adding a counter cache attribute to existing records

gistfile1.rb

class AddCommentsCountToNominations < ActiveRecord::Migration
  def self.up
    add_column :nominations, :comments_count, :integer, null:false, default: 0
    nomination_ids_with_comments = Set.new
    Comment.all.each {|c| nomination_ids_with_comments << c.nomination_id}
    nomination_ids_with_comments.each do |nomination_id|
      Nomination.reset_counters(nomination_id, :comments)
    end
  end

  def self.down
    remove_column :nominations, :comments_count
  end
end