Unit Tests in Elixir - Part 1

Unit tests are important. So are end-to-end tests that mimic real user behavior. All the other stuff in between on the testing pyramid - not so much.

That’s basically my philosophy behind testing. But defining what a unit test even is can be really tricky! I mean, what if that function you’re testing calls a bunch of other functions? Is that function by definition a sort of integration test?

Today I’m going to try and lay down some of the basic things I believe about testing in Elixir. Later on I’ll go over some of the finer points of how to do unit tests for certain tricky things (like unit testing GenServers, unit testing functions that send messages, unit testing functions that touch the file system, unit testing functions that make HTTP calls, etc.).

I don’t want to say these are rules, but I consider them solid guidelines that I follow all the time when I’m writing unit tests.

1) All unit tests should run in parallel

If a test relies on some shared state, it isn’t a unit test. There are many ways that you can design your tests (and your code) to make this possible, and I’ll go over them in depth in future posts. But in general, each unit test should be run within its own world and that means it’s totally fine to run them in parallel.

2) Unit tests cover all behavior in a single function within a single process

This is related to number 1, since if your unit test relies on changes happening outside of a single process it can’t be a unit test. At that point you’re testing integration between two processes which are always doing things asynchronously. This is an integration test by default.

But if your function calls other functions, that’s ok! As long as all the stuff you’re testing for is within a single process, it’s still a unit test in my book. Take the following example:

defmodule MyApp.Transform do
  def validate_parse_and_format_for_csv(raw_json) do
    |> MyApp.Validation.validate_raw()
    |> MyApp.Parse.parse_validated()
    |> MyApp.Format.csv()

Here we have a function that we’re unit testing. The behavior of this function is a composition of behavior of other functions. In theory, it should be enough to simply test that those other functions are called with the right data and then let the unit tests of those functions cover the rest for us, but I don’t like this approach in Elixir for many reasons (and I’m not going to go into that today).

If I were to unit test that function, I’d test it in the following way:

defmodule MyApp.TransformTest do
  use ExUnit.Case, async: true

  describe "validate_parse_and_format_for_csv/1" do
    test "returns a CSV formatted string when the json is valid" do

    test "returns an error tuple when the given json is invalid" do

Yes, technically this test is duplicating tests that are already unit tested in those composite functions, but when we’re writing unit tests we’re defining behavior that we want from a function, not defining interactions between objects (with a couple of exceptions, which I’ll go into on another day). It’s just different in FP vs. OO, and it took me a little while to fully come to terms with that.

3) Don’t test library code

If you’re using a library, you don’t need to test what that library does for you. For example, let’s look at Ecto. You don’t need to test the mere existance of an association, or that the association works as expected. If you’re using a library, you need to trust that it works as documented - and it is documented, right? Otherwise, you shouldn’t be using that library.

4) One describe block tests one function

In ExUnit, you’re only allowed one level of nesting with describe blocks. This is by design! When you’re writing unit tests, you should do it like this:

defmodule MyApp.UsersTest do
  use MyApp.DataCase, async: true

  describe "find_with_posts/1" do
    test "finds a user and preloads their posts" do

    test "returns nil if no user is found" do

  describe "find_with_comments/1" do
    test "finds a user and preloads their comments" do

    test "returns nil if no user is found" do

Always put a describe block showing exactly which function is under test - even if there’s only one test for now! It makes reading tests much easier, especially for those of us who like to use tests as documentation.

5) Use setup blocks sparingly

setup blocks can be helpful when you have a bunch of stuff that’s used within a bunch of tests. One especially great example of this is having the socket set for you when you’re testing Absinthe subscriptions. But using them too much makes individual tests difficult to read. Only use them when you’re able to extract many lines of code (like, at least 5) from multiple tests (like, at least 3). If you have simple things that are shared between tests, pulling those things out to a module attribute is even easier and should be a first step before a setup block.