public static function related_class($relation)
	{
		$class = get_called_class();

		foreach (static::$_valid_relations as $rel_name => $rel_class)
		{
			if (property_exists($class, '_'.$rel_name))
			{
				if (isset(static::${'_'.$rel_name}[$relation]))
				{
					return static::${'_'.$rel_name}[$relation]['model_to'];
				}
			}
		}

		return null;
	}