从codeigniter中的多个表中获取数据？

i have two table, one is post table and other one is comment table.in which i am using post_id as foreign_key. Now i want to get, a post with all its comments. and response should be in this format.

 { 
    "posid":1,
    "post_name":"testpost",
    "comments":[{
                    "comment_id":1,
                    "comment_des":"testcoment"
                },{
                    "comment_id":2,
                    "comment_des":"testcoment2"
                }
            ]
    }

Can any one write simple SQL query for me for this type of response?

I tried below query in codeigniter , but this return multiple result, mean one post two time, because one post contains two comments.

  $this->db->select("p.post_id,p.post_desc,p.post_time ,c.id,c.comment_desc,c.comment_time");
  $this->db->join("asoc_comments as c","p.post_id = c.post_id","INNER");
 $response = $this->db->get("asgn_posts as p")->result();