Specify a Default Laravel Relationship

default-laravel-relations.php

/**
 * Get the author of the post.
 * @see https://laravel.com/docs/5.7/eloquent-relationships#updating-belongs-to-relationships
 */
public function user()
{
    return $this->belongsTo('App\User')->withDefault([
        'name' => 'Guest Author',
    ]);
}