Migrate unread_comments flag (fix inconsistencies)

db/migrate/20240109180000_fix_unread_comments_inconsistencies.rb

@@ -0,0 +1,55 @@
+# Fixes the unread_comments flag for all users. Unintended behavior was
+# introduced in pull request #515. Migration introduced in #587.
+# Behavior fixed in #585.
+#
+# This migration is generally *not* idempotent since users might have interacted
+# with the website since the migration was run and thus they will probably have
+# different unread comments flags as the ones at the time of the migration.
+#
+# This migration is not reversible as we don't store the previous state of
+# the unread_comments flag.
+class FixUnreadCommentsInconsistencies < ActiveRecord::Migration[7.0]
+  def change
+    num_total_users = 0
+    num_fixed_users = 0
+
+    User.find_each do |user|
+      was_user_fixed = fix_unread_comments_flag(user)
+      num_fixed_users += 1 if was_user_fixed
+      num_total_users += 1
+    end
+
+    Rails.logger.debug { "Ran through #{num_total_users} users (unread comments flag)" }
+    Rails.logger.debug { "Fixed #{num_fixed_users} users (unread comments flag)" }
+  end
+
+  # Fixes the unread_comments flag for a given user.
+  # Returns true if the flag needed a change (and was changed), false otherwise.
+  def fix_unread_comments_flag(user)
+    readers = Reader.where(user_id: user.id)
+    return false if readers.blank?
+
+    had_user_unread_comments = user.unread_comments
+    has_user_unread_comments = false
+
+    readers.each do |reader|
+      thread = Commontator::Thread.find_by(id: reader.thread_id)
+      next if thread.blank? # thread_id should never be nil, just to be sure
+
+      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
+
+      if has_user_unread_comments
+        # user has unread comments, so no need to check other threads
+        break
+      end
+
+      user.update(unread_comments: has_user_unread_comments)
+    end
+
+    had_user_unread_comments != has_user_unread_comments
+  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[7.0].define(version: 2023_11_01_100015) do
+ActiveRecord::Schema[7.0].define(version: 2024_01_09_180000) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
   enable_extension "plpgsql"