# File lib/moneta/adapters/mongo.rb, line 92
      def increment(key, amount = 1, options = {})
        @collection.find_and_modify(:query => { '_id' => to_binary(key) },
                                    :update => { '$inc' => { @value_field => amount } },
                                    :new => true,
                                    :upsert => true)[@value_field]
      end