Migrating from a has_one relationship to a has_and_belongs_to_many

migration.rb

class RemoveCategoryIdFromItem < ActiveRecord::Migration
  def self.up
    # Migrate items from has_many
    Item.all.each do |item|
      CategoriesItems.create(:item_id => item.id, :category_id => item.category_id)
    end

    remove_column :items, :category_id
  end

  def self.down
    add_column :items, :category_id, :integer

    # Migrate items to has_many
    CategoriesItems.all.each do |record|
      item = Item.find(record.item_id)
      item.category_id = record.category_id if item.category_id.nil?
    end

  end
end