下拉依赖于codeigniter活动记录

I am having a hard time figuring out how to make a make a drop down dependency(payment terms base on selected class name), when you select the name in the class table you the payment terms base on the sql table (see last table) base selection base on the classes level. I think there is something wrong with my javascript and CI model. I am very new in this codeigniter activerecord, Please help me with my concern and Thank you very much in advance

CLASS TABLE

  classes
  |-------|------------------|
  |  id   |       Name       |
  |-------|------------------|
  |   1   |   Grade 1        |
  |-------|------------------|
  |   2   |   Grade 2        |
  |-------|------------------|

PAYMENT TERMS TABLE

  payment_terms
  |--------|------------------------------------|
  |   id   |       payment_terms                |
  |--------|------------------------------------|
  |   1    |   Grade 1    Full Payment          |
  |--------|------------------------------------|
  |   2    |   Grade 1    Quarterly Payment     |
  |--------|------------------------------------|
  |   3    |   Grade 2    Full Payment          |
  |--------|------------------------------------|
  |   4    |   Grade 2    Quarterly Payment     |
  |--------|------------------------------------|

ACADEMIC YEAR TABLE

  academic_year 
  |--------|------------------------------------|
  |   id   |       academic_year                |
  |--------|------------------------------------|
  |   1    |         2018-2019                  |
  |--------|------------------------------------|

ACADEMIC YEAR AND PAYMENT TERMS TABLE

  academic_year_payment_terms
  |--------|------------------|------------------|
  |   id   | academic_year_id | Payment_term_id  |
  |--------|------------------|------------------|
  |   1    | 1                | 1                |
  |--------|------------------|------------------|
  |   2    | 1                | 2                |
  |--------|------------------|------------------|
  |   3    | 1                | 3                |
  |--------|------------------|------------------|
  |   4    | 1                | 4                |
  |--------|------------------|------------------|

ACADEMIC YEAR PAYMENT TERMS AND CLASS TABLE

  academic_year_payment_terms_class
  |--------|-----------------------------|------------------|
  |   id   | academic_year_payment_terms | class_id         |
  |--------|-----------------------------|------------------|
  |   1    | 1                           | 1                |
  |--------|-----------------------------|------------------|
  |   2    | 2                           | 1                |
  |--------|-----------------------------|------------------|
  |   3    | 3                           | 2                |
  |--------|-----------------------------|------------------|
  |   4    | 4                           | 2                |
  |--------|-----------------------------|------------------|

My Codeigniter View

<div class="form-group">
  <select  id="class_id" name="class_id" class="form-control" >
     <option value=""><?php echo $this->lang->line('select'); ?></option>
         <?php
            foreach ($classlist as $class) { ?>
           <option value="<?php echo $class['id'] ?>"<?php
               if (set_value('class_id') == $class['id']){
                    echo "selected =selected";
               }
               ?>><?php echo $class['class'] ?></option>
            <?php
               $count++;
            }
            ?>
  </select>
</div>
<div class="form-group">
  <select class="form-control" id="academic_year_payment_terms_id" name="academic_year_payment_terms_id">
  </select>
</div>

My JavaScript

<script type="text/javascript">
function getAcademicPaymentTermsByClass(class_id, academic_year_payment_terms_id {
    if (class_id != "" && academic_year_payment_terms_id != "") {
        $('#academic_year_payment_terms_id').html("");
        var base_url = '<?php echo base_url() ?>';
        var div_data = '<option value=""><?php echo $this->lang->line('select'); ?></option>';
        $.ajax({
            type: "GET",
            url: base_url + "enrollment/fetchPaymentTermsByClass",
            data: {'class_id': class_id},
            dataType: "json",
            success: function (data) {
                $.each(data, function (i, obj)
                {
                    var sel = "";
                    if (academic_year_payment_terms_id == obj.academic_year_payment_terms_id) {
                        sel = "selected";
                    }

                    div_data += "<option value=" + obj.academic_year_payment_terms_id+ " " + sel + ">" + obj.payment_terms + "</option>";
                });
                $('#academic_year_payment_terms_id').append(div_data);
            }
        });
    }
}

$(document).ready(function () {
    var class_id = $('#class_id').val();
    var academic_year_payment_terms_id = '<?php echo set_value('academic_year_payment_terms_id') ?>';
    getAcademicPaymentTermsByClass(class_id, academic_year_payment_terms_id);

    $(document).on('change', '#class_id', function (e) {
        $('#academic_year_payment_terms_id').html("");
        var class_id = $(this).val();
        var base_url = '<?php echo base_url() ?>';
        var div_data = '<option value=""><?php echo $this->lang->line('select'); ?></option>';
        $.ajax({
            type: "GET",
            url: base_url + "enrollment/fetchPaymentTermsByClass",
            data: {'class_id': class_id},
            dataType: "json",
            success: function (data) {
                $.each(data, function (i, obj)
                {

                    div_data += "<option value=" + obj.academic_year_payment_terms_id' + ">" + obj.payment_terms' + "</option>";
                });
                $('#academic_year_payment_terms_id').append(div_data);
            }
        });
    });
});

My Enrollment Controller

   function fetchPaymentTermsByClass() {
    $class_id = $this->input->get('class_id');
    $data = $this->Enrollment_model->getPaymentTermsByClass($class_id);
    echo json_encode($data);
   }

My Enrollment model

   public function getPaymentTermsByClass($classid){
    $this->db->select('academic_year_payment_terms_class.id, academic_year_payment_terms_class.academic_year_payment_terms_id, academic_year_payment_terms.payment_terms');
    $this->db->from('academic_year_payment_terms_class');
    $this->db->join('academic_year_payment_terms', 'academic_year_payment_terms.id = academic_year_payment_terms_class.academic_year_payment_terms_id');
    $this->db->where('academic_year_payment_terms_class.class_id', $classid);
    $this->db->order_by('academic_year_payment_terms_class.id');
    $query = $this->db->get();
    return $query->result_array();
    }