How to populate second dropdown based on selection of first dropdown using jQuery/AJAX and PHP/MySQL?

I am trying to create a dynamic set of dropdown boxes, using jQuery/AJAX and PHP/MySQL. The first dropdown box will be populated when the page loads based on values from a database. The second dropdown box should display a set of values based on the selection from the first dropdown box. I know there have been similar questions asked on here before, but I haven't found a solution that matches my scenario.

My query to generate a JSON encoded list of values for the second drop down is functioning, but I am having issues populating it into the actual dropdown form element. Any ideas on where I'm going wrong.

Javascript:

<script>
$().ready(function() {

    $("#item_1").change(function () {   

      var group_id = $(this).val();

       $.ajax({
            type: "POST", 
            url: "../../db/groups.php?item_1_id=" + group_id, 
            dataType: "json",
            success: function(data){
                //Clear options corresponding to earlier option of first dropdown
                $('select#item_2').empty(); 
                $('select#item_2').append('<option value="0">Select Option</option>');
                //Populate options of the second dropdown
                $.each( data.subjects, function(){    
                    $('select#item_2').append('<option value="'+$(this).attr('group_id')+'">'+$(this).attr('name')+'</option>');
                });
                $('select#item_2').focus();
            },
            beforeSend: function(){
                $('select#item_2').empty();
                $('select#item_2').append('<option value="0">Loading...</option>');
            },
            error: function(){
                $('select#item_2').attr('disabled', true);
                $('select#item_2').empty();
                $('select#item_2').append('<option value="0">No Options</option>');
            }
        })  

    }); 
});

</script>