def activity_by_year
        self.all.group_by { |i| i.created_at.year }
      end

      def activity_by_month(year = nil)
        if year
          self.activity_by_year[year].group_by { |i| i.created_at.month }
        else
          by_year = self.activity_by_year
          data = {}
          by_year.each_key do |y|
            data.merge!( { y => by_year[y].group_by { |i| i.created_at.month } } )
          end
          data
        end
      end