document repository lookups

ai/backend-context.md

@@ -20,6 +20,9 @@ intentionally omitted here - update this section as entities land.
   - Do not write unit tests for concrete repository implementations
     (e.g. `Postgres*Repository`). They are exercised by e2e tests.
     Use cases are tested with fake repositories.
+  - Repository methods that find records by a foreign key should accept
+    the related entity, not a raw id (e.g. `findBySet(Set $set)`, not
+    `findBySetId(int $setId)`).
 - Use cases: business logic with Request objects
   - When throwing exceptions, add `@throws` docblock
 - Fakes: in-memory implementations for testing
@@ -50,16 +53,33 @@ to be added to the repo; the Nix flake already provides
 Constructs LLMs default to that this project forbids. Name the trap
 explicitly so you catch yourself before writing it.
 
-| Anti-pattern | Forbidden | Required |
-|---|---|---|
-| Arrow function | `fn ($x) => $x->getId()` | `function ($x) { return $x->getId(); }` |
-| Inline FQCN type | `function f(): \Psr\Http\Message\ResponseInterface` | `use Psr\Http\Message\ResponseInterface;` then `function f(): ResponseInterface` |
-| Inline `::class` | `Container::get(\App\Foo\Bar::class)` | `use App\Foo\Bar;` then `Container::get(Bar::class)` |
-| Default param | `function f(int $count = 10)` | `function f(int $count)` |
-| Default in fake | `public function create(Dto $dto, bool $strict = true)` | no default; every caller passes a value |
-| Lookup returns stored ref | `return $this->items[$id] ?? null;` | rebuild a new instance with the stored fields |
-| Short variable name | `$t`, `$n`, `$res`, `$req`, `$e` | `$text`, `$node`, `$response`, `$request`, `$exception` |
-| Em dash | `// fetches user — by email` | `// fetches user - by email` |
+- Arrow function:
+  - Forbidden: `fn ($node) => $node->getId()`
+  - Required: `function ($node) { return $node->getId(); }`
+- Inline FQCN type:
+  - Forbidden: `function f(): \Psr\Http\Message\ResponseInterface`
+  - Required: import `ResponseInterface`, then return `ResponseInterface`
+- Inline `::class`:
+  - Forbidden: `Container::get(\App\Foo\Bar::class)`
+  - Required: import `Bar`, then call `Container::get(Bar::class)`
+- Default param:
+  - Forbidden: `function f(int $count = 10)`
+  - Required: `function f(int $count)`
+- Default in fake:
+  - Forbidden: `public function create(Dto $dto, bool $strict = true)`
+  - Required: no default; every caller passes a value
+- Lookup returns stored ref:
+  - Forbidden: `return $this->items[$id] ?? null;`
+  - Required: rebuild a new instance with the stored fields
+- FK lookup by id:
+  - Forbidden: `findBySetId(int $setId)`
+  - Required: `findBySet(Set $set)`
+- Short variable name:
+  - Forbidden: one-letter or abbreviated names (`$t`, `$n`, `$res`)
+  - Required: explicit names (`$text`, `$node`, `$response`)
+- Em dash:
+  - Forbidden: comment or docblock containing an em dash character
+  - Required: use `-`
 
 When generating code, scan the diff for these patterns before writing it
 to disk. If you catch one mid-write, rewrite that line.

ai/shared.md

@@ -111,6 +111,8 @@ mechanical, not aspirational - a "yes" to all is required.
       (PHP or TS).
 - [ ] Find/lookup repository methods return new instances, not stored
       references.
+- [ ] Backend repository methods that find by a foreign key accept the
+      related entity, not a raw id.
 - [ ] No em dashes (use hyphens).
 - [ ] Variable names are explicit (no `$t`, `n`, `res`, `req`, `e`, etc.).
 - [ ] No `any` in TypeScript - use concrete types or `unknown` with a