Fixing incorrect topic participants

The alias-moving code had a logic issue, which sometimes created
incorrect topic participations.

This commit fixes the issue, but we also have to recalculate all
affected records. We do this by querying topic participations where we
see somebody registered who didn't really post messages to that topic,
and completely rebuild the records for these threads.

Author: dutow

app/jobs/person_id_propagation_job.rb

@@ -1,10 +1,7 @@
 class PersonIdPropagationJob < ApplicationJob
   queue_as :default
 
-  def perform(alias_id, new_person_id)
-    alias_record = Alias.find_by(id: alias_id)
-    old_person_id = alias_record&.person_id_before_last_save || find_old_person_id(alias_id, new_person_id)
-
+  def perform(alias_id, new_person_id, old_person_id)
     # Update topics where this alias is the creator
     Topic.where(creator_id: alias_id)
          .where.not(creator_person_id: new_person_id)
@@ -27,15 +24,6 @@ def perform(alias_id, new_person_id)
 
   private
 
-  def find_old_person_id(alias_id, new_person_id)
-    # Try to find the old person by looking at topic_participants that reference
-    # topics where this alias sent messages but the participant has a different person_id
-    topic_ids = Message.where(sender_id: alias_id).select(:topic_id).distinct
-    TopicParticipant.where(topic_id: topic_ids)
-                    .where.not(person_id: new_person_id)
-                    .pick(:person_id)
-  end
-
   def merge_topic_participants(old_person_id, new_person_id)
     is_new_contributor = ContributorMembership.exists?(person_id: new_person_id)
     affected_topic_ids = []

app/models/alias.rb

@@ -111,7 +111,8 @@ def contributor_badge
   def queue_person_id_propagation
     return unless person_id.present?
 
-    PersonIdPropagationJob.perform_later(id, person_id)
+    old_person_id = person_id_before_last_save
+    PersonIdPropagationJob.perform_later(id, person_id, old_person_id)
   end
 
   def auto_star_recent_topics

script/fix_topic_participants.rb

@@ -0,0 +1,68 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+# Fix incorrect TopicParticipant records.
+#
+# This script finds TopicParticipant records where the person has no messages
+# in that topic (orphaned records created by a bug in PersonIdPropagationJob),
+# and rebuilds the correct participant data from actual messages.
+#
+# Usage:
+#   ruby script/fix_topic_participants.rb           # dry run, report only
+#   ruby script/fix_topic_participants.rb --fix     # actually fix the records
+
+require_relative '../config/environment'
+
+dry_run = !ARGV.include?('--fix')
+
+puts "Finding topics with incorrect TopicParticipant records..."
+puts "(dry run - use --fix to apply changes)" if dry_run
+puts
+
+# Find TopicParticipant records where the person has no messages in that topic
+orphaned_participants = TopicParticipant.where(<<~SQL)
+  NOT EXISTS (
+    SELECT 1 FROM messages
+    WHERE messages.topic_id = topic_participants.topic_id
+      AND messages.sender_person_id = topic_participants.person_id
+  )
+SQL
+
+affected_topic_ids = orphaned_participants.distinct.pluck(:topic_id)
+
+if affected_topic_ids.empty?
+  puts "No incorrect TopicParticipant records found."
+  exit 0
+end
+
+puts "Found #{orphaned_participants.count} orphaned TopicParticipant records across #{affected_topic_ids.size} topics."
+puts
+
+# Show details of affected topics
+affected_topic_ids.each do |topic_id|
+  topic = Topic.find(topic_id)
+  orphaned = orphaned_participants.where(topic_id: topic_id).includes(:person)
+
+  puts "Topic ##{topic_id}: #{topic.title.truncate(60)}"
+  orphaned.each do |tp|
+    person_name = tp.person&.display_name || "Person ##{tp.person_id}"
+    puts "  - #{person_name} (#{tp.message_count} messages recorded, 0 actual)"
+  end
+end
+
+puts
+
+if dry_run
+  puts "Run with --fix to rebuild these #{affected_topic_ids.size} topics."
+else
+  puts "Rebuilding #{affected_topic_ids.size} topics..."
+
+  affected_topic_ids.each_with_index do |topic_id, index|
+    topic = Topic.find(topic_id)
+    topic.recalculate_participants!
+    puts "  [#{index + 1}/#{affected_topic_ids.size}] Fixed topic ##{topic_id}"
+  end
+
+  puts
+  puts "Done. Rebuilt #{affected_topic_ids.size} topics."
+end