public static function delete_by_user_id($user_id = null)
	{
		if (empty($user_id))
		{
			return true;
		}

		$results = self::all()
			->whereEquals('user_id', $user_id)
			->rows();

		foreach ($results as $result)
		{
			if (!$result->destroy())
			{
				return false;
			}
		}

		return true;
	}