选择下拉值取决于codeigniter php中的第一个下拉值

Question

This is my view file here i use 5 search field like product, group, branch, startdate and end date. All are placed in different table. Both were worked nicely and searched nicely but now i want some additional option that is when i select a particular branch in group field dynamically show values based on branch field value. Now it is showing whole data in db.

NOTE: i want this change appear when dropdown select.

<div class="row">
<div class="col-md-12">
    <div class="tab-content">
        <form method="get" id="frmSearchGroupPur" action="<?php echo 
  base_url('admin/group/listgroup'); ?>">
            <div class="form-body">
                <div class="row">
                    <div class="col-md-4">
                        <div class="form-group">
                            <label class="col-md-5 control-label" 
   style="text-align: left">Product</label>
                            <div class="col-md-7">
                                <?php
                                $product = array();
  //                                    if($data['products']){
  //                                        foreach ($data['products'] as 
  $p) 
  {
 //                                            $product[$p['id']] = 
  $p['name'];
  //                                        }
 //                                    }
                                echo form_dropdown('product', $product, 
 $data['product'], array('class' => 'form-control 
 select2','id'=>'product'));
                                ?>
                            </div>
                        </div>
                    </div>
                        <div class="col-md-4">
                        <div class="form-group">
                            <label class="col-md-5 control-label" 
   style="text-align: left">Branch</label>
                            <div class="col-md-7">
                                <?php
                                $branch = array(NULL=>'Any');
                                if($data['branches']){
                                    foreach ($data['branches'] as $w){
                                        $branch[$w['id']] = $w['name'];
                                    }
                                }
                                echo form_dropdown('branch', $branch, 
 $data['branch'], array('class' => 'form-control','id'=>'branch_dat'));
                                ?>
                            </div>
                        </div>
                    </div>  
                   <div class="col-md-4">
                        <div class="form-group">
                            <label class="col-md-5 control-label" 
   style="text-align: left">Group</label>
                             <div class="col-md-7">
                                <?php
                                $group = array(NULL=>'Any');
                                if($data['groups']){
                                    foreach ($data['groups'] as $g){
                                        $group[$g->id] = $g->name;
                                    }
                                }
                                echo form_dropdown('group', $group, 
  $data['group'], array('class' => 'form-control'));
                                ?>
                            </div>
                        </div>
                    </div>
                </div><br/>
                <div class="row">
                     <div class="col-md-4">
                        <div class="form-group">
                            <label class="col-md-5 control-label" 
 style="text-align: left">Purchase Date From</label>
                            <div class='input-group date col-md-7' 
id='datetimepicker6'>
                                <?php echo 
 form_input('fromdate',$data['fromdate'],array('class' => 'form- 
 control','id'=>'fromdate')); ?>
                                <span class="input-group-addon">
                                    <span class="glyphicon glyphicon- 
 calendar"></span>
                                </span>
                            </div>
                        </div>
                    </div>
                    <div class="col-md-4">
                        <div class="form-group">
                            <label class="col-md-5 control-label" 
  style="text-align: left">Purchase Date To</label>
                            <div class='input-group date col-md-7' 
 id='datetimepicker7'>
                                <?php echo 
  form_input('todate',$data['todate'],array('class' => 'form- 
 control','id'=>'todate')); ?>
                                <span class="input-group-addon">
                                    <span class="glyphicon glyphicon- 
calendar"></span>
                                </span>
                            </div>
                        </div>
                    </div>
                </div><br/>
                <div class="row">
                    <div class="col-md-4">
                        <div class="form-group pull-center">
                            <button type="submit" id="btnFilter" class="btn 
     btn-info">Submit</button>
                            <a href="<?php echo 
    base_url('admin/group/listgroup'); ?>" class="btn btn-default">Reset</a>
                        </div>
                    </div>
                </div>
            </div>
        </form>
    </div>
   </div>

and my model file to get group is

 function getGroupDetails($groupid=0,$state=''){
    $this->db->where('id',$groupid);
    $this->db->where('branchid',$state);
    $query = $this->db->get('group');
    if($query->num_rows()==1){
        return $query->row_array();
    }else{
        return FALSE;
    }

and my controller function is

 function get_group(){
    $state=$this->input->post('brch');
   $result = $this->model_group->getGroupDetails($state);
  // echo json_decode($result);
   }

and my not worked jquery file to change group dynamically is below

  $('branch_dat').change(function(){
  var branch_da=$('#branch_dat').val();
  if(branch_da != "")
  {
  var post_url="admin/sale/get_group";
   $('branch_dat').change(function(){
   var branch_da=$('#branch_dat').val();
  if(branch_da != "")
  {
 var post_url="admin/sale/get_group";
 $.ajax({
  type:"POST",
  url:post_url,
  data:function(params)
  {
    return{ brch : params.brch};
  }
  });
  }
  });