A different approach to testing Elixir with mocks/doubles

After watching J B Rainsberger’s talk “Integrated Tests Are A Scam” I decided to revisit the elixir ecosystem for available tools for mocks/doubles.

During that research I got an idea how it could be done a bit differently. There was no other option than to try it.

This is a proof of concept for a different approach to elixir test doubles. Some of the goals/issues I’ve tried to solve here are:

  • No module compilation hackery
  • Reduce boilerplate to the minimum
  • Be explicit
  • Allow multiple implementations per module

Usage example

Let’s start with a service that talks to real world - the Sandbox.Store

defmodule Sandbox.Store do
  # Include `use Double` in module you want to stub/mock in tests
  # and that's it, nothing else is required.
  use Double

  def users do
    IO.puts "REAL Store.users/0"
    [:real]
  end

  def create(params) do
    IO.puts "REAL Store.create/1"
    {:ok, params}
  end
end

Now let’s make a Sandbox.Controller module that will use our store.

defmodule Sandbox.Controller do
  # This is how dependency injection is done
  # Just call Double.get(SomeModule)
  @store Double.get(Sandbox.Store)

  def index do
    case @store.users() do
      []      -> :zero
      [_one]  -> :one
      _list   -> :many
    end
  end

  def create(kind) do
    @store.create(%{"kind" => kind})
  end

  # this is just for testing the execution in :dev
  def devrun do
    index
    create(name: "Alice")
  end
end

Let’s see if we didn’t break the real implementation

$ mix run -e Sandbox.Controller.devrun
REAL Store.users/0
REAL Store.create/1

It works as expected.

Now, let’s write some tests.

# test/sandbox/controller_test.exs
defmodule Sandbox.ControllerTest do
  use ExUnit.Case

  alias Sandbox.Controller

  ## TEST QUERIES

  test "index when zero" do
    # use Double.return(mod, fun, ret) to stub return value
    Double.return(Sandbox.Store, :users, [])

    assert Controller.index == :zero
  end

  test "index when one" do
    Double.return(Sandbox.Store, :users, [:alice])

    assert Controller.index == :one
  end

  test "index when many" do
    Double.return(Sandbox.Store, :users, [:alice, :bob, :eve, :dave])

    assert Controller.index == :many
  end


  ## TEST COMMANDS

  test "create good" do
    Controller.create(:good)

    # use Double.called?(mod, fun) to check if function was called
    assert Double.called?(Sandbox.Store, :create)
  end

  test "create bad" do
    Controller.create(:bad)

    assert Double.called?(Sandbox.Store, :create, [%{"kind" => :bad}])
  end
end

One more thing to add - in the test_helper.exs

# test/test_helper.exs
Double.start_link() # <-- this line
ExUnit.start()

And finally let’s run the tests:

$ mix test
.....

Finished in 0.03 seconds
5 tests, 0 failures

Randomized with seed 454606

\o/

Conclusions

  • No module compilation hackery

    • Well, we do generate additional module, but there is no recompilation in runtime

  • Reduce boilerplate to the minumum

    • Two additional lines in regular code, no need to define mock/double modules by hand

  • Be explicit

    • The dependency from Controller to Store has been defined explicitly. Also the return values stubs are explicit

  • Allow multiple implementations per module

    • Instead of having written a one TestStore by hand we can dynamically set the return value we expect

Questions?

Since it took more to write this post than the actual code I have a feeling that I’m missing something important here. I’d be more than happy to discuss this, especially if you find any flaws in the approach.

The source code of Double as well all others can be find in the GitHub repo.

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