Display topic at top of list if has new reply - codeigniter

Question

I have a list of my topics by default it will get the topics where the reply_id is 0 And then arrange them by there date_created.
How every if there is a reply to one of the topics then that topic should go to the top of the list.
Question How can I make sure if there is a new reply to a topic then that topic will go to list.
Currently my var dump as you can see have two items how ever the 2nd array item should be at top because it has a new last post time()
Array
(
[0] => Array
(
[title] => How to
[author] => demo
[author_post_time] => Tue Jun 13 , 2017 19:43:01 pm
[last_post_by] => demo
[latest_post_time] => Tue Jun 13 , 2017 19:43:01 pm
)
[1] => Array
(
[title] => How to install codeigniter
[author] => admin
[author_post_time] => Sat Jun 10 , 2017 23:26:48 pm
[last_post_by] => demo
[latest_post_time] => Tue Jun 13 , 2017 20:48:00 pm
)
)
Image of database
Model
<?php
class Topic_model extends CI_Model
{
public function get_active_topics($fid)
{
$data = array();
$this->db->select('t.*, u.username');
$this->db->from('topics t');
$this->db->join('user u', 'u.user_id = t.user_id');
$this->db->where('t.reply_id', '0');
$this->db->order_by('t.date_created', 'desc');
$query = $this->db->get();
foreach ($query->result_array() as $topic_result)
{
$reply = $this->get_reply($topic_result['topic_id']);
// This $date1 just for display only as shown on image
$date1 = date('D M d', $topic_result['date_created']) . ' , ' . date('Y H:i:s a', $topic_result['date_created']);
// This $date2 just for display only as shown on image
$date2 = date('D M d', $reply['date_created']) . ' , ' . date('Y H:i:s a', $reply['date_created']);
$data[] = array(
'title' => $topic_result['title'],
'author' => $topic_result['username'],
'author_post_time' => $date1,
'last_post_by' => isset($reply['username']) ? $reply['username'] : $topic_result['username'],
'latest_post_time' => ($reply['reply_id'] > 0) ? $date2 : $date1,
);
}
return $data;
}
public function get_reply($reply_id)
{
$this->db->select('t.*, u.username');
$this->db->from('topics t');
$this->db->join('user u', 'u.user_id = t.user_id');
$this->db->where('t.reply_id', $reply_id);
$this->db->limit(1);
$query = $this->db->get();
if ($query->num_rows() > 0) {
return $query->row_array();
}
return false;
}
}

Answer 1

Please try to use below mentioned line, I hope this will fix your issue.
Change From
$this->db->order_by('t.date_created', 'desc');
TO
$this->db->order_by('t.reply_id DESC, t.date_created DESC');

Answer 2

Try ordering your table first by date2 and if that doesnt exist (no reply) then date1. You seem to have it implemented already, you're just ordering by date created on the post.