naive implementation of post_count on categories

app/models/category.rb

@@ -102,18 +102,35 @@ class Category < ActiveRecord::Base
   # all categories.
   def self.update_stats
     topics = Topic
-               .select("COUNT(*)")
+               .select("COUNT(*) topic_count")
                .where("topics.category_id = categories.id")
                .where("categories.topic_id <> topics.id")
                .visible
 
-    topic_count = topics.to_sql
+    topics_with_post_count = Topic
+                              .select("topics.category_id, topics.id topic_id, COUNT(*) topic_count, SUM(topics.posts_count) post_count")
+                              .group("topics.category_id, topics.id")
+                              .visible.to_sql
+
     topics_year = topics.created_since(1.year.ago).to_sql
     topics_month = topics.created_since(1.month.ago).to_sql
     topics_week = topics.created_since(1.week.ago).to_sql
 
-    Category.update_all("topic_count = (#{topic_count}),
+
-                         topics_year = (#{topics_year}),
+    Category.exec_sql <<SQL
+    UPDATE categories c
+    SET   topic_count = x.topic_count,
+          post_count = x.post_count
+    FROM (#{topics_with_post_count}) x
+    WHERE x.category_id = c.id AND
+          (c.topic_count <> x.topic_count OR c.post_count <> x.post_count) AND
+          x.topic_id <> c.topic_id
+
+SQL
+
+
+    # TODO don't update unchanged data
+    Category.update_all("topics_year = (#{topics_year}),
                          topics_month = (#{topics_month}),
                          topics_week = (#{topics_week})")
   end

db/migrate/20131017014509_add_post_count_to_categories.rb

@@ -0,0 +1,14 @@
+class AddPostCountToCategories < ActiveRecord::Migration
+  def up
+    add_column :categories, :post_count, :integer, null: false, default: 0
+    execute <<SQL
+    UPDATE categories
+    SET post_count = (SELECT SUM(posts_count) FROM topics
+                      WHERE category_id = categories.id AND deleted_at IS NULL)
+SQL
+  end
+
+  def down
+    remove_column :categories, :post_count
+  end
+end

lib/sql_builder.rb

@@ -77,7 +77,7 @@ class SqlBuilder
     16 => :value_to_boolean
   }
 
-  def map_exec(klass, args = {})
+  def map_exec(klass = OpenStruct, args = {})
     results = exec(args)
 
     setters = results.fields.each_with_index.map do |f, index|

spec/models/category_spec.rb

@@ -255,7 +255,7 @@ describe Category do
 
     context 'with regular topics' do
       before do
-        @category.topics << Fabricate(:topic, user: @category.user)
+        create_post(user: @category.user, category: @category.name)
         Category.update_stats
         @category.reload
       end
@@ -265,6 +265,7 @@ describe Category do
         @category.topics_month.should == 1
         @category.topics_year.should == 1
         @category.topic_count.should == 1
+        @category.post_count.should == 1
       end
 
     end
@@ -282,6 +283,7 @@ describe Category do
         @category.topic_count.should == 0
         @category.topics_month.should == 0
         @category.topics_year.should == 0
+        @category.post_count.should == 0
       end
 
     end