Assuming that the tables are in the same database, you can do it in a single transaction, albeit not in a single save.
Library.transaction do @library.update_attributes(params[:book]) @book.update_attributes(params[:library]) end
Whether you use Library or Book as the receiver of the transaction method shouldn't matter since the transaction applies to the database connection which is shared assuming, as I said, that both tables are in the same DB.
