has_many_ids

has_many_ids.rb

class ActiveRecord::Base
  def self.has_many_ids(association, ids, options={})
    options = {:typecast => :to_sym}.merge(options)

    method_name = :"#{association.to_s.singularize}_#{ids.to_s.pluralize}"

    define_method method_name do
      self.send(association).map(&ids)
    end

    define_method :"#{method_name}=" do |new|
      collection = self.send(association)
      old = self.send(method_name)

      new = new.select(&:present?).map(&options[:typecast])
      (new - old).each do |v|
        collection.create!(ids => v)
      end

      delete_ids = (old - new).map { |value| collection.detect { |v| v.send(ids) == value } }
      collection.destroy(delete_ids)
    end
  end
end