Oo, fancy.

external_relations.rb

module ExternalRelations
  class Linkage
    def initialize(relation, caller, opts)
      opts = opts.clone
      @key = (opts.delete(:id) || :id)
      @caller = caller
      @target = relation.to_s.singularize.camelize.constantize.scoped.where(:"#{@caller.class.to_s.downcase}_id" => @caller.send(@key))
    end

    def <<(new_link_target)
      new_link_target.send(:"#{caller.class.downcase}_id=", @caller.send(@key))
      new_link_target.save
    end

    def method_missing(name, *args, &blk)
      @target.send(name, *args, &blk)
    end
  end

  def self.extended(base)
    class << base
      define_method(:has_external_relation) do |relation, opts|    
        self.send(:define_method, relation) do
          ExternalRelations::Linkage.new(relation, self, (opts || {}))
        end
      end
    end
  end
end