# Engineering Lessons

Recurring bugs, surprising library behaviors, and process insights extracted from completed cycles. Newest at the top. Keep entries short — this is for fast scanning at the start of new cycles, not exhaustive history.

This file has two layers:

- **Deep engineering lessons** (L-NNN): library bugs, architectural insights, multi-paragraph context. Persist forever.
- **Ring buffer** at the bottom: most recent 15 single-sentence lessons emitted by the retrospective skill, consumed by `new-task` / `plan` / `decompose` / `autodev` Step 0. Oldest entries drop off the top.

Categories: `estimation` · `architecture` · `testing` · `dependencies` · `tooling` · `process`

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## L-001 — Dapper `TypeHandler<T>` is bypassed for enum types during read deserialization

**Cycle**: 1 (AZ-484)
**Discovered by**: integration test failure (`Error parsing column 12 (source=google_maps - String)`); root-caused via web search to long-standing Dapper issue [#259](https://github.com/DapperLib/Dapper/issues/259).
**Affects**: Dapper 2.1.35 (and most other versions until the proposed `Settings.PreferTypeHandlersForEnums` opt-in in PR #2200, not yet merged).

**What happens**
Registering `SqlMapper.AddTypeHandler(new MyEnumHandler())` for an `enum` type — even via `SqlMapper.TypeHandler<TEnum>` — works for **writes** (the handler's `SetValue` is invoked for parameter binding) but is **silently bypassed for reads**. Dapper's IL-emitted deserializer checks `IsEnum` first and falls back to `Enum.TryParse(string, ignoreCase: true)`.

**Why this is dangerous**
If the enum's wire string happens to match a member name case-insensitively (e.g., `RegionStatus.Failed` ↔ `"failed"`), the bypass goes unnoticed and round-trip works *accidentally*. The bug only surfaces when the wire format diverges from the C# member name (e.g., `TileSource.GoogleMaps` ↔ `"google_maps"` — `Enum.TryParse("google_maps")` does not match `GoogleMaps` because of the underscore).

**Recommended approach**
- Do **not** rely on `SqlMapper.TypeHandler<TEnum>` for read-side enum mapping unless the wire values match the enum member names case-insensitively.
- For enums whose wire format diverges (snake_case, kebab-case, custom IDs), store the entity field as `string` and provide an explicit converter (`*Converter.ToWireValue` / `FromWireValue`) for use at the service-layer boundary. This is what AZ-484 does for `TileEntity.Source` ↔ `TileSourceConverter`.
- Unit-test the converter directly. Do not assume that round-tripping through Dapper proves anything for enums.

**Detection**
- Unit tests of the type handler in isolation will pass even when the handler is bypassed at runtime.
- Failure surfaces only at integration-test time when the actual SELECT runs.
- If you must keep an enum-typed field, write at minimum one integration test that reads the enum back through Dapper from a real database row.

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## Ring buffer (last 15 entries — newest at top)

- [2026-05-11] [testing] Test helpers shared across unit + integration projects must live in one consolidated location — duplicate near-identical copies will diverge and require parallel fixes (cycle 2: `JwtTokenFactory.cs` and `JwtTestHelpers.cs` had the same `Expires < NotBefore` bug fixed in separate commits).
  Source: _docs/06_metrics/retro_2026-05-11_cycle2.md
- [2026-05-11] [process] Deferred-status NFR entries are allowed at most ONCE per NFR — if a Deferred NFR has not landed by the end of the cycle that follows the one in which it was deferred, the harness work must be promoted to a real PBI before any new NFR is accepted as Deferred (cycle 2 inherited cycle 1's PT-07 + added PT-08 + JWT-attach script-rot).
  Source: _docs/06_metrics/retro_2026-05-11_cycle2.md
- [2026-05-11] [testing] Integration tests must explicitly reset DB state at startup — relying on wallclock seeds or "tests probably won't collide" is a workaround, not isolation; the persistent Postgres volume in docker-compose makes test data accumulation the default state (cycle 2: `UavUploadTests._coordinateCounter` collision was patched with a wallclock seed instead of a real DB-reset hook).
  Source: _docs/06_metrics/retro_2026-05-11_cycle2.md
- [2026-05-11] [testing] Persisted enums need a Dapper read-roundtrip integration test — unit-testing the type handler in isolation does not prove read-side behavior (see L-001).
  Source: _docs/06_metrics/retro_2026-05-11.md
- [2026-05-11] [process] NFR test-spec additions must include the runner-script implementation in the same step, or be tagged "Deferred — harness work tracked in <ticket>"; otherwise scenarios accumulate as Unverified across cycles.
  Source: _docs/06_metrics/retro_2026-05-11.md
- [2026-05-11] [estimation] Task-spec test-site-count estimates must be backed by an explicit grep evidence block, not pattern-matched against neighboring code (AZ-484 spec said ~3 sites in `RegionServiceTests`; actual = 0).
  Source: _docs/06_metrics/retro_2026-05-11.md
