magento2/app/code/Magento/Sales/Model/ResourceModel/Order/Shipment.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Sales\Model\ResourceModel\Order;

use Magento\Framework\App\ResourceConnection as AppResource;
use Magento\SalesSequence\Model\Manager;
use Magento\Sales\Model\ResourceModel\Attribute;
use Magento\Sales\Model\ResourceModel\EntityAbstract as SalesResource;
use Magento\Sales\Model\Spi\ShipmentResourceInterface;

/**
 * Flat sales order shipment resource
 *
 * @author      Magento Core Team <core@magentocommerce.com>
 */
class Shipment extends SalesResource implements ShipmentResourceInterface
{
    /**
     * Event prefix
     *
     * @var string
     */
    protected $_eventPrefix = 'sales_order_shipment_resource';

    /**
     * Fields that should be serialized before persistence
     *
     * @var array
     */
    protected $_serializableFields = ['packages' => [[], []]];

    /**
     * Model initialization
     *
     * @return void
     */
    protected function _construct()
    {
        $this->_init('sales_shipment', 'entity_id');
    }

    /**
     * Perform actions before object save
     *
     * @param \Magento\Framework\Model\AbstractModel|\Magento\Framework\DataObject $object
     * @return $this
     * @throws \Magento\Framework\Exception\LocalizedException
     */
    protected function _beforeSave(\Magento\Framework\Model\AbstractModel $object)
    {
        /** @var \Magento\Sales\Model\Order\Shipment $object */
        if ((!$object->getId() || null !== $object->getItems()) && !count($object->getAllItems())) {
            throw new \Magento\Framework\Exception\LocalizedException(__('We cannot create an empty shipment.'));
        }

        if (!$object->getOrderId() && $object->getOrder()) {
            $object->setOrderId($object->getOrder()->getId());
            $object->setShippingAddressId($object->getOrder()->getShippingAddress()->getId());
        }

        return parent::_beforeSave($object);
    }
}