public static function add_foreign_key($table, $foreign_key)
	{
		if ( ! is_array($foreign_key))
		{
			throw new InvalidArgumentException('Foreign key for add_foreign_key() must be specified as an array');
		}

		$sql = 'ALTER TABLE ';
		$sql .= \DB::quote_identifier(\DB::table_prefix($table, static::$connection)).' ';
		$sql .= 'ADD ';
		$sql .= ltrim(static::process_foreign_keys(array($foreign_key)), ',');

		return \DB::query($sql, \DB::UPDATE)->execute();
	}