Update the Post counter caches when adding the counter cache column

db/migrate/20110705003445_counter_cache_on_post_likes.rb

 class CounterCacheOnPostLikes < ActiveRecord::Migration
+  class Post < ActiveRecord::Base; end
   def self.up
     add_column :posts, :likes_count, :integer, :default => 0
+    execute <<SQL if Post.count > 0
+      UPDATE posts
+      SET posts.likes_count = (SELECT COUNT(*) FROM likes WHERE likes.post_id = posts.id)
+SQL
   end
 
   def self.down