group_by_drop.rb

class GroupByDrop << Liquid::Drop
  def initialize(objects)
    @objects = objects
  end

  def published_at_beginning_of_year_then_month
    objects = ActiveSupport::OrderedHash.new
    objects_by_year = @objects.group_by { |o| o.published_at.beginning_of_year }
    objects_by_year.each do |time, objects_of_year|
      objects[time] = objects_of_year.group_by { |o_year| o_year.published_at.beginning_of_month}
    end
    objects
  end
end