Split ThreadFollower into separate models for posts and threads

app/controllers/comments_controller.rb

@@ -59,11 +59,11 @@ 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)
@@ -330,8 +330,8 @@ 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|
@@ -341,7 +341,7 @@ 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/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

@@ -1,15 +1,4 @@
 class ThreadFollower < ApplicationRecord
-  belongs_to :comment_thread, optional: true
-  belongs_to :post, optional: true
+  belongs_to :comment_thread
   belongs_to :user
-
-  validate :thread_or_post
-
-  private
-
-  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

db/migrate/20251203164131_create_new_thread_followers.rb

@@ -0,0 +1,28 @@
+class CreateNewThreadFollowers < ActiveRecord::Migration[7.2]
+  def change
+    create_table_new_thread_followers
+    move_rows_with_non_nil_post_id
+    remove_post_id_column_from_thread_followers
+  end
+
+  def create_table_new_thread_followers
+    create_table :new_thread_followers do |t|
+      t.bigint :user_id
+      t.bigint :post_id
+
+      t.timestamps
+    end
+    add_index :new_thread_followers, [:user_id, :post_id]
+  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)
+    )
+    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
+  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_10_15_121326) do
+ActiveRecord::Schema[7.2].define(version: 2025_12_03_164131) do
   create_table "abilities", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "community_id"
     t.string "name"
@@ -401,6 +401,14 @@
     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 ["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"
@@ -723,9 +731,7 @@
     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
 
@@ -893,7 +899,6 @@
   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"

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/new_thread_followers.yml

@@ -0,0 +1,4 @@
+# Read about fixtures at https://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
+standard_author_question_one:
+  user: standard_user
+  post: question_one

test/fixtures/thread_followers.yml

@@ -5,7 +5,3 @@ deleter_normal:
 standard_author_normal:
   user: standard_user
   comment_thread: normal
-
-standard_author_question_one:
-  user: standard_user
-  post: question_one

test/models/new_thread_follower_test.rb

@@ -0,0 +1,22 @@
+require 'test_helper'
+
+class NewThreadFollowerTest < ActiveSupport::TestCase
+  test 'save succeeds with user and post' do
+    new_thread_follower = NewThreadFollower.new
+    new_thread_follower.user = users(:basic_user)
+    new_thread_follower.post = posts(:question_one)
+    assert new_thread_follower.save
+  end
+
+  test 'save fails without user' do
+    new_thread_follower = NewThreadFollower.new
+    new_thread_follower.post = posts(:question_one)
+    assert_not new_thread_follower.save
+  end
+
+  test 'save fails without post' do
+    new_thread_follower = NewThreadFollower.new
+    new_thread_follower.user = users(:basic_user)
+    assert_not new_thread_follower.save
+  end
+end

test/models/thread_follower_test.rb

@@ -1,7 +1,22 @@
 require 'test_helper'
 
 class ThreadFollowerTest < ActiveSupport::TestCase
-  # test "the truth" do
-  #   assert true
-  # end
+  test 'save succeeds with user and thread' do
+    new_thread_follower = ThreadFollower.new
+    new_thread_follower.user = users(:basic_user)
+    new_thread_follower.comment_thread = comment_threads(:normal)
+    assert new_thread_follower.save
+  end
+
+  test 'save fails without user' do
+    new_thread_follower = ThreadFollower.new
+    new_thread_follower.comment_thread = comment_threads(:normal)
+    assert_not new_thread_follower.save
+  end
+
+  test 'save fails without thread' do
+    new_thread_follower = ThreadFollower.new
+    new_thread_follower.user = users(:basic_user)
+    assert_not new_thread_follower.save
+  end
 end

test/test_helper.rb

@@ -20,6 +20,7 @@
     WarningTemplate,
     ModWarning,
     ThreadFollower,
+    NewThreadFollower,
     Comment,
     CommentThread,
     Reaction,