extracting pure ruby object from model

due_date_calculator.rb

require "active_support/time"

class DueDateCalculator
  def initialize(season)
    @season = season
  end

  def calculate
    @season.upcoming_round.start_date - 1.day
  end
end

due_date_calculator_spec.rb

require_relative "../../app/services/due_date_calculator"

describe DueDateCalculator do
  describe "#calculate" do
    let(:round) { double(:round, :start_date => Date.new(2013, 12, 28)) }
    let(:season) { double(:season, :upcoming_round => round) }

    subject { described_class.new(season) }

    it "is one date before start of upcoming_round" do
      subject.calculate.should eq Date.new(2013, 12, 27)
    end
  end
end

season.rb

class Season < ActiveRecord::Base
  # ...
  def upcoming_round
    future_rounds.first
  end
  
  private

  def future_rounds
    rounds.to_a.select { |round| Date.today < round.start_date }
  end
  
  # ...
end

season_ticket.rb

class SeasonTicket < ActiveRecord::Base
  # ...
  def due_date
    @due_date ||= DueDateCalculator.new(season).calculate
  end
  # ...
end