Part three of my Three Steps to Code Quality via TDD series. The most important concept when coupled with the previous two points - not every unit will relate to a method or class.
Most introductions into TDD use simple examples. Even the excellent TDD by Example uses a value object in terms of Domain Driven Design. Most introductory articles on the Internet suffer the same fate. While these are great for demonstrations, they don't relate to what most developers need to code on a day to day basis. It's around this point where people proclaim that the benefit of automated testing (even after the fact) is a waste of time.
One of my biggest revelations with TDD was that each unit does not need to equate to a single method or class. For a long time I followed what others did. Each collaborator would be injected and replaced with a test double. Each class would have a corresponding test file. However as I have stated in the introduction, this leads to problems.
We should test units of behaviour, not units of implementation. Given we know we should be using as few dependencies as possible, and we know we should limit visibility, each test should be simple to write. As part of the refactor step if we choose to introduce a new class that is fine. There is no need in most cases to extract this and introduce a test double. Every time this is done we tie the test closer and closer to the implementation details. Every class having a corresponding test file is wrong.
By testing a unit of behaviour we can chop and change the internals of the system under test without breaking anything. This allows the merciless refactoring automated testing advertises as a benefit.
Aren't you describing integration testing?
No. Tests should be isolated as I've documented before, but there is nothing stating they should be isolated from the components they work with. If we isolate at the method or class level we make testing and refactoring much harder. Due to the term "unit" being so closely linked with a class or method, I like the naming convention Google use for their tests - small, medium and large.
Additionally an excellent article from Martin Fowler on the subject of unit testing introduces two new terms, solitary and sociable tests. Neither one style alone works so the type of test you write should be based on context. Unfortunately the industry seems to be fixated on solitary testing.
Work great at the aggregate root level. Does the object do what we expect it to? It can use zero or many collaborators behind the scenes but these are implementation details. Here we would limit the use of test doubles as much as possible but still have fast, isolated tests. As generalization - most automated testing should fall into this category as the core domain of your application is likely to have the most amount of logic present.
Useful at the adapter or system edge. For example, does the controller invoke the correct application service? We don't care how it works behind the scenes. Anything beyond this service would be a test double. These sort of tests are more closely coupled to implementation details so should be used sparingly.
Doesn't this lead to huge tests?
No, not really. As you will limit implementation details leaking into the public API the use of test doubles will reduce. This will shrink test setup and in most cases improve readability. Worrying about large tests shouldn't be a problem with this style of testing. You will not reduce the amount of tests required, however you will find them to be much more stable and resilient than before.