private function retrieveGoods($commodityId, $specification)
{
// match
$commodity = Commodity::find($commodityId);
$goods = $commodity->goods->where('specification', $specification);
// constraint error
if ($goods->count() > 0) {
throw new \Exception('Commodities and specification unique constraint error!');
}
// create Goods if not exist
if ($goods->isEmpty()) {
$goods = new Goods();
$goods->commodity()->associate($commodity);
$goods->specification = $specification;
$goods->save();
}
return $goods;
}
