public function getSite(CNabuCustomer $nb_customer = null, $force = false)
    {
        if ($nb_customer !== null && ($this->nb_site === null || $force)) {
            $this->nb_site = null;
            if ($this instanceof CNabuDataObject && $this->contains(NABU_SITE_FIELD_ID)) {
                $this->nb_site = $nb_customer->getSite($this);
            }
        }

        return $this->nb_site;
    }