Framework: Infinite loop when a model is updated in an event listener, and that event is cuased by a preceding update on that same model.

Created on 23 Apr 2018 · 5Comments · Source: laravel/framework

Laravel Version: 5.6.12
PHP Version: docker image php:7.1-fpm-alpine
Database Driver & Version: docker image mysql:5.7

Description:

When a model is updated in an event listener, and that event is caused by a preceding update on that same model, an infinite loop occurs.
This is only the case when the event listener runs synchronously, not asynchronously on a queue.

Steps To Reproduce:

Create a dummy model.

In the model, listen for a model updated event; and if the "status" attribute changes, trigger a MyModelStatusChangedEvent event.

App\Models\MyModel.php

<?php

namespace App\Models;

use App\Events\MyModelStatusChangedEvent;
use Illuminate\Database\Eloquent\Model;

class MyModel extends Model
{
    public static function boot()
    {
        static::updated(function (MyModel $model) {
            if ($model->isDirty('status')) {
                event(
                    new MyModelStatusChangedEvent(
                        $model,
                        $model->getOriginal('status'),
                        $model->status
                    )
                );
            }
        });

        parent::boot();
    }
}

App\EventsMyModelStatusChangedEvent.php

<?php

namespace App\Events;

use App\Models\MyModel;
use Illuminate\Queue\SerializesModels;
use Illuminate\Foundation\Events\Dispatchable;

class MyModelStatusChangedEvent
{
    use Dispatchable, SerializesModels;

    /**
     * @var MyModel
     */
    public $model;

    /**
     * @var string
     */
    public $oldStatus;

    /**
     * @var string
     */
    public $newStatus;

    /**
     * @param MyModel $model
     * @param string $oldStatus
     * @param string $newStatus
     */
    public function __construct(MyModel $model, $oldStatus, $newStatus)
    {
        $this->model = $model;
        $this->oldStatus = $oldStatus;
        $this->newStatus = $newStatus;
    }
}

Create a listener which triggers an update to the same model when the status changes to 'APPROVED'.

App\Listeners\TouchMyModelStatusChangedDate.php

<?php

namespace App\Listeners;

use App\Events\MyModelStatusChangedEvent;

class TouchMyModelStatusChangedDate
{
    /**
     * Handle the event.
     *
     * @param MyModelStatusChangedEvent $event
     */
    public function handle(MyModelStatusChangedEvent $event)
    {
        if ($event->oldStatus === 'APPROVED') {
            \Log::info('Updating blah to bleh after status change to APPROVED.');
            $event->model->update(['blah' => 'bleh']);
        }
    }
}

In your EventServiceProvider.php, bind the listener to the event:

/**
 * The event listener mappings for the application.
 *
 * @var array
 */
protected $listen = [
    // ....

    MyModelStatusChangedEvent::class => [
        TouchMyModelStatusChangedDate::class,
    ],
];

Trigger an update to the status of a dummy model.

$model = \App\Models\MyModel::find(1);
$model->update(['status' => 'APPROVED']);

You would expect the listener to trigger a single update, but it fires multiple times.
This is because the update happens in a listener, on the same model, between the model firing the "updated" event and a call to both syncChanges and syncOriginal.

The status field is seen as dirty in the 2nd update $event->model->update(['blah' => 'bleh']), even though it's already been updated, so this causes a subsequent MyModelStatusChangedEvent to fire, and so on...

Source