clicking TableTools buttons resets my table data and display only table headings

Niaz

I am using Codeigniter and bootstrap. any suggestion will be appreciated.
## HTML

<div class="tab-content">
            <!----TABLE LISTING STARTS-->
            <div class="tab-pane active" id="list">
                <div class="form-group">
                                <label for="class_id" class="col-sm-2 control-label"><?php echo get_phrase('Select Class');?></label>
                <div class="col-sm-4">
                            <select name="class_id" class="form-control" data-validate="required" id="class_id" 
                                data-message-required="<?php echo get_phrase('value_required');?>"
                                    onchange="return get_class_sections(this.value)">
                              <option value=""><?php echo get_phrase('select class');?></option>
                              <?php 
                              $classes = $this->db->get('class')->result_array();
                                foreach($classes as $row):
                                    ?>
                                    <option value="<?php echo $row['class_id'];?>">
                                            <?php echo $row['name'];?>
                                            </option>
                                <?php
                                endforeach;
                              ?>
                          </select>
                </div>
                </div> 
                    
                <div class="form-group">
                                <label for="section_id" class="col-sm-2 control-label"><?php echo get_phrase('Select Section');?></label>
                <div class="col-sm-4">
                                <select name="section_id" class="form-control" id="section_selector_holder" onchange="return get_class_routines(this.value)">
                                    <option value=""><?php echo get_phrase('select_class_first');?></option>
                                                                        
                                </select>
                </div>
                </div>
            
                
                            <table class="table table-bordered datatable" id="table_export">
                                <thead>
                                    <tr>
                                        <th>Day</th>
                                        <th>Subject</th>
                                        <th>Start Time</th>
                                        <th>End Time</th>
                                    </tr>                   
                                    
                                </thead>
                                <tbody id="putresponse">
                                </tbody>
                            </table>

## JavaScript

<script type="text/javascript">

jQuery(document).ready(function($)
    {
        

        var datatable = $("#table_export").dataTable({
            "sPaginationType": "bootstrap",
            "sDom": "<'row'<'col-xs-3 col-left'l><'col-xs-9 col-right'<'export-data'T>f>r>t<'row'<'col-xs-3 col-left'i><'col-xs-9 col-right'p>>",
            "oTableTools": {
                "aButtons": [
                    
                    {
                        "sExtends": "xls",
                        "mColumns": [0, 1, 2, 3]
                    },
                    {
                        "sExtends": "pdf",
                        "mColumns": [0, 1, 2, 3]
                    },
                    {
                        "sExtends": "print",
                        "fnSetText"    : "Press 'esc' to return",
                        "fnClick": function (nButton, oConfig) {
                            //datatable.fnSetColumnVis(6, false);
                            
                            this.fnPrint( true, oConfig );
                            
                            window.print();
                            
                            $(window).keyup(function(e) {
                                  if (e.which == 27) {
                                      datatable.fnSetColumnVis(4, true);
                                  }
                            });
                        },
                        
                    },
                ]
            },
            
        });
        
        $(".dataTables_wrapper select").select2({
            minimumResultsForSearch: -1
        });
    });
        
</script>

<script type="text/javascript">

    function get_class_routines(section_id) {
        var e = document.getElementById("class_id");
        var class_id = e[e.selectedIndex].value;

        $.ajax({
            url: '<?php echo base_url();?>index.php?admin/get_class_routiness/'+section_id+'/' + class_id,
            success: function(response)
            {
                jQuery('#putresponse').html(response);
            }
        });

    }


</script>

## Controller

function get_class_routiness($section_id, $class_id){
      $query = $this->db->get_where('class_routine', array('class_id' => $class_id , 'section_id' => $section_id ))->result_array();
         if($query){
                        // $html_return = "";
                        foreach ($query as $routine) {
                            $html_return .="<tr><td>"; 
                            $html_return .= $routine['day'];
                            $html_return .="</td><td>";
                            $html_return .= $this->crud_model->get_type_name_by_id('subject',$routine['subject_id']);
                            $html_return .="</td><td>";
                            $html_return .= $routine['time_start'];
                            $html_return .= "</td><td>";
                            $html_return .= $routine['time_end']; 
                            $html_return .="</td></tr>";
                        }
                        
                        echo $html_return;
                    } else {
                        echo 'No record';
                    }
    }