$(document).ready(function(){

    var data = {"X":['maths', 'bio'],
                      "Y":['maths']};

    $('#lecturer_id').change(function(){


      var selecti = $('#lecturer_id >option:selected').text();
            $('#course_id').empty();


              var i = 0 ;
              for(i = 0; i < data[selecti].length; i++){
                 $("#course_id").append('<option value='+[selecti][i]+'>' + data[selecti][i]  +'</option>');
              }
    });
});