): Attachment
    {
        if (!$element instanceof Part) {
            throw new \InvalidArgumentException('The element associated with a PartAttachment must be a Part!');
        }

        $this->element = $element;

        return $this;
    }
}