Ruby で 月ごとに繰り返す

gistfile1.txt

require 'date'

# 実際は ActiveSupport の機能をつかう
class Date
  def beginning_of_month
    self.class.new(self.year, self.month, 1)
  end
end

class MonthlyEnumerator < Enumerator
  def initialize(params = {})
    from = (params[:from] || Date.today).beginning_of_month
    to   =  params[:to]   || Date.today

    super() do |y|
      loop do
        break if from > to

        y << from
        from = from.next_month
      end
    end
  end
end

start = Date.new(2013, 7, 20)
last  = Date.new(2015, 1, 10)

enum = MonthlyEnumerator.new(from: start, to: last)
enum.each do |date|
  puts date
end

__END__
$ ruby example.rb
2013-07-01
2013-08-01
2013-09-01
2013-10-01
2013-11-01
2013-12-01
2014-01-01
2014-02-01
2014-03-01
2014-04-01
2014-05-01
2014-06-01
2014-07-01
2014-08-01
2014-09-01
2014-10-01
2014-11-01
2014-12-01
2015-01-01

Enumerator なんで、当然 次のようなことも出来る

enum = MonthlyEnumerator.new(from: start, to: last)
enum.group_by {|date| date.year}.each do |year, months|
  puts "#{year} 年"
  months.each do |month|
    puts month
  end
end

# =>
2013 年
2013-07-01
2013-08-01
2013-09-01
2013-10-01
2013-11-01
2013-12-01
2014 年
2014-01-01
2014-02-01
2014-03-01
2014-04-01
2014-05-01
2014-06-01
2014-07-01
2014-08-01
2014-09-01
2014-10-01
2014-11-01
2014-12-01
2015 年
2015-01-01