Testing in Elixir

Elixir Testing Vinicius Negrisolo Vinicius Negrisolo

My goal here:

Testing help developers to speed up, so let’s talk testing in Elixir.

Test to be Fast ⚡️

How to Get Productive in Elixir

The Dream

  • Functional
  • Immutability
  • Compiled
  • Performance
  • Pattern Matching
  • Fault Tolerance
  • Concurrent
  • Simpler than OO

The Saga begins

Documentation / Tutorials

Blog posts (Elixir Radar / ElixirWeekly)



Small apps and libraries

Test Feedback Cycle

  • defines development speed
  • ⚡️ fast
  • ✅ reliable

Test Situation

  • test suite
    • brittle / intermittent / gaps
    • 11 min to run / all sequential
  • manual tests on staging
  • Continuous Integration
    • rerun if test fails? really?

How to revert this scenario?

  • test
    • remove brittleness
    • fill the gaps
    • speed and paralelism

Test data

A Big Win!

Test data - Situation

  • module attrs with fixed values
  • setup to build data
  • attrs, setup and assertions were far apart

Test data - Code Situation

@attrs %{
  email: "user@mail.com",
  full_name: "Billy Bob",
  age: 30,
  gender: :male

setup do
  user = @attrs |> User.new() |> Repo.insert!()
  [user: user]

test "do something", %{user: user} do
  assert page.user_full_name == user.full_name

Test data - Factory

  • Keep it simple
  • A single factory definition per schema
  • Random data all the time
  • Build all regular attributes
  • Build all belongs_to relation
  • Everything else defined on tests

Test data - Factory Sample

defmodule Factory do
  use MapBot

  def new(User) do
      email: &"#{&1}_#{Faker.Internet.email()}",
      full_name: Faker.Name.name(),
      age: 15 + :rand.uniform(100),
      gender: Enum.random(~w(male female)a)
user = Factory.build(User, age: 17)
%User{} = Factory.insert!(User, age: 21)

Test data - Factory Benefits

  • no database cycles
  • no need to open factory definitions
  • no assertions on hardcoded factory values
  • schema changes <=> factory changes
  • new test scenarios <=> no factory changes

Unit Testing

  • single level of describe
  • describe/test/assert/refute
  • async false by default
  • async by test file

Unit Testing

defmodule MyApp.Ecto.NormalizerTest do
  use ExUnit.Case, async: true
  alias MyApp.Ecto.Normalizer

  describe "trim/2" do
    test "trims a valid changeset for single atom" do
      attrs = %{name: " Bob ", email: "bob@mail.com"}
      changeset = attrs |> changeset() |> Normalizer.trim(:name)

      assert changeset.changes.name == "Bob"

  defp changeset(attrs) do

Unit Testing - Pattern Matching

  • flip the assertion
  • add/remove the pin

Unit Testing - Pattern Matching

assert changeset.changes.name == "Bob"

assert changeset.changes.name == full_name

assert changeset.changes.name == user.name

assert %{name: "Bob"} = changeset.changes

assert %{name: ^full_name} = changeset.changes

name = user.name
assert %{name: ^name} = changeset.changes

Unit Testing - Data Table

  • controlled input/output
  • scale up tests

Unit Testing - Data Table

defmodule MyApp.Ecto.Type.MoneyTest do
  use ExUnit.Case, async: true
  alias MyApp.Ecto.Type.Money

  @cast_data [
    {nil, {:ok, nil}},
    {"314", {:ok, 314.00}},
    {"314.59", {:ok, 314.59}},
    {314, {:ok, 314.00}},
    {314.59, {:ok, 314.59}}

  describe "cast/1" do
    for {value, expected} <- @cast_data do
      @value value
      @expected expected

      test "casts '#{inspect(@value)}'" do
        assert Money.cast(@value) == @expected

Unit Testing - Properties

  • inputs are random and abundant
  • catches some unthinkable edge cases

Unit Testing - Properties

defmodule MyApp.Ecto.Type.MoneyTest do
  use ExUnit.Case, async: true
  use ExUnitProperties
  alias MyApp.Ecto.Type.Money

  describe "cast/1, dump/1 and load/1" do
    property "cast/1, dump/1, load/1 binary dollars+cents" do
      check all dollars <- integer(),
                cents <- 0..99 |> integer() do
        value = "#{dollars}.#{cents}"
        {expected, ""} = Float.parse(value)

        assert {:ok, value} = Money.cast(value)
        assert {:ok, value} = Money.dump(value)
        assert Money.load(value) == {:ok, expected}

Special Test Cases

Controller / WebSocket / Feature

Controller Testing

  • use MyAppWeb.ConnCase
  • exposes Plug.Conn
  • http request/response

Controller Testing

defmodule MyAppWeb.PostControllerTest do
  use MyAppWeb.ConnCase, async: true

  describe "show/2 when the user is authenticated" do
    setup [:authenticated_conn_setup]

    test "renders show page", %{conn: conn, current_user: user} do
      post = insert!(Post, user: user)
      conn = get(conn, Routes.post_path(conn, :show, post))

      assert response = html_response(conn, 200)
      assert response =~ "Post"

WebSocket Testing

  • use MyAppWeb.ChannelCase
  • exposes Phoenix.Socket
  • channel join/push/reply/broadcast
  • joined channels runs on new pids
  • ecto sandbox allow for async

WebSocket Testing

defmodule MyAppWeb.UserChannelTest do
  use MyAppWeb.ChannelCase, async: true

  describe "handle_in/3 for list-posts when joined on user:lobby" do
    setup [:authenticated_user_socket_setup, :join_user_lobby_setup]

    test "returns all user posts", %{socket: socket, current_user: user} do
      post = insert!(Post, user: user)
      ref = push(socket, "list-posts")

      assert_reply(ref, :ok, response, 500)
      assert response == %{posts: [%{name: post.name}]}

  def join_user_lobby_setup(%{socket: socket}) do
    {:ok, _reply, socket} = subscribe_and_join(socket, UserChannel, "user:lobby")
    Ecto.Adapters.SQL.Sandbox.allow(MyApp.Repo, self(), socket.channel_pid)
    [socket: socket]

Feature Testing

  • covers all the layers
  • slower, but not slow
  • can be parallel
  • wallaby / hound

Feature Testing

defmodule MyAppWeb.Features.PostTest do
  use MyAppWeb.FeatureCase, async: true
  import Wallaby.Query, only: [css: 1, link: 1]

  describe "when the user is authenticated" do
    setup [:authenticated_session_setup]

    test "posts page", %{session: session, current_user: user} do
      post = insert!(Post, user: user)
      session = visit(session, "/posts")

      assert "Posts" in texts_by(session, css("h1"))
      assert "Name" in texts_by(session, css("table th"))
      assert post.name in texts_by(session, css("table td"))

      session = click(session, link(post.name))

      assert current_path(session) == "/posts/#{post.id}"

Solid and Fast Test suite

  • Developers run locally
  • faster feedback cycle
  • oftener and smaller deploys
  • easier to manage deploys
  • less bugs
  • builds your confidence

Don’t underestimate how tests can help us

Test to be Fast ⚡️