Split ThreadFollower into separate models for posts and threads

app/controllers/comments_controller.rb

@@ -60,11 +60,11 @@ def create_thread
         @comment.post.user.create_notification(notification, helpers.comment_link(@comment))
       end
 
-      ThreadFollower.where(post: @post).each do |tf|
-        unless tf.user == current_user || tf.user == @comment.post.user
-          tf.user.create_notification(notification, helpers.comment_link(@comment))
+      NewThreadFollower.where(post: @post).each do |ntf|
+        unless ntf.user == current_user || ntf.user == @comment.post.user
+          ntf.user.create_notification(notification, helpers.comment_link(@comment))
         end
-        ThreadFollower.create(user: tf.user, comment_thread: @comment_thread)
+        ThreadFollower.create(user: ntf.user, comment_thread: @comment_thread)
       end
 
       apply_pings(pings)
@@ -331,8 +331,8 @@ def post
   end
 
   def post_follow
-    if ThreadFollower.where(post: @post, user: current_user).none?
-      ThreadFollower.create(post: @post, user: current_user)
+    if NewThreadFollower.where(post: @post, user: current_user).none?
+      NewThreadFollower.create(post: @post, user: current_user)
     end
 
     respond_to do |format|
@@ -342,7 +342,7 @@ def post_follow
   end
 
   def post_unfollow
-    ThreadFollower.where(post: @post, user: current_user).destroy_all
+    NewThreadFollower.where(post: @post, user: current_user).destroy_all
 
     respond_to do |format|
       format.html { redirect_to post_path(@post) }

app/jobs/clean_up_new_thread_followers_job.rb

@@ -0,0 +1,24 @@
+class CleanUpNewThreadFollowersJob < ApplicationJob
+  queue_as :default
+
+  def perform
+    sql = File.read(Rails.root.join('db/scripts/posts_with_duplicate_new_thread_followers.sql'))
+    posts = ActiveRecord::Base.connection.execute(sql).to_a
+
+    posts.each do |post|
+      user_id, post_id = post
+
+      followers = NewThreadFollower.where(post_id: post_id, user_id: user_id)
+
+      next unless followers.many?
+
+      duplicate = followers.first
+      result = duplicate.destroy
+
+      unless result
+        puts "failed to destroy new thread follower duplicate \"#{duplicate.id}\""
+        duplicate.errors.each { |e| puts e.full_message }
+      end
+    end
+  end
+end

app/models/concerns/user_merge.rb

@@ -71,6 +71,7 @@ def update_thread_references(target_user)
       CommentThread.where(archived_by_id: id).update_all(archived_by_id: target_user.id)
       CommentThread.where(deleted_by_id: id).update_all(deleted_by_id: target_user.id)
       ThreadFollower.where(user_id: id).update_all(user_id: target_user.id)
+      NewThreadFollower.where(user_id: id).update_all(user_id: target_user.id)
     end
 
     def update_post_action_references(target_user)

app/models/new_thread_follower.rb

@@ -0,0 +1,4 @@
+class NewThreadFollower < ApplicationRecord
+  belongs_to :user
+  belongs_to :post
+end

app/models/post.rb

@@ -243,7 +243,7 @@ def deleted_by_owner?
   # @param user [User] user to check
   # @return [Boolean] check result
   def followed_by?(user)
-    ThreadFollower.where(post: self, user: user).any?
+    NewThreadFollower.where(post: self, user: user).any?
   end
 
   # Is the post an imported post?

app/models/thread_follower.rb

@@ -1,22 +1,13 @@
 class ThreadFollower < ApplicationRecord
-  belongs_to :comment_thread, optional: true
-  belongs_to :post, optional: true
+  belongs_to :comment_thread
   belongs_to :user
 
   after_create :bump_thread_last_activity
   before_destroy :bump_thread_last_activity
 
-  validate :thread_or_post
-
   private
 
   def bump_thread_last_activity
     comment_thread&.bump_last_activity(persist_changes: true)
   end
-
-  def thread_or_post
-    if comment_thread.nil? && post.nil?
-      errors.add(:base, 'Must refer to either a comment thread or a post.')
-    end
-  end
 end

config/schedule.rb

@@ -30,6 +30,10 @@
   runner 'scripts/run_thread_followers_cleanup.rb'
 end
 
+every 7.days, at: '02:40' do
+  runner 'scripts/run_new_thread_followers_cleanup.rb'
+end
+
 every 6.hours do
   runner 'scripts/recalc_abilities.rb'
 end

db/migrate/20251226185531_create_new_thread_followers.rb

@@ -0,0 +1,63 @@
+class CreateNewThreadFollowers < ActiveRecord::Migration[7.2]
+  def up
+    create_table_new_thread_followers
+    if column_exists?(:thread_followers, :post_id)
+      move_rows_with_non_nil_post_id
+      remove_post_id_column_from_thread_followers
+    end
+  end
+
+  def down
+    if !column_exists?(:thread_followers, :post_id)
+      add_post_id_column_to_thread_followers
+    end
+    move_rows_back_from_new_thread_followers
+    delete_table_new_thread_followers
+  end
+
+  def create_table_new_thread_followers
+    create_table :new_thread_followers, if_not_exists: true do |t|
+      t.bigint :user_id
+      t.bigint :post_id
+
+      t.timestamps
+    end
+    add_index :new_thread_followers, [:user_id, :post_id], if_not_exists: true
+    add_index :new_thread_followers, :post_id, if_not_exists: true
+  end
+
+  def move_rows_with_non_nil_post_id
+    NewThreadFollower.insert_all(
+      ThreadFollower.select(:user_id, :post_id, :created_at, :updated_at)
+        .where.not(post_id:nil)
+        .to_a
+        .map(&:attributes)
+    )
+    ThreadFollower.where.not(post_id:nil).delete_all
+  end
+
+  def remove_post_id_column_from_thread_followers
+    remove_reference :thread_followers, :post, index: true, foreign_key: true, if_exists: true
+  end
+
+  def add_post_id_column_to_thread_followers
+    add_reference :thread_followers, :post, index: true, foreign_key: true, if_not_exists: true
+  end
+
+  def move_rows_back_from_new_thread_followers
+    ThreadFollower.insert_all(
+      NewThreadFollower.select(:user_id, :post_id, :created_at, :updated_at)
+        .to_a
+        .map(&:attributes)
+    )
+    NewThreadFollower.delete_all
+  end
+
+  def delete_table_new_thread_followers
+    remove_index :new_thread_followers, :post_id, if_exists: true
+    remove_index :new_thread_followers, [:user_id, :post_id], if_exists: true
+    remove_reference :new_thread_followers, :user_id, foreign_key: true, if_exists: true
+    remove_reference :new_thread_followers, :post_id, foreign_key: true, if_exists: true
+    drop_table :new_thread_followers, if_exists: true
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2025_12_21_142105) do
+ActiveRecord::Schema[7.2].define(version: 2025_12_26_185531) do
   create_table "abilities", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "community_id"
     t.string "name"
@@ -403,6 +403,15 @@
     t.index ["user_id"], name: "index_micro_auth_tokens_on_user_id"
   end
 
+  create_table "new_thread_followers", charset: "utf8mb4", collation: "utf8mb4_unicode_ci", force: :cascade do |t|
+    t.bigint "user_id"
+    t.bigint "post_id"
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["post_id"], name: "index_new_thread_followers_on_post_id"
+    t.index ["user_id", "post_id"], name: "index_new_thread_followers_on_user_id_and_post_id"
+  end
+
   create_table "notifications", charset: "utf8mb4", collation: "utf8mb4_unicode_ci", force: :cascade do |t|
     t.text "content"
     t.string "link"
@@ -725,9 +734,7 @@
     t.bigint "user_id"
     t.datetime "created_at", precision: nil, null: false
     t.datetime "updated_at", precision: nil, null: false
-    t.bigint "post_id"
     t.index ["comment_thread_id"], name: "index_thread_followers_on_comment_thread_id"
-    t.index ["post_id"], name: "index_thread_followers_on_post_id"
     t.index ["user_id"], name: "index_thread_followers_on_user_id"
   end
 
@@ -895,7 +902,6 @@
   add_foreign_key "tag_synonyms", "tags"
   add_foreign_key "tags", "communities"
   add_foreign_key "tags", "tags", column: "parent_id"
-  add_foreign_key "thread_followers", "posts"
   add_foreign_key "user_abilities", "abilities"
   add_foreign_key "user_abilities", "community_users"
   add_foreign_key "user_websites", "users"

db/scripts/posts_with_duplicate_new_thread_followers.sql

@@ -0,0 +1,11 @@
+select
+  user_id,
+  post_id,
+  count(*) as count
+from
+  new_thread_followers
+group by
+  user_id,
+  post_id
+having
+  count > 1;

db/scripts/threads_with_duplicate_followers.sql

@@ -4,8 +4,6 @@ select
   count(*) as count
 from
   thread_followers
-where
-  post_id is null
 group by
   user_id,
   comment_thread_id

scripts/run_new_thread_followers_cleanup.rb

@@ -0,0 +1 @@
+CleanUpNewThreadFollowersJob.perform_later

test/controllers/comments/post_follow_test.rb

@@ -11,13 +11,13 @@ class CommentsControllerTest < ActionController::TestCase
     question = posts(:question_one)
 
     # Assert user follows post
-    assert_equal 1, ThreadFollower.where(['post_id = ? AND user_id = ?', question, user]).count
+    assert_equal 1, NewThreadFollower.where(['post_id = ? AND user_id = ?', question, user]).count
 
     try_post_unfollow(question)
     assert_response(:found)
 
     # Assert user does not follow post
-    assert_equal 0, ThreadFollower.where(['post_id = ? AND user_id = ?', question, user]).count
+    assert_equal 0, NewThreadFollower.where(['post_id = ? AND user_id = ?', question, user]).count
   end
 
   test 'non-follower can follow post' do
@@ -26,13 +26,13 @@ class CommentsControllerTest < ActionController::TestCase
     question = posts(:question_one)
 
     # Assert user does not follow post
-    assert_equal 0, ThreadFollower.where(['post_id = ? AND user_id = ?', question, user]).count
+    assert_equal 0, NewThreadFollower.where(['post_id = ? AND user_id = ?', question, user]).count
 
     try_post_follow(question)
     assert_response(:found)
 
     # Assert user follows post
-    assert_equal 1, ThreadFollower.where(['post_id = ? AND user_id = ?', question, user]).count
+    assert_equal 1, NewThreadFollower.where(['post_id = ? AND user_id = ?', question, user]).count
   end
 
   test 'follower cannot duplicate the following of a post' do
@@ -41,12 +41,12 @@ class CommentsControllerTest < ActionController::TestCase
     question = posts(:question_one)
 
     # Assert user follows post
-    assert_equal 1, ThreadFollower.where(['post_id = ? AND user_id = ?', question, user]).count
+    assert_equal 1, NewThreadFollower.where(['post_id = ? AND user_id = ?', question, user]).count
 
     try_post_follow(question)
     assert_response(:found)
 
     # Assert user still only follows post once
-    assert_equal 1, ThreadFollower.where(['post_id = ? AND user_id = ?', question, user]).count
+    assert_equal 1, NewThreadFollower.where(['post_id = ? AND user_id = ?', question, user]).count
   end
 end

test/fixtures/community_users.yml

@@ -140,3 +140,17 @@ sample_post_scorer:
   is_admin: false
   is_moderator: false
   reputation: 1
+
+sample_merge_source:
+  user: merge_source
+  community: sample
+  is_admin: false
+  is_moderator: false
+  reputation: 42
+
+sample_merge_target:
+  user: merge_target
+  community: sample
+  is_admin: false
+  is_moderator: false
+  reputation: 24

test/fixtures/new_thread_followers.yml

@@ -0,0 +1,8 @@
+# Read about fixtures at https://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
+standard_author_question_one:
+  user: standard_user
+  post: question_one
+
+merge_source_question_one:
+  user: merge_source
+  post: question_one

test/fixtures/posts.yml

@@ -547,3 +547,17 @@ deleted_spam_question:
   deleted: true
   deleted_at: 2019-01-01T00:00:00.000000Z
   deleted_by: moderator
+
+without_new_thread_followers:
+  post_type: question
+  title: This post does not have any new thread followers
+  body: This is the body of the post used to test new thread followers
+  body_markdown: <p>This is the body of the post used to test new thread followers</p>
+  tags_cache:
+    - support
+  tags:
+    - test
+  user: standard_user
+  community: sample
+  category: main
+  license: cc_by_sa

test/fixtures/thread_followers.yml

@@ -6,6 +6,6 @@ standard_author_normal:
   user: standard_user
   comment_thread: normal
 
-standard_author_question_one:
-  user: standard_user
-  post: question_one
+merge_source_normal:
+  user: merge_source
+  comment_thread: normal

test/fixtures/user_abilities.yml

@@ -30,6 +30,14 @@ pp_eo:
   community_user: sample_post_scorer
   ability: everyone
 
+ms_eo:
+  community_user: sample_merge_source
+  ability: everyone
+
+mt_eo:
+  community_user: sample_merge_target
+  ability: everyone
+
 stu_ur:
   community_user: sample_standard_user
   ability: unrestricted
@@ -62,6 +70,14 @@ pp_ur:
   community_user: sample_post_scorer
   ability: unrestricted
 
+ms_ur:
+  community_user: sample_merge_source
+  ability: unrestricted
+
+mt_ur:
+  community_user: sample_merge_target
+  ability: unrestricted
+
 c_fc:
   community_user: sample_closer
   ability: flag_close

test/fixtures/users.yml

@@ -208,3 +208,17 @@ post_scorer:
   sign_in_count: 1337
   username: post_scorer
   confirmed_at: 2020-01-01T00:00:00.000000Z
+
+merge_source:
+  email: merge-source@example.com
+  encrypted_password: '$2a$11$roUHXKxecjyQ72Qn7DWs3.9eRCCoRn176kX/UNb/xiue3aGqf7xEW'
+  sign_in_count: 1337
+  username: merge_source
+  confirmed_at: 2020-01-01T00:00:00.000000Z
+
+merge_target:
+  email: merge-target@example.com
+  encrypted_password: '$2a$11$roUHXKxecjyQ72Qn7DWs3.9eRCCoRn176kX/UNb/xiue3aGqf7xEW'
+  sign_in_count: 1337
+  username: merge_target
+  confirmed_at: 2020-01-01T00:00:00.000000Z