依赖下拉列表不起作用

在下面的代码中，我有一个学生表，当我想更新时，我会从课程主题表中选择一个下拉的课程代码，它应该显示课程主题中相应的subject_code。但我的实际结果是，它在课程代码和主题代码中显示了课程主题的所有值。请任何人帮帮我。

视图：student_detail_view

<section id="tables">
<script>
 jQuery(document).ready(function() {
  oTable = jQuery('#studenttable').dataTable({
   "bJQueryUI": true,
   "sPaginationType": "full_numbers"
  });
   }); 
  </script> 
  <script type="text/javascript" charset="utf-8">
 function get_studentdetails() {
    var exam_name = jQuery('#exam_name_id').val();
    //alert("exam_name"+exam_name);
    jQuery.ajax({
        data: {
            exam_name: exam_name,
        },
        type: 'POST',
        url: 'student_site/studentupdate',          
        success: function(data){
        //alert("inside change");
            console.log(data);
            jQuery('#details').html(data);
            jquery('#deleting').html(data);
        }
    });
    jQuery(document).ready(function(){
    jQuery.ajax({
        data: {
            exam_name: exam_name,
        },
        type: 'POST',
        url: 'student_site/studentcreate',
        success: function(data){
        //alert("inside change");
            console.log(data);
            jQuery('#crud').html(data);
            jquery('#deleting').html(data);
        }
    }); 
    });
}     
function CheckBoxVerification(From)
{
alert('hai');
    alert('hai:' $("table input[type=checkbox]:checked").length);
    if(From == "Insert")
    {
        if(!!$('#rowInsert input:checked').length == false)
        {
            alert('please select at least one check box to Create ...!');
        }
    }
    if(From == "Update")
    {
        if(!!$('#rowUpdate input:checked').length == false)
        {
            alert('please select at least one check box to Update ...!');
        }
    }
}
  function get_subjectdetails() {
    //var index = jQuery('#index').val();
    var course_name = jQuery('#course_name_id').val();
    //alert("course_name"+course_name);
    var exam_name = jQuery('#course_name_id>option:selected').text();
    var exam_name = jQuery('#exam_name_id').val();
    var ssubject_code = jQuery('#ssubject_code_id').val();
    //var partsArray = exam_name.split('.');
    //alert("ssubject_code"+ssubject_code);
    //alert("course_name"+course_name);
    //alert("exam_name"+exam_name);
    jQuery.ajax({
        data: 'exam_name='+exam_name+'&course_name=' + course_name,
        type: 'POST',
        url: 'student_site/subject_records',
        success: function(data){
        //alert("inside change");
            console.log(data);
            //alert ("data"+data);
            //for(var j = course_name; j < ssubject_code; j++)
        //{
            jQuery('#ssubject_code').empty().append(data);
        //}
        }
    });
} 
</script>
<?php 
    $attributes=array(
        'name'=>'updatecustomer',
        'id'=>'updatecustomer'
        );
    echo form_open('student_site/manage_student',$attributes);
?>
<div id="validation_failed">
    <?php
        echo validation_errors();
    ?>
    <?php $data = array();
        foreach ($course_records as $row)
        {
            $data[$row->course_code] = $row->course_code; 
        } 
        $subject_data = array();
        foreach ($all_coursesubject_records as $row)
        {
            $subject_data[$row->subject_code] = $row->subject_code; 
        }  

     ?>
     <div id="Processy">
<table class="display table table-bordered table-striped" id='studenttable'>
<thead>
<tr font style='font-size:13px'>
<th>&nbsp;</th>
<th>&nbsp;</th>
<th>Register Number</th>
<th>Name </th>
<th>Course Code</th>
<th>Subject Code</th>
</tr></thead>
<?php if(isset($records)) : foreach($records as $row) : ?>
<tr >
<td>
<?php echo anchor('student_site/delete/'.$row->id, 'Delete',array('onClick'=>"return confirm('Are you sure want to delete..?')")); ?>
</td>
<td id="rowUpdate">
<input type=checkbox name="editstudent[]" id="editstudent[]" value="<?php echo $row->id ?>">
</td>
<td ><input class="inputmedium span2" type="text" name="register_number_<?php echo $row->id ?>" id="register_number_<?php echo $row->id ?>" value="<?php echo $row->register_number; ?>" ></td>
<td ><input class="inputmedium span2" type="text" name="name_<?php echo $row->id ?>" id="name_<?php echo $row->id ?>" value="<?php echo $row->name; ?>" ></td>
<td >
<?php 
        $js = 'class="dropdown_class" id="course_code_id'.$row->id.'" onChange="get_subjectdetails()" '; 
        $js_name = 'course_code_id'.$row->id;
        echo form_dropdown($js_name, $data, $row->course_code, $js);
?>
</td>
<td>    
<?php 
        $js = 'class="dropdown_class" id="subject_code_id'.$row->id.'"'; 
        $js_name = 'subject_code_id'.$row->id;
        echo form_dropdown($js_name, $subject_data, $row->subject_code, $js);
?>
<div id="ssubject_code" ></div>
<input type="hidden" name="ssubject_code" id="ssubject_code" value="ssubject_code"/>
</td>
</tr>
<?php endforeach; ?>
</table>
</div>
<center>
<br /><input type="submit" class="btn-success btn" value="Update Student"></center>
<?php else : ?>
<h2>No records were returned.</h2>
<?php endif; ?>