February 12th, 2012
- TDD is about specifying behavior, not structure.
- Validations are behavior, and should be specified.
- Associations are structure, and need not be.
This is my personal viewpoint, though it is not mine alone. YMMV.
ActiveRecord provides a declarative interface for describing the structure and behavior of a model:
While syntactically similar, these two declarations do fundamentally different things.
Validations are behavior
validates_presence_of :title declaration changes the behavior of
save method (and other methods that use
save), and should be specified
explicitly. Here’s an example using shoulda matchers:
Even though the matcher’s name looks just like the likely implementation, the
validate_presence_of matcher specifies that you can not save an
without a non-nil value for
title, not that the
validates_presence_of(:title) declaration exists.
Associations are structure
has_many declaration exposes a
comments method to clients that appears
to be a collection of
Comment objects. Doing Test-Driven Development, you
would add this declaration when a specified behavior requires it e.g.
This example needs a
comments method that returns a collection in order to
pass. If it doesn’t exist already (because no other example drove you to add
it), this would be all the motivation you need to introduce it. You don’t need
an example that says
it "should have_many(:comments)".
Testing the framework
Some will argue that we don’t need to spec validations either, suggesting that
it "should validate_presence_of(:title)" is testing the Rails framework,
which we trust is already tested. If you think of TDD as a combination of
specification, documentation, and regression testing, then this argument falls
short on the specification/documentation front because the validation is
behavior and, thus, the spec should specify the validation.
Even if you view testing as nothing more than a safety net against regressions,
the argument still falls down in the face of refactoring. If we add a
class that also
we want to extract that behavior to a
Postable module that gets included in
Review, we’d want a regression test to fail if we failed
to include either of those declarations in the
But declarations are already declarative!
Another argument is that declarations supply sufficient documentation. e.g. we
can look at
rental_contract.rb and know that it validates the presence of
This is an interesting argument that I think has some merit, but I think it would require an extraordinarily disciplined and consistent approach of using declarations 100% of the time in model files such that each one is the spec for that model, e.g.
100% may sound extreme, but as soon as we define a single method body in any one of the models, the declarative nature of the file begins to degrade, and so does its fitness for the purpose of specification. Plus, if we can only understand the expected behavior of a model by looking at both its spec and its implementation, we’ve lost some of the power of a test-driven approach.
What do you think?
Do you spec associations? If so, what value do you get from doing so? If not, have you run into situations where you wished you had?
Same questions for validations.