modifying constants in a class to aid testing

We needed a way to test pagination and sorting of lists across different pages in Rails controllers without having to create enough records to cross to the second page. So we created a method to alter the value of a constant in a class that we are testing.

It enables us to test pagination and sorting (amongst other things) like this:

class CarsController...

  PER_PAGE = 20

  def index
    @pages, @cars = paginate :cars, :per_page => PER_PAGE, :order => "make, model" 
  end

class CarsControllerTest < Test::Unit::TestCase...

  def test_index_sorts_by_make_and_then_model_for_page_1
    car1 = Car.create!(standard_params.merge(:make => "Fiat", :model => "Uno"))  
    car2 = Car.create!(standard_params.merge(:make => "Fiat", :model => "Bravo"))
    car3 = Car.create!(standard_params.merge(:make => "Citroen", :model => "C2"))  

    with_constants CarsController, :PER_PAGE => 2 do
      get :index
    end
    
    assert_equal [car3, car2], assigns(:assets)
  end
  
  def test_index_sorts_by_make_and_then_model_for_page_2
    car1 = Car.create!(standard_params.merge(:make => "Fiat", :model => "Uno"))  
    car2 = Car.create!(standard_params.merge(:make => "Fiat", :model => "Bravo"))
    car3 = Car.create!(standard_params.merge(:make => "Citroen", :model => "C2"))  

    with_constants CarsController, :PER_PAGE => 2 do
      get :index, :page => 2
    end
    
    assert_equal_arrays [car1], assigns(:assets)
  end

with_constants takes a class name and a hash of constant => value pairs, and yields to a block which is the code you want to call with the altered constants:

def with_constants(klass, constants_hash)
  begin
    old_constants = {}
    constants_hash.each_pair do |name, value|
      old_constants[name] = klass.send(:remove_const, name)
      klass.const_set(name, value)
    end
    yield      
  ensure
    constants_hash.each_pair do |name, value|
      klass.send(:remove_const, name)
      klass.const_set(name, old_constants[name])
    end
  end
end