Each test file gets its own throwaway Postgres database by mutating
process.env.DATABASE_URL in tests/setup.ts's beforeAll. That per-file
DB switch is not safe under Vitest's default parallel file execution:
timing between two files' beforeAll/query calls can interleave enough
that both end up issuing real queries against the same throwaway DB.
This was invisible before because no two test files shared tables
with a real FK between them. It became visible once Request gained a
real FK to Employee: tests/employees.test.ts and tests/requests.test.ts
started intermittently deleting/reading each other's rows (FK
violations on deleteMany, wrong row counts), reproducible on every
run. Setting fileParallelism: false makes each file run to completion
before the next starts, so the per-file throwaway DB is genuinely
private. Confirmed: 31/31 tests pass reliably under plain npm test
(previously reproducibly 4 failing).
toanhoang