Migrate unread_comments flag (fix inconsistencies) (#587)

* Add dummy migration

* Implement migration for unread comment flag

* Remove unnecessary comment

* Declare migration as not idempotent

* Use array.length instead of counting

* Throw error to prevent revert of migration

* Fix severe flaws in unread comments migration

* Simplify Reader retrieval

* Use the more explicit `.nil?` method

* Update migration date

* Fix annoying bug: don't use `.select!` but `.select`

* Polish migration

e.g. update comment, more suitable name for the method etc.

* Rename method according to #585

db/migrate/20240116180000_fix_unread_comments_inconsistencies.rb

@@ -0,0 +1,56 @@
+# 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 up
+    num_fixed_users = 0
+
+    User.find_each do |user|
+      had_user_unread_comments = user.unread_comments # boolean
+      has_user_unread_comments = user_unread_comments?(user)
+
+      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.debug { "Ran through #{User.count} users (unread comments flag)" }
+    Rails.logger.debug { "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 down
+    raise ActiveRecord::IrreversibleMigration
+  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_16_180000) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
   enable_extension "plpgsql"