调用FastAPI接口出现HTTP 422 Unprocessable Entity错误原因排查

问题背景 #

用FastAPI开发/patient/add_patient接口后，调用时返回HTTP 422 Unprocessable Entity错误。已知Users表的password字段允许为空，代码中已将其赋值为None，同时需要理解错误信息中loc is body的含义。

相关代码 #

patient路由文件

from fastapi import APIRouter, HTTPException
from pydantic import BaseModel
from sqlalchemy import select
from sqlalchemy.exc import IntegrityError
from sqlalchemy.ext.asyncio import AsyncSession
from starlette import status
from starlette import status

# Local Imports
from database import SessionLocal
# from .auth import CreateUserRequest, db_dependency, user_dependency
from models import Patients, Users


router = APIRouter(
    prefix='/patient',
    tags=['patient']
)

async def get_db():
    async with engine.begin() as conn:
        await conn.run_sync(Base.metadata.create_all)

    db = SessionLocal()
    try:
        yield db
    finally:
        await db.close()


# Annotation for our database for dependency injection
db_dependency = Annotated[AsyncSession, Depends(get_db)]

async def get_curent_user(token: Annotated[str, Depends(oauth2_bearer)]):
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        email: str = payload.get('sub')
        user_id: int = payload.get('id')
        role_id: int = payload.get('role_id')
        if email is None or user_id is None:
            raise HTTPException(
                status_code=status.HTTP_401_UNAUTHORIZED,
                detail="Could not validate user."
            )
        return {'username': email, 'id': user_id, 'role_id': role_id}
    except JWTError:
        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED,
        detail='Could not validate user.')

user_dependency = Annotated[dict, Depends(get_curent_user)]


class CreateUserRequest(BaseModel):
    username: str # Email is username
    password: str
    phone: str
    dob: date
    role_id: int


class CreatePatinetReuqest(BaseModel):
    name: str
    registration_date: date
    user_id: int


@router.post("/add_patient/")
async def add_patient(db: db_dependency, create_user_request: CreateUserRequest):
    # Create a new user (patient)
    create_user_model = Users(
        email=create_user_request.username,
        password=None,
        phone=create_user_request.phone,
        date_of_birth=create_user_request.dob,
        role_id=0,
    )
    try:
        db.add(create_user_model)
        await db.commit()
    except IntegrityError as er:
        raise HTTPException(
            status_code=status.HTTP_409_CONFLICT,
            detail=f"User {create_user_request.username} already exists. Choose a different username."
        )
    await db.refresh(create_user_model)

    # Create a corresponding patient entry
    add_patient_model = Patients(
        registration_date=date.today(),
        user_id=create_user_model.id
    )

    try:
        db.add(add_patient_model)
        await db.commit()
    except IntegrityError as er:
        raise HTTPException(
            status_code=status.HTTP_409_CONFLICT,
            detail=f"Patient with {add_patient_model.patient_id} already exists."
        )
    await db.refresh(add_patient_model)

    return {
        "message": "Patient added successfully",
        "patient_id": add_patient_model.patient_id
    }

main.py文件

from fastapi import FastAPI

from routers import patient


app = FastAPI()
app.include_router(patient.router)

错误截图

错误原因分析 #

1. Pydantic模型校验失败 #

接口函数add_patient接收的CreateUserRequest模型中，password被定义为必填的str类型，但调用接口时可能未传入该字段，或者传入值不符合要求。尽管代码中最终将Users的password设为None，但FastAPI会先通过Pydantic校验请求体，只要请求缺少必填字段就会触发422错误。

2. 代码中存在其他潜在问题 #

• CreatePatinetReuqest存在拼写错误（应为CreatePatientRequest），不过该模型未被当前接口使用，暂不影响。
• get_db函数中使用的engine未导入，会导致运行时错误。
• Annotated、Depends、oauth2_bearer、jwt、SECRET_KEY、ALGORITHM、date等依赖未导入，会引发NameError。

loc is body的含义 #

错误信息中的loc（location）字段用于定位错误发生的位置，loc: ['body']表示错误出现在请求体（Request Body）中，即发送的JSON请求数据不符合接口要求的Pydantic模型规则，比如缺少必填字段、字段类型不匹配、格式错误等。

修正方案 #

1. 调整Pydantic模型，允许password可选 #

既然Users表的password字段允许为空，将CreateUserRequest中的password字段改为可选类型，并设置默认值为None：

from pydantic import BaseModel, Optional
from datetime import date

class CreateUserRequest(BaseModel):
    username: str  # Email is username
    password: Optional[str] = None  # 设为可选字段，默认值None
    phone: str
    dob: date
    role_id: int

2. 补全缺失的导入 #

在patient路由文件顶部添加缺失的依赖导入：

from fastapi import APIRouter, HTTPException, Depends
from pydantic import BaseModel, Optional
from sqlalchemy import select
from sqlalchemy.exc import IntegrityError
from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine
from sqlalchemy.ext.declarative import declarative_base
from starlette import status
from typing import Annotated
from jose import JWTError, jwt
from datetime import date

# 补充engine和Base的定义（若database.py未包含）
engine = create_async_engine("你的异步数据库连接URL")
Base = declarative_base()

3. 优化接口逻辑（可选） #

如果该接口专门用于添加患者，可直接在代码中固定role_id为0，去掉模型中的role_id字段，简化请求参数：

class CreateUserRequest(BaseModel):
    username: str  # Email is username
    password: Optional[str] = None
    phone: str
    dob: date

# 创建用户时直接使用固定的role_id
create_user_model = Users(
    email=create_user_request.username,
    password=create_user_request.password,
    phone=create_user_request.phone,
    date_of_birth=create_user_request.dob,
    role_id=0,
)

内容的提问来源于stack exchange，提问作者Ciasto piekarz