gistfile1.rb

class PurgePostCategories < ActiveRecord::Migration
  
  @posts = [3,6,9,10,11,14,15,19,23,24,38,40,42,65,66,70,71,103,106,107,108,109,197,198,199,200,201]
  c = Category.find_by_title('Useful')
  
  def self.up    
    @posts.each do |post|
      Post.find(post).categories.delete(c)
    end
  end

  def self.down
    @posts.each do |post|
      Post.find(post).categories << c
    end
  end
end