8_10.rb

class Bicycle
  attr_reader :size, :parts

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

  def spares
    parts.spares
  end
end

class Parts
  attr_reader :parts

  def initialize(parts)
    @parts = parts
  end

  def spares
    parts.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


#####  Someone has to construct each individual part
chain =
  Part.new(name: "chain", description: "11-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", needs_spare: false)

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

#####
road_bike_parts =
  Parts.new([chain, road_tire, tape])

#####  After constructing every part, someone has to know
#####   group the correct ones for style of bike.
road_bike =
  Bicycle.new(
    size:  "L",
    parts: Parts.new([chain,
                      road_tire,
                      tape]))

puts road_bike.size
# => L

puts road_bike.spares.inspect
# => [#<Part:0x00007ff560124cf8 @name="chain", @description="11-speed", @needs_spare=true>, #<Part:0x00007ff560124bb8 @name="tire_size", @description="23", @needs_spare=true>, #<Part:0x00007ff5601248c0 @name="tape_color", @description="red", @needs_spare=true>]

# to get it nicely formated, check if you regenerate
# => [#<Part:0x007fbd04a174d0
#         @name="chain",
#         @description="11-speed",
#         @needs_spare=true>,
#     #<Part:0x007fbd04a17390
#         @name="tire_size",
#         @description="23",
#         @needs_spare=true>,
#     #<Part:0x007fbd04a171b0
#         @name="tape_color",
#         @description="red",
#         @needs_spare=true>]

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

puts mountain_bike.size
# => L

puts mountain_bike.spares.inspect
# => [#<Part:0x00007ff560124cf8 @name="chain", @description="11-speed", @needs_spare=true>, #<Part:0x00007ff5601247a8 @name="tire_size", @description="2.1", @needs_spare=true>, #<Part:0x00007ff560124460 @name="front_shock", @description="Manitou", @needs_spare=true>]

# to get it nicely formated, check if you regenerate
# => [#<Part:0x007fbd04a174d0
#       @name="chain",
#       @description="11-speed",
#       @needs_spare=true>,
#     #<Part:0x007fbd04a17070
#       @name="tire_size",
#       @description="2.1",
#       @needs_spare=true>,
#     #<Part:0x00007fc8ac124590
#       @name="front_shock",
#       @description="Manitou",
#       @needs_spare=true>]

puts mountain_bike.spares.size
# => 3

puts mountain_bike.parts.size
# => undefined method `size' for #<Parts:0x00007ff5600dea00>