protected function sortAssignments(array $assignments)
    {
        usort($assignments, function (StockAssignmentInterface $a1, StockAssignmentInterface $a2) {
            $u1 = $a1->getStockUnit();
            $u2 = $a2->getStockUnit();

            return $this->compareStockUnit($u1, $u2);
        });

        return $assignments;
    }