PERF: optimise featuring of topics and users

Previously this would delete all features and recreate, causing
uneeded network traffic and db pain

app/models/category_featured_topic.rb

@@ -5,18 +5,25 @@ class CategoryFeaturedTopic < ActiveRecord::Base
   # Populates the category featured topics
   def self.feature_topics
     transaction do
-      Category.all.each do |c|
-        feature_topics_for(c)
-        CategoryFeaturedUser.feature_users_in(c)
+      current = {}
+      CategoryFeaturedTopic.select(:topic_id, :category_id).order(:rank).each do |f|
+        (current[f.category_id] ||= []) << f.topic_id
+      end
+      Category.select(:id, :topic_id).all.each do |c|
+
+        feature_topics_for(c, current[c.id] || [])
+        CategoryFeaturedUser.feature_users_in(c.id)
       end
     end
   end
 
-  def self.feature_topics_for(c)
+  def self.feature_topics_for(c, existing=nil)
     return if c.blank?
 
     query = TopicQuery.new(self.fake_admin, per_page: SiteSetting.category_featured_topics, except_topic_ids: [c.topic_id], visible: true)
-    results = query.list_category(c).topic_ids.to_a
+    results = query.list_category(c).topic_ids
+
+    return if results == existing
 
     CategoryFeaturedTopic.transaction do
       CategoryFeaturedTopic.delete_all(category_id: c.id)

app/models/category_featured_user.rb

@@ -6,7 +6,14 @@ class CategoryFeaturedUser < ActiveRecord::Base
     5
   end
 
-  def self.feature_users_in(category)
+  def self.feature_users_in(category_or_category_id)
+    category_id =
+      if Fixnum === category_or_category_id
+        category_or_category_id
+      else
+        category_or_category_id.id
+      end
+
     # Figure out most recent posters in the category
     most_recent_user_ids = exec_sql "
       SELECT x.user_id
@@ -21,12 +28,17 @@ class CategoryFeaturedUser < ActiveRecord::Base
       ) AS x
       ORDER BY x.created_at DESC
       LIMIT :max_featured_users;
-    ", category_id: category.id, max_featured_users: max_featured_users
+    ", category_id: category_id, max_featured_users: max_featured_users
+
+    user_ids = most_recent_user_ids.map{|uc| uc['user_id'].to_i}
+    current = CategoryFeaturedUser.where(category_id: category_id).order(:id).pluck(:user_id)
+
+    return if current == user_ids
 
     transaction do
-      CategoryFeaturedUser.delete_all category_id: category.id
-      most_recent_user_ids.each do |uc|
-        create(category_id: category.id, user_id: uc['user_id'])
+      CategoryFeaturedUser.delete_all category_id: category_id
+      user_ids.each do |user_id|
+        create(category_id: category_id, user_id: user_id)
       end
     end
 

app/models/topic_list.rb

@@ -42,8 +42,8 @@ class TopicList
   end
 
   def topic_ids
-    return [] if @topics_input.blank?
-    @topics_input.map {|t| t.id}
+    return [] unless @topics_input
+    @topics_input.pluck(:id)
   end
 
   def attributes