class Invoice
{
/*
..... other stuff
*/
    public function makeValid()
    {
        if ($this->invoice_number && $this->invoice_date) {
            return;
        }

        DB::transaction((function () {
            // lock $this
            Invoice::where('id', $this->id)
                ->lockForUpdate()
                ->first();

            // get and lock the invoice with the highest invoice number
            $largestCurrentInvoice = Invoice::whereNotNull('invoice_number')
                ->orderBy('invoice_number', 'DESC')
                ->lockForUpdate()
                ->first();

            $this->invoice_number = $largestCurrentInvoice
                ? $largestCurrentInvoice->invoice_number + 1
                : config('company.invoice_no_base');

            $this->invoice_date = Carbon::now();

            $this->save();
        }));
}