standard_params to improve readability of tests

I beleive this came from the Pragmatic Programmers.

For each of our ActiveRecord model objects, we have a private test method within the test class called "standard_params". It returns a Hash containing parameters which, if passed to the constructor of the ActiveRecord object, will result in the construction of a valid object.

The benefits of this method are two-fold:

1. It improves the readability of tests. If the ActiveRecord object has a long list of attributes, and you are testing behaviour which is dependent only on one or two of these attributes, you do not have to explicitly specify all of the parameters - you simply merge in the ones that you really care about.

For example:

Let's say an Aeroplane object is invalid unless it has a 6 digit :model_number. It also has a list of required attributes, such as :name, :manufacturer, :safety_rating and :capacity. It would be invalid without any of these attributes.

Without the standard_params method, our test for invalidity without a six-digit model number would look like this:

def test_invalid_without_six_digit_model_number
   aeroplane = Aeroplane.new(:model_number => "12345", :name => "A630", :manufacturer => "Airbus", :safety_rating => "5", :capacity => "200")
   assert_equal false, aeroplane.valid?
   assert_equal false, aeroplane.errors.invalid?(:model_number)
end

With standard_params it would change to:

def test_invalid_without_six_digit_model_number
   aeroplane = Aeroplane.new(standard_params.merge(:model_number => "12345"))
   assert_equal false, aeroplane.valid?
   assert_equal false, aeroplane.errors.invalid?(:model_number)
end

private

def standard_params
   {
       :model_number => "123456",
       :name => "A630",
       :manufacturer => "Airbus",
       :safety_rating => "5",
       :capacity => "200")
   }
end

:model_number is the only attribute explicitly specified in the test, thus removing a lot of "noise" in the test setup data - something which aids readability.

The second benefit is:

2. Tests are DRY-er (in a good way). I'm a firm believer that tests do not necessarily need to be DRY - readability is far more important. "Extract method" in tests can often be your worst enemy if done in a naive way. I think it is acceptable in this case because the part of the setup data that is extracted is not important to the test - that which *is* important is merged explicitly into the Hash in the test method itself. And so the usual benefits of DRY-ness result - when another attribute is added to the model object which itself has associated validations, the test methods that require construction of valid objects do not have to change - only the standard_params method needs to change.

We also added a "without" method to Hash, which removes elements specified in the parameters:

module HashExtension
 def without(*keys)
   clone = self.dup
   keys.each { |key| clone.delete(key) }
   clone
 end 
end
Hash.send :include, HashExtension

We use this in combination with standard_params to test for validates_presence_of clauses. Eg

def test_invalid_without_name
   aeroplane = Aeroplane.new(standard_params.without(:name))
   assert_equal false, aeroplane.valid?
   assert_equal false, aeroplane.errors.invalid?(:name)   
end