add.ctp

<script type='text/javascript'>
function addTrack()
{
    var fieldsets = $('#add-release-form .tracks fieldset');
    var numberOfTracks = fieldsets.length + 1;

    $.ajax(
    {
        url: '<?php echo Router::url(array('action' => 'updateAddForm')); ?>/' + numberOfTracks,
        type: 'GET',
        dataType: 'json',
        success: function(data)
        {
            updateForm(data, fieldsets);
        }
    });
}

// snip

function updateForm(data, fieldsets)
{
    var form = $('#add-release-form');

    var tokenKey = form.find('input[name=\'data[_Token][key]\']');
    tokenKey.attr('id', data.token.key.id);
    tokenKey.attr('value', data.token.key.value);

    var tokenFields = form.find('input[name=\'data[_Token][fields]\']');
    tokenFields.attr('id', data.token.fields.id);
    tokenFields.attr('value', data.token.fields.value);

    fieldsets.each(function(index)
    {
        // snip
    });

    if(data.fieldsets.length > fieldsets.length)
    {
        var added = $(data.fieldsets[data.fieldsets.length - 1]);
        form.find('.tracks').append(added);
        return added;
    }

    return null;
}
</script>