Threaded Messaging System

Am building a threaded messaging system, i have the following tables

conversations_inbox
id
R_id
S_id
message
read
post_time
conv_id

conversation_outbox 
id
R_id
S_id
message
read
post_time
conv_id

conversation_meta
id
sender_id
reciever_id
datetime

The conversations_inbox and conversation_outbox have a foreign key conv_id which tracks a messaging thread between two users I hit a snag when i was working on the messaging inbox, am trying to group the messages in the inbox by the sender_id(S_id) instead 开发者_Go百科of show each new message a user receives as new row.So all the messages to user in the box is grouped under the user that sent it and the newest message from that user is displayed.

i have tried

function get_user_conversations($user_id) {
       //Load Models
       $this->load->model('conversation_model');
       //Load helper
       $this->load->helper('date');
       //database query
       $q = $this->db->select('R_id,message,post_time,read,conv_id')
                     ->from('conversations_inbox')
                     ->where('R_id',$user_id)
                     ->group_by('S_id')
                     ->order_by('post_time','desc')
                     ->get();
        $conversations = $q->result();
        return $conversations;
   }

But it only returns one row of data for each Sender id Please i'd really appreciate if someone could point me in the right direction

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First I'll state that before now I've never heard of codeigniter...

I'm thinking its because you call ->order_by('post_time','desc') after group_by, try switching those two around, order them while they are not grouped is the thought...

Just found this forum with a similar problem