car_generator.rb

def build_cars(count)
  models = { "Ford" => [ "Taurus", "Explorer", "Mustang", ], "Toyota" => [ "Camry", "Highlander" ], "Chevrolet" => [ "HHR", "Camaro"]}
  makes  = %w[ Ford Toyota Chevrolet ]
  colors = %w[ Green Black Red Silver ]

  Array.new(count) { build_a_car(makes, models, colors) }
end

def build_a_car(makes, models, colors)
  make  = makes.sample
  model = models[make].sample
  color = colors.sample

  { make: make, model: model, year: rand(1990..2015), mileage: rand(1..250_000), color: color }
end

cross_joining_in_ruby.rb

require 'rspec'

cars = [
  { make: "Toyota", model: "Camry", year: 1996, mileage: 201293, color: "Green" },
  { make: "Toyota", model: "Camry", year: 2011, mileage: 11920, color: "Black" },
  { make: "Chevrolet", model: "HHR", year: 2007, mileage: 128743, color: "Sandstone Metallic" },
  { make: "Toyota", model: "Highlander", year: 2009, mileage: 47560, color: "Silver" },
]

def sample(count, list)
  makes  = list.map { |i| i[:make]}.uniq
  models = list.map { |i| i[:model]}.uniq

  pairings = makes.product(models).cycle

  samples = [ ]

  count.times do
    make, model = pairings.next
    possibility = list.find { |car| car[:make] == make && car[:model] == model }
    redo unless possibility
    samples << possibility
  end

  samples
end


describe 'sample' do

  it 'returns the correct number' do
    sample = sample(3, cars)
    expect(sample.count).to eq(3)
  end

  it 'returns a good distribution of makes' do
    cars.permutation.to_a.each do |permutation|
      sample = sample(3, permutation)
      makes  = sample.map { |vehicle| vehicle[:make] }
      expect(makes.sort).to eq(["Toyota", "Toyota", "Chevrolet"].sort)
    end
  end

  it 'returns a good distribution of models' do
    cars.permutation.to_a.each do |permutation|
      sample  = sample(3, permutation)
      models  = sample.map { |vehicle| vehicle[:model] }
      expect(models.sort).to eq(["Camry", "Highlander", "HHR"].sort)
    end
  end

end