生成/保存帐单时，我想减少库存

Question

我正在使用这种方法按照ID保存客户数据和销售量。现在，我想在保存或生成帐单时减少库存。

public function addBill(Request $request)
{
    // $this->validate($request,[
    //     'cus_id'=>'required'
    // ]);
    $customer = new Customer;
    $customer->cus_id_no = $request->cus_id_no;
    $customer->name = $request->name;
    $customer->p_number = $request->p_number;
    $customer->address = $request->address;
    $customer->cus_type = $request->cus_type;
    $customer->total = $request->total;
    $customer->tax = $request->tax;
    $customer->g_total = $request->g_total;
    $customer->paid = $request->paid;
    $customer->remaining = $request->g_total - $request->paid;

    if($customer->save()){
        $id=$customer->id;
        foreach($request->productname as $key=>$v)
        {
            $data = array('cus_id'=>$id,
                      'pro_id'=>$v,
                      'actual_cost'=>$request->actual_cost[$key],
                      'qty'=>$request->qty[$key],
                      'price'=>$request->price[$key],
                      'dis'=>$request->dis[$key],
                      'amount'=>$request->amount[$key]
                    );
            Sale::insert($data);
        }
    }
}

股票是我的型号名称，股票是我的表格名称。