module PolymorphicParent
  def self.included(controller)
    controller.extend(ClassMethods)
  end

  module ClassMethods
    attr_reader :parents

    def parent_resources(*parents)
      @parents = parents
    end
  end

  protected
    def parent_id(parent)
      request.path_parameters["#{ parent }_id"]
    end

    def parent_type
      self.class.parents.detect { |parent| parent_id(parent) }
    end

    def parent_class
      parent_type && parent_type.to_s.classify.constantize
    end

    def parent_object
      parent_class && parent_class.find_by_id(parent_id(parent_type))
    end
end