<?php
namespace Swissup\CheckoutFields\Observer;

use Magento\Framework\Event\ObserverInterface;

class SalesOrderSaveAfter implements ObserverInterface
{
/**
* @var \Swissup\CheckoutFields\Model\ResourceModel\Field\Value\CollectionFactory
*/
protected $fieldValuesCollectionFactory;

/**
* @var \Swissup\CheckoutFields\Model\FieldFactory
*/
protected $fieldFactory;

/**
* Store Manager
*
* @var \Magento\Store\Model\StoreManagerInterface
*/
protected $storeManager;

/**
* @param \Swissup\CheckoutFields\Model\ResourceModel\Field\Value\CollectionFactory $fieldValuesCollectionFactory
* @param \Swissup\CheckoutFields\Model\FieldFactory $fieldFactory
* @param \Magento\Store\Model\StoreManagerInterface $storeManager
*/
public function __construct(
\Swissup\CheckoutFields\Model\ResourceModel\Field\Value\CollectionFactory $fieldValuesCollectionFactory,
\Swissup\CheckoutFields\Model\FieldFactory $fieldFactory,
\Magento\Store\Model\StoreManagerInterface $storeManager
)
{
$this->fieldValuesCollectionFactory = $fieldValuesCollectionFactory;
$this->fieldFactory = $fieldFactory;
$this->storeManager = $storeManager;
}

public function execute(\Magento\Framework\Event\Observer $observer)
{
$order = $observer->getEvent()->getOrder();
$storeId = $this->storeManager->getStore()->getId();

$fieldsCollection = $this->fieldValuesCollectionFactory
->create()
->addStoreFilter($storeId)
->addQuoteFilter($order->getQuoteId())
->load();

foreach ($fieldsCollection as $fieldValue) {
$fieldValue->setOrderId($order->getId())->save();
}

return $this;
}
}