Counter cache migration hack

counter_cache_migration.rb

class AddCommentsCountToPosts < ActiveRecord::Migration
  def change
    change_table :posts do |t|
      t.integer :comments_count, default: 0
    end

    reversible do |dir|
      dir.up { data }
    end
  end

  def data
    execute <<-SQL.squish
        UPDATE posts
           SET comments_count = (SELECT count(1)
                                   FROM comments
                                  WHERE comments.post_id = posts.id)
    SQL
  end
end