8_20.rb

class Bicycle
  attr_reader :size, :parts

  def initialize(size:, parts:)
    @size       = size
    @parts      = parts
  end

  def spares
    parts.spares
  end
end

require 'forwardable'
class Parts
  extend Forwardable
  def_delegators :@parts, :size, :each
  include Enumerable

  def initialize(parts)
    @parts = parts
  end

  def spares
    select {|part| part.needs_spare}
  end
end

class Part
  attr_reader :name, :description, :needs_spare

  def initialize(name:, description:, needs_spare: true)
    @name         = name
    @description  = description
    @needs_spare  = needs_spare
  end
end

##########################

chain =
  Part.new(name: "chain", description: "10-speed")

road_tire =
  Part.new(name: "tire_size", description: "23")

tape =
  Part.new(name: "tape_color", description: "red")

mountain_tire =
  Part.new(name: "tire_size", description: "2.1")

rear_shock =
  Part.new(name: "rear_shock", description: "Fox")

front_shock =
  Part.new(
    name:         "front_shock",
    description:  "Manitou",
    needs_spare:  false)

##########################
road_bike =
  Bicycle.new(
    size:  "L",
    parts: Parts.new([chain,
                      road_tire,
                      tape]))

mountain_bike =
  Bicycle.new(
    size:  "L",
    parts: Parts.new([chain,
                      mountain_tire,
                      front_shock,
                      rear_shock]))

puts mountain_bike.spares.size   # => 3
puts mountain_bike.parts.size    # => 4

puts mountain_bike.parts + road_bike.parts
# => undefined method `+' for #<Parts:0x007fc1d59fe040>