forum: optimize unread forum topics on forum index.

Avoid doing one SQL query per topic when checking for new topics on the
forum index.

This also changes it so that forum topics aren't always marked as new
for anonymous users.

app/models/forum_topic.rb

@@ -14,8 +14,10 @@ class ForumTopic < ApplicationRecord
   belongs_to :creator, class_name: "User"
   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_many :moderation_reports, through: :posts
   has_one :original_post, -> {order("forum_posts.id asc")}, class_name: "ForumPost", foreign_key: "topic_id", inverse_of: :topic
+
   before_validation :initialize_is_deleted, :on => :create
   validates_presence_of :title
   validates_associated :original_post
@@ -55,6 +57,11 @@ class ForumTopic < ApplicationRecord
       where("min_level <= ?", CurrentUser.level)
     end
 
+    def read_by_user(user)
+      return none if user.last_forum_read_at.nil? || user.last_forum_read_at < '2000-01-01'
+      merge(user.visited_forum_topics.where("forum_topic_visits.last_read_at >= forum_topics.updated_at OR ? >= forum_topics.updated_at", user.last_forum_read_at))
+    end
+
     def sticky_first
       order(is_sticky: :desc, updated_at: :desc)
     end
@@ -89,16 +96,6 @@ class ForumTopic < ApplicationRecord
   end
 
   module VisitMethods
-    def read_by?(user = nil)
-      user ||= CurrentUser.user
-
-      if user.last_forum_read_at && updated_at <= user.last_forum_read_at
-        return true
-      end
-
-      ForumTopicVisit.where("user_id = ? and forum_topic_id = ? and last_read_at >= ?", user.id, id, updated_at).exists?
-    end
-
     def mark_as_read!(user = CurrentUser.user)
       return if user.is_anonymous?
 

app/models/user.rb

@@ -94,6 +94,8 @@ class User < ApplicationRecord
   has_many :wiki_page_versions, foreign_key: :updater_id
   has_many :feedback, :class_name => "UserFeedback", :dependent => :destroy
   has_many :forum_post_votes, dependent: :destroy, foreign_key: :creator_id
+  has_many :forum_topic_visits, dependent: :destroy
+  has_many :visited_forum_topics, through: :forum_topic_visits, source: :forum_topic
   has_many :moderation_reports, as: :model
   has_many :pools, foreign_key: :creator_id
   has_many :posts, :foreign_key => "uploader_id"