Observer to watch transactions associated with an invoice and mark paid/unpaid if transaction sums meet invoice total.

Transaction.php

<?php

....
class Transaction extends Model
{

.....

    public function monitor_transaction()
    {
        $invoice = Invoice::find($this->invoice_id);
        if ($invoice) {
            $total_due = $invoice->total;
            $sum = $invoice->transactions->sum('amount');
            Log::debug('Sum: '.$sum);
            if ($sum >= $total_due) {
                $invoice->status = 'paid';
                $invoice->save();
                Log::debug('Marked as paid');
            } else {
                $invoice->status = 'unpaid';
                $invoice->save();
                Log::debug('Marked as unpaid');
            }
        }
    }
}

TransactionsToInvoice.php

<?php

namespace App\Observers;

use App\Models\Transaction;

class TransactionsToInvoice
{
    /**
     * Handle the Transaction "created" event.
     */
    public function created(Transaction $transaction)
    {
        $transaction->monitor_transaction();
    }

    /**
     * Handle the Transaction "updated" event.
     */
    public function updated(Transaction $transaction)
    {
        $transaction->monitor_transaction();
    }

    /**
     * Handle the Transaction "deleted" event.
     */
    public function deleted(Transaction $transaction)
    {
        $transaction->monitor_transaction();
    }
}