Rewritten as a Rails concern with updated syntax. Also added a relation_identifier and relation_position that gets dynamically attached to related objects.

has_relations.rb

module RelationExtensions
  extend ActiveSupport::Concern

  included do
    # -- Constants ------------------------------------------------------------
    
    # -- Attributes -----------------------------------------------------------
    attr_accessor :relation_identifier, :relation_position

    # -- AR Extensions --------------------------------------------------------
    
    # -- Relationships --------------------------------------------------------
    has_many :relations, as: :from

    # -- Validations ----------------------------------------------------------

    # -- Scopes ---------------------------------------------------------------
    
    # -- Callbacks ------------------------------------------------------------
    
    # -- Class Methods --------------------------------------------------------
    
    # -- Instance Methods -----------------------------------------------------

    def related(params = {})
      relation_type = params[:relation_type]
      identifier = params[:identifier]
      scope = relations.order(:position)
      if relation_type.present?
        scope = scope.where(to_type: relation_type.to_s.classify)
      end
      if identifier.present?
        scope = scope.where(identifier: identifier)
      end
      scope.collect do |relation|
        to = relation.to
        to.relation_identifier = relation.identifier
        to.relation_position = relation.position
        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

end

relation.rb

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

sample_model.rb

class SampleUsage < ActiveRecord::Base
  include RelationExtensions
end

kur