# File lib/mongo_mapper/plugins/modifiers.rb, line 12
        def decrement(*args)
          criteria, keys, options = criteria_and_keys_from_args(args)
          values, to_decrement = keys.values, {}
          keys.keys.each_with_index { |k, i| to_decrement[k] = -values[i].abs }
          collection.update(criteria, {'$inc' => to_decrement}, :multi => true)
        end