php - Fatal error: Call to undefined method Model_...::function() on controller -Codeigniter -

i got message fatal error: call undefined method model_...::function() happened on 1 function

so here's code

controller :

function paidgen(){   $reportcode=$this->input->get('paid');       $data = array(         'status' => 'proccess'     );    $getcode= $this->model_expreport->changestat($data,$reportcode);    $data['getcode2'] = $getcode;     $this->loadpage($data);  }  function loadpage($data){   parent::header(); $this->load->view('reportlands',$data);    parent::footer(); } 

view :

<input name="paid[]" value="<?php echo $o->reportcode ?>" type="checkbox" class="checkpaid" /> <span class="cr"><i class="cr-icon glyphicon glyphicon-ok"></i></span>   <form method="get" action="<?php echo site_url()? >cexpreport/paidgen" class="pull-right">     <input name="paid[]" value="<?php echo $o->reportcode ?>" type="checkbox" class="checkpaid" /></form> 

i separate input checkbox form javascript think that's not problem, because code work well

model :

 function changestat($data,$reportcode){  $this->db->where('reportcode', $reportcode); $this->db->update('expreport', $data);  } 

hope can give me suggestions ,thank much