has_relations.rb

module HasRelations
  module ClassMethods
    def has_relations
      include InstanceMethods
      has_many :relations, :as => :from
    end
  end
  
  module InstanceMethods
    def related(relation_type = nil)
      if relation_type
        relations.find_all_by_to_type(relation_type.to_s.classify).map(&:to)
      else
        relations.map(&:to)
      end
    end

    def add_related(model)
      raise ArgumentError, "Cannot add related object since it is not an ActiveRecord model" unless model.kind_of?(ActiveRecord::Base)
      relations.create(:to => model) unless related.member?(model)
      model
    end
  end
  
  def self.included(receiver) #:nodoc:
    receiver.extend ClassMethods
  end
end

ActiveRecord::Base.class_eval { include HasRelations }

relation.rb

class Relation < ActiveRecord::Base
  belongs_to :from, :polymorphic => true
  belongs_to :to,   :polymorphic => true
end

sample_model.rb

class SampleUsage < ActiveRecord::Base
  has_relations
end