Extending TestCase functionality to add the more elegent must method, through some ruby metaprogramming magic

must.rb

# Ruby Dose Facebook Page

require 'test/unit'


class Test::Unit::TestCase
  def self.must(name, &block)
    # convert "any method description" to any_method_description
    test_name = "test_#{name.gsub(/\s+/, '_')}".to_sym

    # don't define a new method if its already defined
    defined = instance_method(test_name) rescue false
    raise "#{test_name} is already defined on #{self}" if defined

    if block_given?
      # define the actual test_something_is_valid TestCase method
      # pass it its functionality through a closure block
      define_method(test_name, &block)
    else
      flunk "No implementation provided for #{name}"
    end
  end
end

# only use it if on another file otherwise leave it in the same file and don't require anything
# require_relative 'must'

module Store
  class Product
    def initialize(options = nil)
      options.each { |p, v| instance_variable_set("@#{p}", v) } unless options.nil?
    end
    def price
      defined = instance_variable_defined?(:@price)
      raise "price is not set for this #{self.class.name.downcase}" unless defined
      "#{@price} $"
    end
  end
end

class ProductTest < Test::Unit::TestCase
  def setup
    @empty_product = Store::Product.new
    @full_product = Store::Product.new(price: 58)
  end
  must 'raise error if price is not set' do
    assert_raise 'price is not set for this store::product' do
      @empty_product.price
    end
  end
  must 'return price plus dollar sign if price is set' do
    assert_equal @full_product.price, '58 $'
  end
end