FIX: topic id cache should be a multi process cache

app/models/category.rb

@@ -61,8 +61,8 @@ 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
+  after_save :reset_topic_ids_cache
+  after_destroy :reset_topic_ids_cache
 
   scope :latest, -> { order('topic_count DESC') }
 
@@ -86,16 +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
 
+  @topic_id_cache = DistributedCache.new('category_topic_ids')
+
   def self.topic_ids
-    @topic_ids ||= Set.new(Category.pluck(:topic_id).compact)
+    @topic_id_cache['ids'] || reset_topic_ids_cache
   end
 
-  def self.clear_topic_ids_cache
-    @topic_ids = nil
+  def self.reset_topic_ids_cache
+    @topic_id_cache['ids'] = Set.new(Category.pluck(:topic_id).compact)
   end
 
-  def clear_topic_ids_cache
-    Category.clear_topic_ids_cache
+  def reset_topic_ids_cache
+    Category.reset_topic_ids_cache
   end
 
   def self.last_updated_at