我的分页不起作用(Codeigniter）

我的分页不起作用。我不知道如何修复它。
问题是，每当我点击第二页时，值甚至不会改变URL
这是我的控制器代码

    $data['topic'] = $this->Model_forum->get_topic_per_category(5,$start,$cat); // start= 0 
    $config['base_url'] = base_url().'Forum/forum_per_category/'.$cat.'/';
    $config['total_rows']= $this->Model_forum->count_topic($cat);
    $config['per_page']=5;
    $config['full_tag_open'] = '<ul class="pager">';
    $config['full_tag_close'] = '</ul>';
    $config['next_link'] = 'Next &rarr;';
    $config['next_tag_open'] = '<li class="next page">';
    $config['next_tag_close'] = '</li>';
    $config['prev_link'] = '&larr; Previous';
    $config['prev_tag_open'] = '<li class="prev page" ">';
    $config['prev_tag_close'] = '</li>';
    $config['cur_tag_open'] = '<li class="active"><a href="">';
    $config['cur_tag_close'] = '</a></li>';
    $config['num_tag_open'] = '<li class="page">';
    $config['num_tag_close'] = '</li>';
    $this->pagination->initialize($config);
    $data['pages'] = $this->pagination->create_links();
    $data['category'] = $this->Model_forum->get_category($cat);
    $this->load->view('forum_per_category',$data);
    $this->load->view('includes/footer');

我的型号代码

function get_topic_per_category($num,$start,$cat)
{
     $offset = $this->uri->segment(4);
     $this->db->select('ft.*,', FALSE);
     $this->db->select('IFNULL(COUNT(fc.topic_sk),0) as count', FALSE);
     $this->db->from('forum_topic as ft');
     $this->db->join("forum_comment as fc", ' fc.topic_sk = ft.topic_sk','left');
     $this->db->group_by('ft.topic_sk');
     $this->db->order_by('pinned',"DESC");
     $this->db->order_by("date_posted","DESC");
     $this->db->limit($num,$offset);
     $query= $this->db->get();
     return $query->result();
}