Can't get $(this) working in jQueryUI autocomplete

I'm trying to create a generic autocomplete script using jQueryUI. The autocomplete should work for every:

<input type='text' class='autocomplete' id='foo'/>
<input type='text' class='autocomplete' id='bar'/>
...

Now I'm trying to access 'foo' or 'bar' in the source function using $(this), but when alerting I always get 'undefined'.

$('input.autocomplete').autocomplete({
    source: function(req, add){
        var id = $(this).attr('id');
        alert(id);
    }
});

What am I doing wrong?