naive implementation of post_count on categories

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