Maintain single thread between two users in privatemsg module similar to facebook

If you need Facebook like single thread messaging between two users even while using new message form, using privatemsg in your drupal website.
I have a patch for the privatemsg module.

There is a better way though, If you dont want to patch the privatemsg module,
you can use hook_privatemsg_message_presave_alter();

You can try db_query() to get an existing thread between current user and the recipient and then change the $message->thread_id there.

For sake of easier illustration I have created a patch

Index: privatemsg.module
===================================================================
--- privatemsg.module 
+++ privatemsg.module 
@@ -1832,7 +1836,7 @@
       ->fields($args)
       ->execute();
     $message->mid = $mid;
-
+    
     // Thread ID is the same as the mid if it's the first message in the thread.
     if (!isset($message->thread_id)) {
       $message->thread_id = $mid;
@@ -1841,6 +1845,21 @@
     // 2) Save message to recipients.
     // Each recipient gets a record in the pm_index table.
     foreach ($message->recipients as $recipient) {
+      // To continue old msg thread between two users when submitting new msg
+      if(count($message->recipients) == 1){
+         $res = db_query("select author, recipient, thread_id , name from {pm_message} msg
+              left join {pm_index} ind on msg.mid = ind.mid
+              left join {users} u on u.uid = ind.recipient
+              where ((recipient = :recipient and author = :uid) or (author = :recipient and recipient = :uid)) 
+              and (select count(distinct(recipient)) from {pm_index} where thread_id=ind.thread_id ) < 3
+              order by thread_id desc",array(':recipient'=>$recipient->recipient, ':uid' => $message->author->uid));
+         if($thread = $res->fetchObject()){
+           $thread_id = $thread->thread_id;
+           $_REQUEST['destination'] = 'messages/view/' . $thread_id;
+           $message->thread_id = $thread_id;
+         }
+         
+      }
       $query->values(array(
         'mid' => $mid,
         'thread_id' => $message->thread_id,
Index: privatemsg.pages.inc
===================================================================
--- privatemsg.pages.inc (revision 14816)
+++ privatemsg.pages.inc (working copy)
@@ -638,6 +638,10 @@
       elseif (!empty($redirect)) {
         $form_state['redirect'] = $redirect;
       }
+    }else{
+      if($_REQUEST['destination'])
+        $form_state['redirect'] = $_REQUEST['destination'];
     }
   }
   catch (Exception $e) {

Here I have tested if the message is sent to a single recipient, and done the query to get one existing thread between the current user and recipient and then assign the thread_id to the $message object.

<?php 
if(count($message->recipients) == 1){
         $res = db_query("select author, recipient, thread_id , name from {pm_message} msg
              left join {pm_index} ind on msg.mid = ind.mid
              left join {users} u on u.uid = ind.recipient
              where ((recipient = :recipient and author = :uid) or (author = :recipient and recipient = :uid)) 
              and (select count(distinct(recipient)) from {pm_index} where thread_id=ind.thread_id ) < 3
              order by thread_id desc",array(':recipient'=>$recipient->recipient, ':uid' => $message->author->uid));
         if($thread = $res->fetchObject()){
           $thread_id = $thread->thread_id;
           $_REQUEST['destination'] = 'messages/view/' . $thread_id;
           $message->thread_id = $thread_id;
         }
         
      }
?>

Nothing much on privatemsg.pages.inc I have added the redirect to the thread view page, assigned previously in _privatemsg_send().

+           $_REQUEST['destination'] = 'messages/view/' . $thread_id;

so that after new msg is sent the user gets into the thread view.

+      if($_REQUEST['destination'])
+        $form_state['redirect'] = $_REQUEST['destination'];