gistfile1.rb

require 'rubygems'
require 'active_support'

def rates
  o = Object.new
  def o.summer_months_nbr
    [6, 7, 8, 9]
  end
  o
end

# monthly_usage should not be zero based based, but rather 1..12 like actual months.
#
# Returns a simple demo hash with usage per month. Eg. january: { 1 => 1 }
#
# You could also unshift an extra object onto an array so the month numbers line up. YMMV
def monthly_usage
  Hash[*(1..12).zip(1..12).flatten]
end

# Rails
def summer_monthly_usage
  @summer_monthly_usage ||= monthly_usage.values_at(*rates.summer_months_nbr).sum
end

p summer_monthly_usage # => 30

# 1.9
def summer_monthly_usage
  @summer_monthly_usage ||= monthly_usage.values_at(*rates.summer_months_nbr).inject(&:+)
end

p summer_monthly_usage # => 30

# 1.8
def summer_monthly_usage
  @summer_monthly_usage ||= monthly_usage.values_at(*rates.summer_months_nbr).inject { |sum, u| sum + u }
end

p summer_monthly_usage # => 30