Mark flawed unread comments migration as stale

db/migrate/20240116180000_fix_unread_comments_inconsistencies.rb

@@ -10,45 +10,80 @@
 # the unread_comments flag.
 class FixUnreadCommentsInconsistencies < ActiveRecord::Migration[7.0]
   def up
-    num_fixed_users = 0
+    # 🛑 This migration contains a bug, please don't use it again. Instead,
+    # refer to the migration in 20240215100000_fix_unread_comments_inconsistencies_again.rb.
+    #
+    # 💡 Explanation of the bug:
+    # The bug resides within the method `user_unread_comments?`. It was supposed
+    # to reflect the logic of the method `comments` in app/controllers/main_controller.rb:
+    #
+    #  def comments
+    #    @media_comments = current_user.subscribed_media_with_latest_comments_not_by_creator
+    #    @media_comments.select! do |m|
+    #      (Reader.find_by(user: current_user, thread: m[:thread])
+    #            &.updated_at || 1000.years.ago) < m[:latest_comment].created_at &&
+    #        m[:medium].visible_for_user?(current_user)
+    #    end
+    #    @media_array = Kaminari.paginate_array(@media_comments)
+    #                           .page(params[:page]).per(10)
+    #  end
+    #
+    # The method `user_unread_comments?` in this migration, however, does not
+    # reflect the logic of the method `comments` in app/controllers/main_controller.rb
+    # precisely. Consider what happens when for each reader instance, the
+    # check reader.updated_at < latest_thread_comment_time is false. In this case,
+    # we don't encounter an early "return true" and therefore go on with the second
+    # part. There, we want to check if there are any *unseen* media. What we actually do
+    # is to check if there are any media with comments (that are not by the creator
+    # and that are visible to the user). But we missed the part where we check if
+    # the user has already seen these comments, i.e. an additional reader query
+    # is missing. This is why suddenly, many more people encounter the original
+    # issue after having run this migration.
+    #
+    # This migration took ~40 minutes to run for ~7000 users at 2024-02-15, 0:40.
+    raise ActiveRecord::IrreversibleMigration
+
+    # For archive reasons, here is the original code:
 
-    User.find_each do |user|
-      had_user_unread_comments = user.unread_comments # boolean
-      has_user_unread_comments = user_unread_comments?(user)
+    # num_fixed_users = 0
 
-      has_flag_changed = (had_user_unread_comments != has_user_unread_comments)
-      user.update(unread_comments: has_user_unread_comments) if has_flag_changed
-      num_fixed_users += 1 if has_flag_changed
-    end
+    # User.find_each do |user|
+    #   had_user_unread_comments = user.unread_comments # boolean
+    #   has_user_unread_comments = user_unread_comments?(user)
 
-    Rails.logger.warn { "Ran through #{User.count} users (unread comments flag)" }
-    Rails.logger.warn { "Fixed #{num_fixed_users} users (unread comments flag)" }
+    #   has_flag_changed = (had_user_unread_comments != has_user_unread_comments)
+    #   user.update(unread_comments: has_user_unread_comments) if has_flag_changed
+    #   num_fixed_users += 1 if has_flag_changed
+    # end
+
+    # Rails.logger.warn { "Ran through #{User.count} users (unread comments flag)" }
+    # Rails.logger.warn { "Fixed #{num_fixed_users} users (unread comments flag)" }
   end
 
   # Checks and returns whether the user has unread comments.
-  def user_unread_comments?(user)
-    # Check for unread comments -- directly via Reader
-    readers = Reader.where(user: user)
-    readers.each do |reader|
-      thread = Commontator::Thread.find_by(id: reader.thread_id)
-      next if thread.nil?
-
-      latest_thread_comment_by_any_user = thread.comments.max_by(&:created_at)
-      next if latest_thread_comment_by_any_user.blank?
-
-      latest_thread_comment_time = latest_thread_comment_by_any_user.created_at
-      has_user_unread_comments = reader.updated_at < latest_thread_comment_time
-
-      return true if has_user_unread_comments
-    end
-
-    # User might still have unread comments but no related Reader objects
-    # -> Check for unread comments -- via Media
-    unseen_media = user.subscribed_media_with_latest_comments_not_by_creator.select do |m|
-      m[:medium].visible_for_user?(user)
-    end
-    unseen_media.present?
-  end
+  # def user_unread_comments?(user)
+  #   # Check for unread comments -- directly via Reader
+  #   readers = Reader.where(user: user)
+  #   readers.each do |reader|
+  #     thread = Commontator::Thread.find_by(id: reader.thread_id)
+  #     next if thread.nil?
+
+  #     latest_thread_comment_by_any_user = thread.comments.max_by(&:created_at)
+  #     next if latest_thread_comment_by_any_user.blank?
+
+  #     latest_thread_comment_time = latest_thread_comment_by_any_user.created_at
+  #     has_user_unread_comments = reader.updated_at < latest_thread_comment_time
+
+  #     return true if has_user_unread_comments
+  #   end
+
+  #   # User might still have unread comments but no related Reader objects
+  #   # -> Check for unread comments -- via Media
+  #   unseen_media = user.subscribed_media_with_latest_comments_not_by_creator.select do |m|
+  #     m[:medium].visible_for_user?(user)
+  #   end
+  #   unseen_media.present?
+  # end
 
   def down
     raise ActiveRecord::IrreversibleMigration