Can't display all item if order by field nama - codeigniter

Question

I have model code function in Codeigniter like below, if me sort order by RAND() can display all item but if order by field name like part_no can't display all item, in next page same display with page 1.
Model
function listProductPerCategory($id, $limit, $start){
$this->db->select('
category.id, category.category, product.id, product.id_category,
product.name, product.picture, product.description, product.permalink, product.part_no
');
$this->db->join('category', 'product.id_category = category.id');
$this->db->where('product.id_category', $id);
$this->db->order_by('product.part_no');
$this->db->limit($limit, $start);
$query = $this->db->get($this->tableProduct);
return $query->result();
}
Controller
function listcategory($page = NULL) {
$id = $this->input->get('list');
$data['categoryHead'] = $this->M_home->listCategory();
$this->load->view('frontend/template/header', $data);
$data['productsLast'] = $this->M_home->listProductLast();
$data['productsLast2'] = $this->M_home->listProductLast2();
$data['category'] = $this->M_home->listCategory();
//paging
$config['base_url'] = base_url('categoryproduct'.'?'.http_build_query($_GET));
$config['total_rows'] = $this->M_home->listProductPerCategory_num_rows($id);
$config['per_page'] = $per_page = 12;
$config['uri_segment'] = 2;
$config['first_link'] = 'First';
$config['last_link'] = 'Last';
$config['next_link'] = 'Next';
$config['prev_link'] = 'Prev';
$config['page_query_string'] = TRUE;
//end paging
$this->pagination->initialize($config);
$data['paging'] = $this->pagination->create_links();
$page = ($this->uri->segment(2)) ? $this->uri->segment(2) : 0;
$data['listCategory'] = $this->M_home->listProductPerCategory($id, $per_page, $page);
$this->load->view('frontend/category', $data);
$this->load->view('frontend/template/footer');
}

Answer 1

You have understand that
In SQL LIMIT, start is starting number of record not PAGE.
But from what you passing to listProductPerCategory is page number
So from page 1 to page 2 it is only one record move on next page not 12 records.
So you need to fix in function listProductPerCategory (dont forget to change variable name which may you confuse yourself , change $start -> $page)
So you should get this as below
function listProductPerCategory($id, $limit, $page){
if($page < 1){
$page = 1;
}
if($page > 1){
$start = $limit * $page;
}else{
$start = 0;
}
$this->db->select('
category.id, category.category, product.id, product.id_category,
product.name, product.picture, product.description, product.permalink, product.part_no
');
$this->db->join('category', 'product.id_category = category.id');
$this->db->where('product.id_category', $id);
$this->db->order_by('product.part_no');
$this->db->limit($limit, $start);
$query = $this->db->get($this->tableProduct);
return $query->result();
}

Answer 2

Controller -
Try this -
function listcategory($page = 0){
$id = $this->input->get('list');
$data['categoryHead'] = $this->M_home->listCategory();
$this->load->view('frontend/template/header', $data);
$data['productsLast'] = $this->M_home->listProductLast();
$data['productsLast2'] = $this->M_home->listProductLast2();
$data['category'] = $this->M_home->listCategory();
//paging
$config['base_url'] = base_url('categoryproduct'.'?'.http_build_query($_GET));
$config['total_rows'] = $this->M_home->listProductPerCategory_num_rows($id);
$config['per_page'] = $per_page = 12;
$config['num_links'] = 2;
$config['uri_segment'] = 3;
$config['first_link'] = 'First';
$config['last_link'] = 'Last';
$config['next_link'] = 'Next';
$config['prev_link'] = 'Prev';
$config['page_query_string'] = TRUE;
//end paging
$this->pagination->initialize($config);
$data['paging'] = $this->pagination->create_links();
$data['page'] = $page;
// $page = ($this->uri->segment(2)) ? $this->uri->segment(2) : 0;
$data['listCategory'] = $this->M_home->listProductPerCategory($id, $per_page, $page);
$this->load->view('frontend/category', $data);
$this->load->view('frontend/template/footer');
Order by condition is wrong in your model.
Try this -
$this->order_by('title','ASC or DESC or RANDOM');