elixir-testing

Elixir/Phoenix testing (ExUnit)

Pairs with elixir-conventions. Tests should be assertive: state the exact expected shape so a failure points at the problem.

RULES

1. Use the case templates: DataCase for context/DB tests, ConnCase for controller/API tests. They wire the Ecto SQL sandbox and helpers.
2. async: true only when the test owns its data via the sandbox and touches no shared global state (named processes, ETS, external services). Otherwise async: false.
3. Assertive assertions over Enum.all?. A for-comprehension of asserts (or per-element asserts) tells you which element failed; assert Enum.all?(...) just says false.
4. Match the exact result, don't assert truthiness. assert {:ok, %User{email: "a@b.c"}} = create_user(attrs) beats assert create_user(attrs).
5. Test changesets via errors_on/1: assert specific field errors, not just refute changeset.valid?.
6. Fixtures build valid baseline data; override per test. Keep them small and composable.
7. Don't sleep to wait. Use assert_receive, Oban.Testing, or sandbox-aware sync points.

Assertive collection checks

# Don't: failure is a useless `false`
assert Enum.all?(users, & &1.active)

# Do: failure names the offending element
for user <- users, do: assert user.active

Context / changeset test

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

  test "create_user/1 requires an email" do
    assert {:error, changeset} = Accounts.create_user(%{})
    assert %{email: ["can't be blank"]} = errors_on(changeset)
  end

  test "create_user/1 inserts a user" do
    assert {:ok, %User{email: "a@b.c"}} = Accounts.create_user(%{email: "a@b.c", password: "secret123"})
  end
end

JSON API test (ConnCase)

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

  setup :register_and_authenticate   # assigns a token-authed conn

  test "GET /api/posts/:id returns the post", %{conn: conn} do
    post = post_fixture()
    conn = get(conn, ~p"/api/posts/#{post.id}")
    assert %{"data" => %{"id" => id}} = json_response(conn, 200)
    assert id == post.id
  end

  test "GET /api/posts/:id is 404 for a stranger's post", %{conn: conn} do
    other = post_fixture(user: user_fixture())
    assert json_response(get(conn, ~p"/api/posts/#{other.id}"), 404)
  end
end

Fixtures

def user_fixture(attrs \\ %{}) do
  {:ok, user} =
    attrs
    |> Enum.into(%{email: "user#{System.unique_integer([:positive])}@test.dev", password: "secret123"})
    |> Accounts.create_user()
  user
end

The Ecto sandbox

• DataCase/ConnCase check out a sandboxed connection per test and roll it back after, so tests don't see each other's writes.
• For code under test that runs in another process (Task/GenServer/Oban), grant it sandbox access (Ecto.Adapters.SQL.Sandbox.allow/3) or use shared mode with async: false.

Before you finish

Run mix test, mix format --check-formatted, and mix compile --warnings-as-errors. These belong in CI; run them locally before pushing.