Testing Database Interactions

The Database Testing Challenge

Database tests are inherently more complex than pure unit tests because they have state. Each test must start from a known, clean state and leave no residue that would affect subsequent tests.

Three common strategies:

1. Transaction wrapping — each test runs in a transaction that rolls back after the test
2. Table truncation — truncate tables in beforeEach
3. Database-per-test — create a fresh database schema for each test

Transaction Wrapping (Recommended)

Wrapping each test in a transaction and rolling back is the fastest approach. Begin a transaction in beforeEach and roll back in afterEach — instant cleanup with no disk writes.

Factory Functions

Repetitive test data setup leads to brittle, hard-to-read tests. Factory functions create test data with sensible defaults and allow specific field overrides. Tests become clean and readable.

Testing Constraints and Validations

Test database constraints explicitly: unique violations should throw errors matching /unique constraint|duplicate/i, and foreign key violations should throw /foreign key/i.

Testing with Supabase

Use the local Supabase development server for tests. Use the service role key for test setup to bypass RLS. Use regular auth tokens when testing RLS behavior — verify that User B cannot read User A's data.

Key Takeaways

• Transaction wrapping is the fastest database test isolation strategy — each test rolls back all changes
• Factory functions reduce test data boilerplate and make tests readable
• Test database constraints explicitly — unique violations, foreign key failures, not-null constraints
• Use the service role key for test setup to bypass RLS; use regular auth tokens when testing RLS behavior
• Never rely on data from other tests — each test must be fully independent