I'm using Laravel 5.1. To make it simple, I have the following code

Migration:

Schema::create('sitemap_data', function (Blueprint $table) {
    // Primary and foreign keys
    $table->increments('id');
    $table->string('postUrl');
    // Database functions
    $table->timestamps();
});

And this is the code somewhere else I'm using

$sitemapData = SitemapData::firstOrNew([
                'postUrl' => $post
            ]);
$sitemapData->save();

Now, according to the Laravel documentation

Again, the updated_at timestamp will automatically be updated, so there is no need to manually set its value

The updated_at value should be updated in the table. However, this is not happening.

It get's only set on the first insert, but not on updating. When I do it manually, like this

$sitemapData = SitemapData::firstOrNew([
                    'postUrl' => $post
                ]);
$sitemapData->updated_at = Carbon::now();
$sitemapData->save();

it works. However, the docs say this should happend automatically, so what is wrong here?

I've searched some sites on stackoverflow for this question, but the ones I found where for Laravel 3 or 4.1, 4.2 etc.

How would I do that correctly?