PERF: cache category topic ids

app/models/category.rb

@@ -61,6 +61,9 @@ class Category < ActiveRecord::Base
   has_many :category_tag_groups, dependent: :destroy
   has_many :tag_groups, through: :category_tag_groups
 
+  after_save :clear_topic_ids_cache
+  after_destroy :clear_topic_ids_cache
+
   scope :latest, -> { order('topic_count DESC') }
 
   scope :secured, -> (guardian = nil) {
@@ -83,6 +86,18 @@ class Category < ActiveRecord::Base
   # we may consider wrapping this in another spot
   attr_accessor :displayable_topics, :permission, :subcategory_ids, :notification_level, :has_children
 
+  def self.topic_ids
+    @topic_ids ||= Set.new(Category.pluck(:topic_id).compact)
+  end
+
+  def self.clear_topic_ids_cache
+    @topic_ids = nil
+  end
+
+  def clear_topic_ids_cache
+    Category.clear_topic_ids_cache
+  end
+
   def self.last_updated_at
     order('updated_at desc').limit(1).pluck(:updated_at).first.to_i
   end

lib/suggested_topics_builder.rb

@@ -7,7 +7,7 @@ class SuggestedTopicsBuilder
   def initialize(topic)
     @excluded_topic_ids = [topic.id]
     @category_id = topic.category_id
-    @category_topic_ids = Category.pluck(:topic_id).compact
+    @category_topic_ids = Category.topic_ids
     @results = []
   end
 

lib/topic_query.rb

@@ -699,7 +699,7 @@ class TopicQuery
 
     def random_suggested(topic, count, excluded_topic_ids=[])
       result = default_results(unordered: true, per_page: count).where(closed: false, archived: false)
-      excluded_topic_ids += Category.pluck(:topic_id).compact
+      excluded_topic_ids += Category.topic_ids.to_a
       result = result.where("topics.id NOT IN (?)", excluded_topic_ids) unless excluded_topic_ids.empty?
 
       result = remove_muted_categories(result, @user)