api.rb

class RailsHelpers
  def form_for(object)
    is_record_new = api_hook_record_new(object)
  end
  
  private
  
  def api_hook_record_new(object)
    # option 1 : raise "Should be overridden by ORM"
    # option 2 : object.new_record?
  end
end
 
module RailsDataMapper
  def api_hook_record_new(object)
    if object.is_a?(DataMapper)
      object.do_dm_stuff
    else
      super
    end
  end
end

orm.rb

class RailsHelpers
  def form_for(object)
    is_record_new = ActiveORM::Base.determine_orm_and_initialize(object).new_record?
  end
end
 
class RailsDataMapper < ActiveORM::Base
  def initialize(object)
    @object = object
  end
  
  def new_record?
    @object.new?
  end
end