# Codegen — Generate Models & Routes From Your Spec

> **TL;DR** — point `openapi-first models` / `openapi-first routes` at your spec and get Pydantic models plus operation-matching route stubs. The output is deterministic and is a *starting point* — not a maintained artifact.

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## 🧭 1. Why Celebrate Codegen?

Two pain points kill OpenAPI projects:

1. **The "write the spec, then write the same thing as Pydantic models" step** — exactly where server/client param types drift (your route says `item_id: int`, your client sends a string…).
2. **Typing handler names by hand** when `bind_routes` requires each `operationId` to resolve to a function of the exact same name.

Codegen collapses both. One command, one source file, the same shapes everywhere.

---

## 🚀 2. Model Generation

```bash
openapi-first models openapi.yaml -o app/_gen/models.py
```

The generated module mirrors the spec's `components.schemas` **by name**:

| Spec | Generated |
|------|-----------|
| `components.schemas.User` | `class User(BaseModel)` |
| `components.schemas.Item` | `class Item(BaseModel)` |
| `required + type` from schema | Pydantic `Field(...)` / type hints ✓ |

**Never hand-edit those files.** If the spec changes, regenerate — just like re-running a compiler after editing source.

### Why constructor-time validation still applies

Codegen doesn't change the design: the generated models are plain Pydantic, and the **client/server still validate against the spec** at startup. Codegen is a *convenience accelerator* on top of the fail-fast guarantees in [04 – Design](../04_design.md) and [06 – Error Handling](../06_error_handling.md).

> **Wire model vs. domain model.** Generated models are the **wire contract**. Keep them separate from your persisted/domain model (see the standards *CRUD Template*): give the wire schema an `id: str`, but model the Mongo document so the id is optional.

---

## 🔁 3. Route Generation

```bash
openapi-first routes openapi.yaml -o app/interfaces \
  --use-models --models-module app._gen.models \
  --inject-request --package
```

One file per resource (derived from the first path segment). Flags:

| Flag | Effect |
|------|--------|
| `--use-models` | Import request-body `$ref` models from `--models-module` |
| `--models-module MOD` | Dotted module for those imports (default `models`) |
| `--inject-request` | Prepend `request: Request` to every handler (DI convention) |
| `--package` | Emit `<resource>/routes.py` packages (with `__init__.py`) instead of flat `<resource>.py` |
| `--template-style` | Emit `async def` handlers with a JSON body parameter named `body` (the standards-template convention) plus a service-resolution hint, instead of sync `def` handlers taking `payload` |

Why bother? Because `bind_routes` (in [02 – Components](../02_components.md)) needs an `operationId` → handler **exactly** matching the spec. Codegen guarantees you never type a handler name wrong — it wears the same `operationId` the spec declares.

### Resource names and extra media types

- A path such as `/fetch-requests` yields a **sanitized** module name `fetch_requests.py` (a raw hyphen could never be imported).
- `multipart/form-data` and `application/x-www-form-urlencoded` bodies expand to one parameter per schema property: `UploadFile = File(...)` for `format: binary`, `Form(...)` otherwise — not an opaque `payload: dict`. Fields are required only when listed in the schema's `required`.

The stubs `raise NotImplementedError`; fill each body in (typically by resolving a service from `request.app.state`).

### Handler shapes

Both shapes bind equally — the router resolves the handler by `operationId` and introspects the FastAPI signature, so it never inspects the request body:

- **Default (sync stub):** `def create_item(payload: ItemInput, response: Response):` — the body parameter is `payload`.
- **`--template-style`:** `async def create_item(request: Request, body: ItemInput, response: Response):` — matches the standards template (`async def`, `body` parameter, a `request.app.state.db_resources["<resource>_svc"]` hint).

Use `--template-style` when generating against the standards template so the scaffold matches the hand-written handlers you will keep.

> **Note:** `models` / `routes` are build-time tooling. They do **not** run at runtime, and templates (the Bake-your-stuff section of [02 – Templates](02_templates.md)) scaffold a working server out-of-the-box for greenfield projects.

---

## 🧪 4. Idempotent Output

Generated output is deterministic w.r.t. the spec:
- Same spec → byte-identical files (unless you hand-edit — you won't)
- Ordering follows spec declaration order
- No timestamps, no machine names, no hidden randomness

That determinism is what lets you `git diff` after a spec change and see **exactly** what moved.

---

## 🧩 Related

- [01 – Overview](../01_overview.md) · [02 – Templates](02_templates.md) · [03 – Client](03_client.md) · [04 – Design](../04_design.md) · [07 – Testing](../07_testing.md)