Unit Testing with a Fake Repository Volume 07 - Advanced Application Architecture | Phase 24
The repository pays rent in tests NestJS makes provider replacement straightforward. Instead of teaching a service test every Prisma method and nested relation shape, we can inject a tiny fake that implements the repository contract.
In-memory fake class FakeTaskRepository implements TaskRepository { items = new Map<string, TaskRecord>()
async findById(id: string) {
return this.items.get(id) ?? null
}async update(id: string, input: UpdateTaskRecord) {
const current = this.items.get(id)
if (!current) throw new Error("missing fake record")const next = { ...current, ...input } this.items.set(id, next) return next }
// implement only the contract, not Prisma }
Nest testing module const moduleRef = await Test.createTestingModule({ providers: [ TasksService, { provide: TASK_REPOSITORY, useValue: fakeRepository, }, ], }).compile()
What this test proves • Status-transition rules.
• 403/404 domain decisions.
• Derived completedAt behavior.
• Due-date policy.
• Service orchestration independent of PostgreSQL.
What it does not prove A fake repository does not prove that Prisma relation filters, indexes, migrations, constraints, or SQL behavior are correct. Those belong to repository integration tests.
Tests are executable behavioral contracts. Good tests protect meaningful invariants and failure cases, not implementation trivia.
Restate the goal of “Unit Testing with a Fake Repository” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: behavior contract → arrange → act → assert → regression signal. Mark the one boundary this lesson changes.
Inspect the current repository state and record the exact files, route, table, or runtime involved. Avoid creating an abstraction before you can name the existing owner.
Implement the smallest vertical slice. Add one coherent change, run the narrowest verification command, and read the output before continuing.
Connect the slice to TaskFlow’s phase artifact: One TaskFlow aggregate/use case refactored behind a purposeful repository interface with real database integration tests.. Verify behavior from the outside as a user or API client would.
Create one controlled failure that is safe to reproduce. Predict the symptom first, capture evidence, locate the earliest broken boundary, then fix only that cause.
Close the reference and rebuild or explain the core change from memory. Record what you would monitor or test before trusting the same change in production.
it("rejects deleting another member's task", async () => {
const response = await asUser(bob).delete(`/tasks/${aliceTask.id}`);
expect(response.status).toBe(403);
expect(await taskStillExists(aliceTask.id)).toBe(true);
});Do not copy this as a finished answer. Predict what each boundary does, type the smallest useful slice, run it, then compare the result with your prediction.
Look for shared database state, timers, globals, unordered execution, or missing cleanup before adding retries.
Move the highest-risk boundary to an integration test with the real adapter/database.
Without copying the final source, create a second small scenario that applies “Unit Testing with a Fake Repository” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Write repository methods in application language rather than generic findAll/create/update/delete.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Mapping everything into duplicate types without a domain need
Predict the symptom before causing the failure. Then locate the earliest boundary where reality diverges from expectation.Why does this responsibility live here, what input does it trust, what output proves success, and what would go wrong if another layer owned it?
If your explanation is vague, repeat the boundary trace before moving on.Close the reference. Reproduce the core idea from memory or explain the exact request/data flow aloud. Then check your diff or notes for accidental shortcuts that bypass validation, authorization, error handling, or verification earned in earlier phases.