Belongs To Relationships: Default Models

/**
 * Get the author of the post.
 */
public function user()
{
    return $this->belongsTo('App\User')->withDefault();
}

/**
 * Get the author of the post.
 */
public function user()
{
    return $this->belongsTo('App\User')->withDefault([
        'name' => 'Guest Author',
    ]);
}

/**
 * Get the author of the post.
 */
public function user()
{
    return $this->belongsTo('App\User')->withDefault(function ($user) {
        $user->name = 'Guest Author';
    });
}