Undefined property: Siswa::$siswa_model

<?php
class Siswa extends CI_Controller{
	private $limit=10;

	function _construct()
	{
		parent::__construct();
		#load library dan helper
		$this->load->library(array('table','form_validation'));
		$this->load->helper(array('form','url'));
		$this->load->model('Siswa_model','',TRUE);
	}

	function index($offset=0,$order_column='id',$order_type='asc')
	{
		if(empty($offset)) $offset=0;
		if(empty($order_column)) $order_column='id';
		if(empty($order_type)) $order_type='asc';

		#load data
		$siswas = $this->siswa_model->get_paged_list($this->limit,$offset,$order_column,$order_type) ->result();
		//generate pagination
		$this->load->library('pagination');
		$config['base_url']=site_url('siswa/index/');
		$config['total_rows']=$this->siswa_model->count_all();
		$config['per_page']=$this->limit;
		$config['uri_segment']=3;
		$this->pagination->initialize($config);
		$data['pagination']=$this->pagination->create_links();

		//generate table siswa
		$this->load->library('table');
		$this->table->set_empty("&nbsp;");
		$new_order=($order_type=='asc'?'desc':'asc');
		$this->table->set_heading('No' ,
		anchor('siswa/index'.$offset.'/nama/'.$new_order,'nama'),
		anchor('siswa/index'.$offset.'/alamat/'.$new_order,'alamat'),
		anchor('siswa/index'.$offset.'/kelamin/'.$new_order,'kelamin'),
		anchor('siswa/index'.$offset.'/tgllahir'.$new_order,'tgllahir(dd-mm-yyyy)'),
		'Actions'
		);

		$i=0+$offset;
		foreach ($siswas as $siswa){
			$this->table->add_row(++$i,
			$siswa->nama,
			$siswa->alamat,
			strtoupper($siswa->kelamin)=='M'?'Laki-Laki':'Perempuan',
			date('d-m-y',strtotime($siswa->tgllahir)),
			anchor('siswa/view'.$siswa->id,'view',array('class'=>'view')).''.
			anchor('siswa/update'.$siswa->id,'update',array('class'=>'update')).''.
			anchor('siswa/delete'.$siswa->id,'delete',array('class'=>'delete','onclick'=>"return confirm('yakin hapus data?')"))
			);
		}

		$data['table']=$this->table->generate();

		if ($this->uri->segment(3)=='delete_success')
			$data['message']='data berhasil dihapus';
		else if ($this->uri->segment(3)=='add_success')
			$data['message']='data berhasil ditambah';
		else
			$data['message']='';
			//load view
			$this->load->view('siswaList',$data);
	}

	function add(){
		//set propertis
		$data['title']='tambah siswa baru';
		$data['action']= site_url('siswa/add');
		$data['link_back']=anchor('siswa/index/','back to list siswas',array('class'=>'back'));

		$this->_set_rules();
		//run validation
		if ($this->form_validation->run()==FALSE){
			$data['message']='';
			//set propertis
			$data['title']='add new siswa';
			$data['message']='';
			$data['siswa']['id']='';
			$data['siswa']['nama']='';
			$data['siswa']['alamat']='';
			$data['siswa']['kelamin']='';
			$data['siswa']['tgllahir']='';
			$data['link_back']=anchor('siswa/index/','lihat daftar Siswa',array('class'=>'back'));
			$this->load->view('siswaEdit',$data);

		}else{
			//simpan data
			$siswa=array('nama'=>$this->input->post('nama'),
			'alamat'=>$this->input->post('alamat'),
			'kelamin'=>$this->input->post('kelamin'),
			'tgllahir'=>date('y-m-d',strtotime($this->input->post('tgllahir')))
			);
			$id=$this->siswa_model->save($siswa);
			//set form input nama="id"
			$this->validation->id=$id;
			redirect('siswa/index/add-success');
		}
	}

	function view(){
		$data['title']='siswa detail';
		$data['link_back']=anchor('siswa/index/','lihat daftar siswas',array('class'=>'back'));

		//get detail
		$data['siswa']=$this->siswa_model->get_by_id($id) ->row();
		//load view
		$this->load->view('siswaView',$data);
	}

	function update($id){
		//set propertis
		$data['title']='update siswa';
		$this->load->library('form_validation');
		$this->_set_rules();
		$data['action']=('siswa/update'.$id);
			if($this->form_validation->run()===FALSE){
				$data['message']='';
				$data['siswa']=$this->siswa_model->get_by_id($id)>row_array();
				$_POST['kelamin']=strtoupper($data['siswa']['kelamin']);
				$data['siswa']['tgllahir']=date('d-m-y',strtotime($data['siswa']['tgllahir']));

				$data['title']='update siswa';
				$data['message']='';
			}else{
				//simpan data
				$id=$this->input->post('id');
				$siswa=array('nama'=>$this->input->post('nama'),
				'alamat'=>$this->input->post('alamat'),
				'kelamin'=>$this->input->post('kelamin'),
				'tgllahir'=>date('y-m-d',strtotime($this->input->post('tgllahir'))));
				$this->siswa_model->update($id,$siswa);
				$data['siswa']=$this->siswa_model->get_by_id($id)->row_array();

				//set bershasil update
				$data['message']='update data suuces';

			}
			$data['link_back']=anchor('siswa/index/','lihat daftar siswa',array('class'=>'back'));
			//load view
			$this->load->view('siswaEdit',$data);

	}

function delete($id){
	//query delete
	$this_siswa_model->delete($id);
	redirect('siswa/index/delete_success','refresh');
}

//fungsi validation
function _set_rules(){
	$this->form_validation->set_rules('nama','Nama','required|trim');
	$this->form_validation->set_rules('kelamin','password','required');
	$this->form_validation->set_rules('alamat','Alamat','required|callback_valid_date');
	$this->form_validation->set_rules('tgllahir','tanggal lahir','required');
}

//date validation callback
function valid_date($str){
	if(!preg_match('/^[0-9]{4}-[0-9]{2}-[0-9]{2}$/',$str))
	{
		$this->form_validation->set_message('valid_date','date format not valid. yyyy-mm-dd');
		return false;
	}
	else{
		return true;
	}
}


}