public function add($related_product_id, $product_id)
    {
        $result = null;
        $this->hook->attach('product.related.add.before', $related_product_id, $product_id, $result, $this);

        if (isset($result)) {
            return (bool) $result;
        }

        $this->db->insert('product_related', array('product_id' => $product_id, 'item_product_id' => $related_product_id));
        $this->db->insert('product_related', array('product_id' => $related_product_id, 'item_product_id' => $product_id));

        $this->hook->attach('product.related.add.after', $related_product_id, $product_id, $result, $this);
        return (bool) $result;
    }