AJAX call and data return through JSON in ROR

Hello all,

This could be a tootally boonie question. i am trying to do this very
simple app.
I have a simple form where user enters data,once entered, i add that
data in a div 'data' . which works fine but i want to save that data
perodically(all the data in DIV 'data') or if user hits a save button
with each data row.

Now one thing i am not sure about that when user hits button 'save
data; for a row,i execute the following

new Ajax.Request('/saveData', {method: 'post',asynchronous:true,
evalScripts:true,onSuccess:DoASNeeded()});

which saves the data but i am not sure how to retrieve the primarky
key of saved record and update it async. to that row. i am not sure
how to retrieve PK of saved record and use it later for any
modifications.

please help