CodeIgniter: Counting results from a joined table

Now I know how to join two tables, I'd like to actually count the number of rows in the second table with active records.

Say, I have these two tables:

blog     comments
-------  ----------
id       id
title    blog_id
content  comment

No I want to take the three last blog entries and count the number of comments per blog entry, all in one query. I tried something like this, but it doesn't work:

$this->db->select('*')
         ->from('blog')
         ->order_by('blog.id', 'desc')
         ->limit(3);

$this->db-&g开发者_StackOverflow社区t;join('comments', 'blog_entry_id = blog.id')
         ->group_by('blog_entry_id')
         ->count_all_results('comments'); 

What should I do? What am I doing wrong?

Got it! :)

$this->db->select('blog.*, COUNT(comments.id) as num_comments')
         ->from('blog')
         ->order_by('blog.id', 'desc')
         ->limit(3);

$this->db->join('comments', 'blog_entry_id = blog.id')
         ->group_by('blog_entry_id');

Yes, this is correct way,

$this->db->where('p.status','Active');
        $this->db->where('p.admin_status','Active');
        $this->db->select("p.id,p.date,p.user_id,p.title,p.contents,p.category_id,p.status,p.admin_status,u.username,i.mediaid,COUNT(c.id) as comments")->from('posts as p')->limit($num,$start)->order_by($order,$format);     
        $this->db->join('users u','u.id=p.user_id','left');
        $this->db->join('user_meta m', 'm.user_id = p.id','left');
        $this->db->join('user_profile_image i', 'i.userid = p.id','left');
        $this->db->join('comments c', 'c.postid = p.id','left');        
        return $this->db->get()->result();