atelier/domains/testing/fixtures.md

Test Fixtures — Derived Rules

Derives from domains/testing/first-principles.md P7 (Realism), P2 (Independence), P3 (Determinism).

Fixtures are Real Data (P7 Realism)

• A fixture resembles production data in shape, distribution, and edge cases.
• A fixture with name: "test" and email: "a@b.c" hides bugs that real data surfaces.
• Use realistic names, realistic emails, realistic dates. "Jane Doe", "jane.doe@example.com", "2026-03-15".

Factory Over Fixture (P2 Independence, P3 Determinism)

• A fixture file is shared state. A factory is fresh state per test.
• Prefer factories (e.g., factory.User() returning a new instance) over shared fixture files.
• A shared fixture is mutated by one test, breaks another. Independence is violated.

Builders for Complex Data

• A builder (UserBuilder().withEmail().withAdmin().build()) composes only the fields the test needs.
• A builder with defaults: every field has a sensible default; tests override only what they test.
• A builder is the test's API to data. Stable, composable, readable.

Setup and Teardown (P2 Independence)

• Every test cleans up after itself. No test leaves state for the next.
• setUp/tearDown (or beforeEach/afterEach) restore the world.
• A test that depends on the order of execution is not independent.

Determinism (P3)

• No Date.now(), no Math.random() in fixtures. Inject the clock, inject the RNG.
• A fixture that uses "now" is non-deterministic. It passes today and fails tomorrow.
• Fix timestamps: createdAt: new Date("2026-01-01T00:00:00Z").

Edge Case Fixtures (P9 Edge Case Coverage)

• A fixture set includes: the empty case, the single-item case, the max-size case, the unicode case.
• A fixture set includes invalid data: malformed email, negative age, future date.
• Edge case fixtures are first-class, not "extra credit."

What Violates Fixture Discipline

Violation	Principle
name: "test" fixture	P7 Realism
Shared fixture file mutated across tests	P2 Independence
createdAt: new Date() (now) in fixture	P3 Determinism
No edge-case fixtures	P9 Edge Case Coverage
A 500-line fixture file	P3 (complexity)