ajax won't let me obtain affected_rows

If you are choosing OOP style for mysqli query then you have to use mysqli object to use any method/property of mysqli.

TRY:
$rowsAffected = $mysqli->affected_rows;

affected_rows is a property of mysqli object and not query. You can refer http://php.net/manual/en/mysqli.affected-rows.php for more information.

That looks like how I am already doing it. I've tried it with and without those parenthesis on the end.