Model objects are falsely marked as not-new after transaction rollback

Consider the following code from a controller action:

begin
  @auction.transaction do
    @auction.save!
    @lot.auction = @auction
    @lot.save!
    flash[:notice] = 'Auction was successfully created.'
    redirect_to(@auction) 
  end
rescue  
  render :action => "new"
end

If no validations fail on @auction ActiveRecord considers it saved even if the transaction cannot be finished successfully (e.g. when @lot.save raises an exception). Hence @auction.new_record? will return false although the record is not saved in the DB.

This leads a problem in the view:

form_for(@auction)

will generate a put (to controller action 'update') instead of a post (to controller action 'create').

I think the right way to fix this is to remember which model objects have been saved in a transaction and to revert their state after rollback. Any suggestions how this could be done the best?