Use this:

class Post extends Eloquent {

    // if you don't want it to be appended always, don't set this one - see below
    protected $appends = ['rating'];

    protected $hidden = ['ratingsSum'];

    public function ratingsSum() {
        return $this->hasOne('Rating')
            ->selectRaw('sum(rating) as aggregate, post_id')
            ->groupBy('post_id');
    }

    public function getRatingAttribute()
    {
		if ( ! array_key_exists('ratingsSum', $this->relations)) $this->load('ratingsSum');

		$relation = $this->getRelation('ratingsSum');

		return ($relation) ? $relation->aggregate : 0;
    }
}

Then you can do this:

$posts = Post::with('ratingsSum')->get();
$posts->toArray();
array(
  ...
  'rating' => '8'
)

// but mind that without eager loading you will face n+1 queries issue:
$posts = Post::get();
$posts->toArray(); // db call for ratingsSum() for each post

If the above (using $appends) doesn't suit you, don't set it and instead do this:

$posts = Post::with('ratingsSum')->get();
foreach ($posts as $post) { $post->setAppends(['rating']); } // when you need it

or leave $appends but don't load rating on demand in the accessor

    public function getRatingAttribute()
    {
		$relation = array_get($this->relations, 'ratingsSum');

		return ($relation) ? $relation->aggregate : null;
    }

// then
$posts = Post::get();
// 'rating' => null

$posts = Post::with('ratingsSum')->get();
// 'rating' => '8'