write-local-datasource

# Write Local DataSource ## Overview The local data layer spans eight phases per entity: DataSource interface, SQLDelight schema, DB migrations, DAO interface, DAO implementation, local mapper, LocalDataSource implementation, and DI registration. All phases must follow strict conventions. ## Phases 1. **DataSource Interface** — Contract the RepositoryImpl depends on; uses repository models (not local types) → see `references/DATASOURCE_INTERFACE.md` 2. **SQLDelight Schema** — `.sq` schema file with named queries and field prefixes → see `references/SCHEMA.md` 3. **DB Migrations** — `.sqm` file required for any structural change to an existing table → see `references/MIGRATIONS.md` 4. **DAO Interface** — Observable streams vs. single reads contract → see `references/DAO.md` 5. **DAO Implementation** — `asFlow().mapToList()` and `executeAsOneOrNull()` patterns → see `references/DAO.md` 6. **Local Mapper** — SQLDelight type ↔ repository model (`toRepo`/`fromRepo`) → see `references/MAPPER.md` 7. **LocalDataSource Implementation** — Injects DAO + mapper; never accesses `*Queries` directly → see `references/LOCAL_DATASOURCE.md` 8. **DI Registration** — `singleOf` for DAOs/DataSources, `factoryOf` for mappers → see `references/DI.md` ## Rules | Rule | Details | |------|---------| | **Flow vs. suspend in DAO** | `Flow<List<T>>` for reactive reads; `suspend` for mutations and point-in-time reads | | **No direct Queries access** | LocalDataSource injects the DAO — never `*Queries` directly | | **executeAsOneOrNull** | Always nullable for single reads — never `executeAsOne()` | | **cleanTable required** | Every table needs a `cleanTable:` query for E2E test teardown | | **Field prefix** | Column names prefixed with table name in snake_case (e.g., `category_id`) | | **DI scope** | `singleOf` for DataSources and DAOs; `factoryOf` for mappers | | **Migration required** | Any structural change to an existing table needs a `.sqm` file | ## Common Mistakes | Mistake | Fix | |---------|-----| | `suspend fun findAll()` for a Flow return | Use `fun findAll(): Flow<List<T>>` — no suspend for reactive reads | | Calling `.first()` in DaoImpl for a Flow return | Return `Flow` directly; callers decide when to collect | | Using `executeAsOne()` for single reads | Always `executeAsOneOrNull()` — assume nullable | | Omitting `cleanTable:` in the `.sq` file | Required for E2E tests to reset state between runs | | Local mapper in `data/repository/mapper/` | Belongs in `data/local/mapper/` with `toRepo`/`fromRepo` | | Registering DataSource or DAO as `factoryOf` | Always `singleOf` — they share database state | | Adding `DatabaseProvider` registration again | Already registered once; duplicate causes a Koin conflict | | Accessing `*Queries` in LocalDataSource | Injects the DAO — never the queries object | | Changing `CREATE TABLE` without a `.sqm` file | Existing users never see the change; always add a migration | | Adding a `NOT NULL` column without `DEFAULT` | SQLite rejects the statement on existing rows | | Editing an existing `.sqm` file | `.sqm` files are immutable; create a new file for each change | | Wrong `.sqm` file number | Number must equal the count of existing `.sqm` files | | Wrapping migration SQL in `BEGIN`/`END TRANSACTION` | The driver manages the transaction; wrapping can cause crashes | ## Testing After completing the local data layer, use the `write-unit-tests` skill to test data sources and use cases. Use the `write-e2e-tests` skill for full-flow coverage — E2E tests use DAOs directly (`by inject()`) to seed and clean data. ## Verify ```bash .claude/skills/write-local-datasource/scripts/verify_migrations.sh ```