Testing Essentials

RULES — Follow these with no exceptions

1. Use DataCase for database tests, ConnCase for LiveView/controller tests — never mix them
2. Test both happy path AND error/invalid cases for every function
3. Use async: true only when safe — safe: pure functions, changesets, helpers; unsafe: DB contexts with shared rows, LiveView, Application.put_env, external services
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

---

Context Test Skeleton

describe "create_post/1" do
  test "with valid attrs creates a post" do
    assert {:ok, %Post{} = post} = Blog.create_post(%{title: "Hello"})
    assert post.title == "Hello"
  end

  test "with invalid attrs returns error changeset" do
    assert {:error, %Ecto.Changeset{} = changeset} = Blog.create_post(%{})
    assert %{title: ["can't be blank"]} = errors_on(changeset)
  end
end

---

LiveView Test Skeleton

describe "index" do
  test "lists posts", %{conn: conn} do
    post = post_fixture()
    {:ok, _lv, html} = live(conn, ~p"/posts")
    assert html =~ post.title
  end

  test "unauthorized user is redirected", %{conn: conn} do
    {:error, {:redirect, %{to: path}}} = live(conn, ~p"/admin/posts")
    assert path == ~p"/login"
  end
end

describe "create" do
  test "saves post with valid attrs", %{conn: conn} do
    {:ok, lv, _html} = live(conn, ~p"/posts/new")

    lv
    |> form("#post-form", post: %{title: "New Post"})
    |> render_submit()

    assert has_element?(lv, "p", "Post created")
  end

  test "shows errors with invalid attrs", %{conn: conn} do
    {:ok, lv, _html} = live(conn, ~p"/posts/new")

    lv
    |> form("#post-form", post: %{title: ""})
    |> render_submit()

    assert has_element?(lv, "p.alert", "can't be blank")
  end
end

---

Changeset Test Skeleton

describe "changeset/2" do
  test "valid attrs" do
    assert %Ecto.Changeset{valid?: true} = Post.changeset(%Post{}, %{title: "Hello"})
  end

  test "requires title" do
    changeset = Post.changeset(%Post{}, %{})
    assert %{title: ["can't be blank"]} = errors_on(changeset)
  end
end

---

Setup Chaining

Compose reusable setup functions with setup [:func1, :func2]. Each function receives and returns a context map.

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

  import MyApp.AccountsFixtures
  import MyApp.BlogFixtures

  setup [:register_and_log_in_user, :create_post]

  test "owner can edit post", %{conn: conn, post: post} do
    {:ok, lv, _html} = live(conn, ~p"/posts/#{post}/edit")
    assert has_element?(lv, "#post-form")
  end

  defp create_post(%{user: user}) do
    %{post: post_fixture(user_id: user.id)}
  end
end

Chain order matters — later functions receive assigns from earlier ones.

---

Timestamp Testing

Never hardcode dates — use relative timestamps to prevent flaky tests as time passes.

# Bad — breaks after 2026
assert post.published_at == ~U[2026-01-15 12:00:00Z]

# Good — relative to now
now = DateTime.utc_now(:second)
assert DateTime.diff(post.inserted_at, now, :second) < 5

# Good — build relative dates for filtering/sorting
past = DateTime.add(DateTime.utc_now(:second), -7, :day)
future = DateTime.add(DateTime.utc_now(:second), 7, :day)
old_post = post_fixture(published_at: past)
new_post = post_fixture(published_at: future)
assert Blog.list_published_posts() == [old_post]

---

See testing-guide.md for comprehensive examples covering async tests, Mox mocking, file upload testing, and Ecto query testing.