# Codegen — Generate Models & Routes From Your Spec

> **TL;DR** — point `openapi-first codegen` at your spec and get `models` + `routes` scaffolds you can bind with one line. The codegen output is deterministic, and it's a *starting point* — not a maintained artifact.

---

## 🧭 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. **Writing the docs/spec catalog by hand**, once you have more than a handful of operations.

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

---

## 🚀 2. Model Generation

```bash
openapi-first codegen models --module my_project.models --input openapi.yaml
```

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

| Spec | Generated |
|------|-----------|
| `components.schemas.User` | `class User(BaseModel)` |
| `components.schemas.Item` | `class Item(BaseModel)` |
| every `$ref` (schema) | a `type: ClassVar` alias ✓ |
| `required + type` from schema | Pydantic `Field(...)` / type hints ✓ |

**Never hand-edit those files.** If the spec changes, regenerate — just like you'd re-run `cargo build` after editing `Cargo.toml`.

### 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).

---

## 🔁 3. Route Generation / Verification

```bash
# dry-run verification against your routes module
openapi-first codegen routes --module my_routes --input openapi.yaml --check

# scaffold an operation skeleton (generates the handler with a TODO)
openapi-first codegen routes --module my_routes --input openapi.yaml
```

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` as the spec says.

> **Note:** `codegen` is build-time tooling. It does **not** run at runtime, and templates (the Bake-your-stuff section of [02 – Templates](02_templates.md)) make scaffolding a server out-of-the-box even simpler 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)