how to generate test data with factories

TL;DR

Factories build valid objects with sensible defaults and per-test overrides. Define one factory per model, override only what the test needs, and use traits for variants.

Error

(Not an error; a technique guide. The problem it solves: brittle, duplicated fixture setup.)

Steps

1. Pick a factory library (factorybot, factoryboy, fishery, rosie). Expected: one library chosen.
2. Define factories with valid defaults for every required field. Expected: create(:user) just works.
3. Override per test: create(:user, admin: true). Expected: minimal, readable setup.
4. Use traits for common variants (:with_orders). Expected: named variants, not inline blobs.
5. Prefer build over create when persistence is unneeded. Expected: faster tests.

When to use

• Replacing static fixtures.
• Complex object graphs in tests.

When not to use

• Truly static reference data (fixtures are fine).
• You need prod-shaped bulk data (separate concern).

Tool compatibility

• factorybot (Ruby), factoryboy (Python), fishery (JS).

Variant phrasings

Test factories vs fixtures

Factories win for variation; fixtures for static reference.

Factory pattern in tests

The general technique; defaults plus overrides.

Why it happens

Fixtures rot and duplicate. Factories generate fresh, valid objects with local overrides, keeping tests readable.

Edge cases

• Factories can hide required setup; keep defaults minimal and explicit.
• Circular factory dependencies need lazy evaluation.
• create hits the DB; build_stubbed is faster for pure unit tests.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_rGcB6d1sF5-EdvldgIjt6g