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As the new Prestashop is using Symfony, this is how to query the database:

$this->entityManager = ServiceLocator::get('\\PrestaShop\\PrestaShop\\Core\\Foundation\\Database\\EntityManager');

$id_order_state = 5;

$orders = $this->entityManager->getRepository('Order')
  ->findBy([
  	'current_state' => $id_order_state
  ]);

You can use this in a front controller like this:

<?php

use PrestaShop\PrestaShop\Adapter\ServiceLocator;

class MymoduleTestcontrollerModuleFrontController extends ModuleFrontController
{
    private $entityManager;

    public function __construct()
    {
        parent::__construct();
        $this->entityManager = ServiceLocator::get('\\PrestaShop\\PrestaShop\\Core\\Foundation\\Database\\EntityManager');
    }

    public function initContent()
    {
        parent::initContent();                          

        $id_order_state = 5;

        $orders = $this->entityManager->getRepository('Order')
            ->findBy([
                'current_state' => $id_order_state
            ]);

        echo '<pre>'; print_r($orders); echo '</pre>'; die;
    }
}