Don't use the parent id attribute anymore but respect https://xmpp.org/extensions/xep-0201.html#new for the Reply mechanism

app/Message.php

@@ -39,7 +39,7 @@ class Message extends Model
 
     public function parent()
     {
-        return $this->belongsTo('App\Message', 'parentthread', 'thread');
+        return $this->belongsTo('App\Message', 'parentmid', 'mid');
     }
 
     public function from()
@@ -222,8 +222,15 @@ class Message extends Model
             if ($stanza->thread) {
                 $this->thread = (string)$stanza->thread;
 
-                if ($stanza->thread->attributes()->parent) {
-                    $this->parentthread = (string)$stanza->thread->attributes()->parent;
+                // Resolve the parent message if it exists
+                $parent = $this->user->messages()
+                    ->jid($this->jidfrom)
+                    ->where('thread', $this->thread)
+                    ->orderBy('published', 'asc')
+                    ->first();
+
+                if ($parent) {
+                    $this->parentmid = $parent->mid;
                 }
             }
 

app/widgets/Chat/Chat.php

@@ -414,7 +414,9 @@ class Chat extends \Movim\Widget\Base
                           ->first();
 
             if ($reply) {
-                $m->parentthread = $reply->thread;
+                // See https://xmpp.org/extensions/xep-0201.html#new
+                $m->thread = $reply->thread;
+                $m->parentmid = $reply->mid;
             }
         }
 
@@ -449,7 +451,6 @@ class Chat extends \Movim\Widget\Base
 
         $p->setId($m->id);
         $p->setThreadid($m->thread);
-        $p->setParentthreadid($m->parentthread);
         $p->setOriginid($m->originid);
 
         if ($muc) {

database/migrations/20201106101328_add_parent_mid_to_messages_table.php

@@ -0,0 +1,35 @@
+<?php
+
+use Movim\Migration;
+use Illuminate\Database\Schema\Blueprint;
+
+class AddParentMidToMessagesTable extends Migration
+{
+    public function up()
+    {
+        $this->disableForeignKeyCheck();
+
+        $this->schema->table('messages', function (Blueprint $table) {
+            $table->integer('parentmid')->unsigned()->nullable();
+            $table->dropColumn('parentthread');
+
+            $table->foreign('parentmid')
+                  ->references('mid')->on('messages')
+                  ->onDelete('set null');
+        });
+
+        $this->enableForeignKeyCheck();
+    }
+
+    public function down()
+    {
+        $this->disableForeignKeyCheck();
+
+        $this->schema->table('messages', function (Blueprint $table) {
+            $table->dropColumn('parentmid');
+            $table->string('parentthread', 128)->nullable();
+        });
+
+        $this->enableForeignKeyCheck();
+    }
+}

lib/moxl/src/Stanza/Message.php

@@ -20,8 +20,7 @@ class Message
         $parentId = false,
         array $reactions = [],
         $originId = false,
-        $threadid = false,
-        $parentthreadid = false
+        $threadid = false
     ) {
         $session = Session::start();
 
@@ -55,10 +54,6 @@ class Message
         if ($threadid) {
             $thread = $dom->createElement('thread', $threadid);
 
-            if ($parentthreadid) {
-                $thread->setAttribute('parent', $parentthreadid);
-            }
-
             $root->appendChild($thread);
         }
 

lib/moxl/src/Xec/Action/Message/Publish.php

@@ -18,7 +18,6 @@ class Publish extends Action
     protected $_attachid = false;
     protected $_originid = false;
     protected $_threadid = false;
-    protected $_parentthreadid = false;
 
     public function request()
     {
@@ -26,11 +25,11 @@ class Publish extends Action
         if ($this->_muc) {
             Muc::message($this->_to, $this->_content, $this->_html, $this->_id,
                          $this->_file, $this->_attachid, [], $this->_originid,
-                         $this->_threadid, $this->_parentthreadid);
+                         $this->_threadid);
         } else {
             Message::message($this->_to, $this->_content, $this->_html, $this->_id,
                              $this->_replace, $this->_file, $this->_attachid, [],
-                             $this->_originid, $this->_threadid, $this->_parentthreadid);
+                             $this->_originid, $this->_threadid);
         }
     }