test.rb

def record_purchase(customer, basket)
  basket.each do |product|
    product.buyers << customer
  end
  
  # Render these into the relation map -- this can be done out of process too
  basket.each do |x|
    all_products.each do |y|
      intersection[x][y] = x.buyers.intersect(y.buyers).count
    end      
  end
end

def get_related_products(product)
  return intersection[product].sort
end