testing-essentials

Testing Essentials

RULES — Follow these with no exceptions

1. Follow the project's existing test setup patterns (e.g. shared setup helpers like setup :store_test_session) — don't inline DataCase/ConnCase boilerplate that the project already abstracts away
2. Test both happy path AND error/invalid cases for every function
3. Use async: true by default; disable it only for real shared state — global state, Application.put_env, named processes, external services. LiveView/ConnCase tests are async-safe under the Ecto SQL Sandbox (Phoenix's own generators emit async: true).
4. Define test data in fixtures (test/support/) — never build it inline across multiple tests
5. Use has_element?/2 and element/2 for LiveView assertions — not html =~ "text" for structure checks
6. Always test the unauthorized case for any protected resource
7. Test the public context interface, not internal implementation details
8. Use describe blocks to group tests by function or behavior

---

TDD Workflow

Write the failing test first. Run it to confirm it fails for the right reason. Implement the minimum code to make it pass. Never write implementation before the test exists.

mix test test/my_app/accounts_test.exs  # Should fail first
# ... implement ...
mix test test/my_app/accounts_test.exs  # Should pass

---

Test Module Setup

DataCase — for context and schema tests

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

  alias MyApp.Accounts
  import MyApp.AccountsFixtures
end

ConnCase — for LiveView and controller tests

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

  import Phoenix.LiveViewTest
  import MyApp.AccountsFixtures
end

---

Fixture Pattern

Define all test data in test/support/fixtures/:

defmodule MyApp.AccountsFixtures do
  def user_fixture(attrs \\ %{}) do
    {:ok, user} =
      attrs
      |> Enum.into(%{
        email: "user#{System.unique_integer([:positive])}@example.com",
        password: "hello world!"
      })
      |> MyApp.Accounts.register_user()

    user
  end
end