Riddle me this: How can you compose sort_by in Ruby?

composable_sort.rb

require 'minitest/autorun'

class ComposableSort
  def initialize(stuff)
    @stuff = stuff
  end

  def sorted
    @stuff.sort_by { |item|
      criteria.flat_map { |criterium| criterium.call(item) }
    }
  end
end

class SortByName < ComposableSort
  def criteria
    [->(item) { [item.name] }]
  end
end

class SortByNameThenScore < SortByName
  def criteria
    super + [->(item) { [-(item.score || -1)] }]
  end
end

StuffThing = Struct.new(:name, :score)

describe SortByNameThenScore do
  it 'sorts first by name' do
    a_name = StuffThing.new('AA', 43)
    b_name = StuffThing.new('BB', 43)
    stuff = [b_name, a_name]
    sorter = SortByNameThenScore.new(stuff)
    sorter.sorted.must_equal([a_name, b_name])
  end

  it 'sorts secondarily by score' do
    a_score = StuffThing.new('AA', 45)
    b_score = StuffThing.new('AA', 20)
    stuff = [b_score, a_score]
    sorter = SortByNameThenScore.new(stuff)
    sorter.sorted.must_equal([a_score, b_score])
  end

  it 'sorts also with structs' do
    high_score = StuffThing.new('AA', 50)
    no_score = StuffThing.new('AA', nil)
    stuff = [no_score, high_score]
    sorter = SortByNameThenScore.new(stuff)
    sorter.sorted.must_equal([high_score, no_score])
  end
end