forum: fix topics being incorrectly marked as unread (again).

Second attempt at 71690cacc. Fix topics on page 2+ being still marked as
unread after the user has marked all topics as read.

app/models/forum_topic.rb

@@ -15,6 +15,7 @@ class ForumTopic < ApplicationRecord
   belongs_to_updater
   has_many :posts, -> {order("forum_posts.id asc")}, :class_name => "ForumPost", :foreign_key => "topic_id", :dependent => :destroy
   has_many :forum_topic_visits
+  has_one :forum_topic_visit_by_current_user, -> { where(user_id: CurrentUser.id) }, class_name: "ForumTopicVisit"
   has_many :moderation_reports, through: :posts
   has_one :original_post, -> {order("forum_posts.id asc")}, class_name: "ForumPost", foreign_key: "topic_id", inverse_of: :topic
 
@@ -58,14 +59,18 @@ class ForumTopic < ApplicationRecord
     end
 
     def read_by_user(user)
-      return none if user.last_forum_read_at.nil? || user.last_forum_read_at < '2000-01-01'
+      last_forum_read_at = user.last_forum_read_at || "2000-01-01".to_time
 
       read_topics = user.visited_forum_topics.where("forum_topic_visits.last_read_at >= forum_topics.updated_at")
-      old_topics = where("? >= forum_topics.updated_at", user.last_forum_read_at)
+      old_topics = where("? >= forum_topics.updated_at", last_forum_read_at)
 
       where(id: read_topics).or(where(id: old_topics))
     end
 
+    def unread_by_user(user)
+      where.not(id: ForumTopic.read_by_user(user))
+    end
+
     def sticky_first
       order(is_sticky: :desc, updated_at: :desc)
     end
@@ -110,17 +115,10 @@ class ForumTopic < ApplicationRecord
         ForumTopicVisit.create(:user_id => user.id, :forum_topic_id => id, :last_read_at => updated_at)
       end
 
-      has_unread_topics =
-        ForumTopic
-        .permitted
-        .active
-        .where("forum_topics.updated_at >= ?", user.last_forum_read_at)
-        .joins("left join forum_topic_visits on (forum_topic_visits.forum_topic_id = forum_topics.id and forum_topic_visits.user_id = #{user.id})")
-        .where("(forum_topic_visits.id is null or forum_topic_visits.last_read_at < forum_topics.updated_at)")
-        .exists?
+      unread_topics = ForumTopic.permitted.active.unread_by_user(user)
 
-      unless has_unread_topics
-        user.update_attribute(:last_forum_read_at, Time.now)
+      if !unread_topics.exists?
+        user.update!(last_forum_read_at: Time.zone.now)
         ForumTopicVisit.prune!(user)
       end
     end
@@ -138,6 +136,16 @@ class ForumTopic < ApplicationRecord
     user.level >= min_level
   end
 
+  # XXX forum_topic_visit_by_current_user is a hack to reduce queries on the forum index.
+  def is_read?
+    return true if CurrentUser.is_anonymous?
+
+    topic_last_read_at = forum_topic_visit_by_current_user&.last_read_at || "2000-01-01".to_time
+    forum_last_read_at = CurrentUser.last_forum_read_at || "2000-01-01".to_time
+
+    (topic_last_read_at >= updated_at) || (forum_last_read_at >= updated_at)
+  end
+
   def create_mod_action_for_delete
     ModAction.log("deleted forum topic ##{id} (title: #{title})", :forum_topic_delete)
   end