feat: PydanticのRequestValidationErrorをRFC 9457 Problem Details形式に変換

docs/how-to/new-project.md

@@ -53,10 +53,12 @@ Create `src/app.py`:
 
 ```python
 from fastapi import FastAPI
+from fastapi.exceptions import RequestValidationError
 
 from nene2.config import AppSettings
 from nene2.log import setup_logging
 from nene2.middleware import ErrorHandlerMiddleware
+from nene2.middleware.error_handler import request_validation_error_handler
 from nene2.middleware.request_id import RequestIdMiddleware
 from nene2.middleware.request_logging import RequestLoggingMiddleware
 from nene2.middleware.request_size_limit import RequestSizeLimitMiddleware
@@ -104,6 +106,9 @@ def create_app(settings: AppSettings | None = None) -> FastAPI:
             window=settings.throttle_window,
         )
 
+    # Convert Pydantic BaseModel validation errors to RFC 9457 Problem Details
+    app.add_exception_handler(RequestValidationError, request_validation_error_handler)  # type: ignore[arg-type]
+
     return app
 
 

src/example/app.py

@@ -1,6 +1,7 @@
 """Application factory — wires dependencies and registers routes."""
 
 from fastapi import FastAPI
+from fastapi.exceptions import RequestValidationError
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse
 from sqlalchemy import create_engine
@@ -23,6 +24,7 @@
     SecurityHeadersMiddleware,
     ThrottleMiddleware,
 )
+from nene2.middleware.error_handler import request_validation_error_handler
 from nene2.validation.exceptions import ValidationException
 
 from .comment.exceptions import CommentNotFoundExceptionHandler
@@ -150,6 +152,10 @@ def create_app(settings: AppSettings | None = None) -> FastAPI:
         ValidationException,
         ErrorHandlerMiddleware.handle_validation_exception,
     )
+    app.add_exception_handler(
+        RequestValidationError,
+        request_validation_error_handler,
+    )
 
     note_repo, tag_repo, comment_repo, db_executor = _build_repositories(cfg)
 

src/nene2/middleware/error_handler.py

@@ -9,12 +9,13 @@
 from typing import Any
 
 from fastapi import Request
+from fastapi.exceptions import RequestValidationError
 from fastapi.responses import JSONResponse
 from starlette.middleware.base import BaseHTTPMiddleware, RequestResponseEndpoint
 from starlette.responses import Response
 
 from nene2.http.problem_details import problem_details_response
-from nene2.validation.exceptions import ValidationException
+from nene2.validation.exceptions import ValidationError, ValidationException
 
 from .domain_exception import DomainExceptionHandlerProtocol
 
@@ -79,3 +80,33 @@ async def handle_validation_exception(_request: Request, exc: Exception) -> JSON
             "The request contains invalid values.",
             extra={"errors": [e.to_dict() for e in exc.errors]},
         )
+
+
+async def request_validation_error_handler(_request: Request, exc: Exception) -> JSONResponse:
+    """Convert FastAPI RequestValidationError to nene2 Problem Details (422).
+
+    Register with FastAPI to replace the default Pydantic validation error format::
+
+        from fastapi.exceptions import RequestValidationError
+        from nene2.middleware.error_handler import request_validation_error_handler
+
+        app.add_exception_handler(RequestValidationError, request_validation_error_handler)
+    """
+    if not isinstance(exc, RequestValidationError):
+        raise TypeError(f"Expected RequestValidationError, got {type(exc)}")
+
+    errors: list[ValidationError] = []
+    for raw in exc.errors():
+        loc = raw.get("loc", ())
+        field = ".".join(str(part) for part in loc if part != "body") or "request"
+        message = str(raw.get("msg", "Invalid value."))
+        code = str(raw.get("type", "invalid"))
+        errors.append(ValidationError(field=field or "request", message=message, code=code))
+
+    return problem_details_response(
+        "validation-failed",
+        "Validation Failed",
+        422,
+        "The request contains invalid values.",
+        extra={"errors": [e.to_dict() for e in errors]},
+    )

tests/nene2/middleware/test_error_handler.py

@@ -1,12 +1,15 @@
 """Tests for ErrorHandlerMiddleware."""
 
 from fastapi import FastAPI
+from fastapi.exceptions import RequestValidationError
 from fastapi.responses import JSONResponse
 from fastapi.testclient import TestClient
+from pydantic import BaseModel, Field
 from starlette.responses import Response
 
 from nene2.http.problem_details import problem_details_response
 from nene2.middleware import DomainExceptionHandlerProtocol, ErrorHandlerMiddleware
+from nene2.middleware.error_handler import request_validation_error_handler
 from nene2.validation.exceptions import ValidationError, ValidationException
 
 
@@ -86,3 +89,41 @@ def test_unregistered_domain_exception_falls_through_to_500() -> None:
     client = TestClient(_make_app(domain_handlers=[]), raise_server_exceptions=False)
     r = client.get("/domain-error")
     assert r.status_code == 500
+
+
+def _make_app_with_pydantic_handler() -> FastAPI:
+    class _Body(BaseModel):
+        rating: int = Field(ge=1, le=5)
+        price: int = Field(ge=0)
+
+    app = FastAPI()
+    app.add_middleware(ErrorHandlerMiddleware)
+    app.add_exception_handler(
+        RequestValidationError,
+        request_validation_error_handler,  # type: ignore[arg-type]
+    )
+
+    @app.post("/items")
+    async def create_item(body: _Body) -> JSONResponse:
+        return JSONResponse({"rating": body.rating, "price": body.price})
+
+    return app
+
+
+def test_pydantic_validation_error_returns_problem_details_format() -> None:
+    client = TestClient(_make_app_with_pydantic_handler(), raise_server_exceptions=False)
+    r = client.post("/items", json={"rating": 99, "price": -1})
+    assert r.status_code == 422
+    body = r.json()
+    assert body["type"].endswith("validation-failed")
+    assert body["status"] == 422
+    assert isinstance(body["errors"], list)
+    assert len(body["errors"]) == 2
+
+
+def test_pydantic_validation_error_field_names_are_extracted() -> None:
+    client = TestClient(_make_app_with_pydantic_handler(), raise_server_exceptions=False)
+    r = client.post("/items", json={"rating": 99, "price": 100})
+    assert r.status_code == 422
+    fields = [e["field"] for e in r.json()["errors"]]
+    assert "rating" in fields