Drupal database api delta

Question

I am working on a module which directly updates the data of a set CCK taxonomy checkboxs via ajax.

Everything is working with the exception of the saving to database. Using the following code:

$data = array(
    'vid' => $nid,
    'nid' => $nid,
    'field_grouping_value' => $tid
);
drupal_write_record('content_field_grouping', $data);

I get error messages which complain I am dublicating the value of the 'delta' field. My problem is the code is attempting to write my new data but the delta field is a joint key which does not auto-increament.

Ordinarily, I would just increment the delta field but Drupal handles it so dynamically I'm worried I will do more harm than good:

http://i.stack.imgur.com/yTwoN.gif">

If nid 3 had another field in the above image, it's delta field will have a value of 2. I then delete the field with nid of 3 and delta of 0. Instead of being left with the delta field of values 1 and 2, everything gets reset to 0 and 1

Just so I don't damage my site, I wonder if anyone can advice how I can resolve this issue and get Drupal to dynamically deal with the delta field ..... or is it unnecessary ????