<=> (DataMapper::Query::Sort)

    # File lib/dm-core/query/sort.rb, line 18
          def <=>(other)
            other_value = other.value

            cmp = case
              when @value.nil? && other_value.nil?
                0
              when @value.nil?
                1
              when other_value.nil?
                -1
              else
                @value <=> other_value
            end

            @ascending ? cmp : cmp * -1
          end