FastAPI同时使用两个Pydantic响应模型报错问题

安全阿岳

2026-8-9

FastAPI接口多表查询响应模型报错问题

我用FastAPI实现接口，初始版本的main.py中，/licence_ol/接口使用list[EvergreenOutput]作为响应模型可正常运行。当修改响应模型为list[EvergreenOutput, MappingOutput]，并调整查询语句同时查询Evergreen和Mapping表后，接口返回500内部服务器错误，报错信息显示EvergreenOutput的product_id字段缺失。去掉类型注解时，响应结构为包含Evergreen和Mapping对象的列表。

初始可运行的main.py代码

@app.get(
    "/licence_ol/",
    summary="Query Licence info by product id",
    response_description="Successful Query",
    response_model=list[EvergreenOutput],
    tags=[Tags.items]
)
def get_product_id(session: Session = Depends(get_session))-> list[EvergreenOutput]: 

    query = select(Evergreen).where(Evergreen.product_id == Mapping.eim_product_id).limit(200)
    
    return session.exec(query).fetchall()

schemas.py代码

from typing import Optional
from datetime import date
from sqlmodel import SQLModel, Field

class Mapping(SQLModel,table=True):
    id_seq: Optional[int] = Field(default=None,primary_key=True)
    eim_product_id: Optional[int] = None
    mem_product_id: Optional[int] = None
    vendor: Optional[str] = None
    name: Optional[str] = None
    eim_name: Optional[str] = None
    product_alias: Optional[str] = None
    lev: Optional[int] = None
    manufacturer: Optional[str] = None

class MappingOutput(SQLModel):
    eim_product_id: Optional[int] = None
    mem_product_id: Optional[int] = None
    vendor: Optional[str] = None
    name: Optional[str] = None
    eim_name: Optional[str] = None
    product_alias: Optional[str] = None
    lev: Optional[int] = None
    manufacturer: Optional[str] = None


class Evergreen(SQLModel,table=True):
    id_seq: Optional[int] = Field(default=None,primary_key=True)
    product_id: int 
    phase_type: Optional[str] = None
    phase_start: Optional[date] = None
    phase_end: Optional[date] = None
    product_name: Optional[str] = None
    software_product_version_name: Optional[str] = None
    software_product_version_id: Optional[int] = None

class EvergreenOutput(SQLModel):
    product_id: int 
    phase_type: Optional[str] = None
    phase_start: Optional[date] = None
    phase_end: Optional[date] = None
    product_name: Optional[str] = None
    software_product_version_name: Optional[str] = None
    software_product_version_id: Optional[int] = None

修改后报错的main.py代码

@app.get(
    "/licence_ol/",
    summary="Query Licence info by product id",
    response_description="Successful Query",
    response_model=list[EvergreenOutput,MappingOutput],
    tags=[Tags.items]
)
def get_product_id(session: Session = Depends(get_session))-> list[EvergreenOutput,MappingOutput]: 

    query = select(Evergreen,Mapping).where(Evergreen.product_id == Mapping.eim_product_id).limit(200)
    
    return session.exec(query).fetchall()

错误信息

INFO:     127.0.0.1:53172 - "GET /licence_ol/ HTTP/1.1" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\uvicorn\protocols\http\httptools_impl.py", line 404, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\uvicorn\middleware\proxy_headers.py", line 78, in __call__
    return await self.app(scope, receive, send)
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\fastapi\applications.py", line 270, in __call__
    await super().__call__(scope, receive, send)
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\starlette\applications.py", line 124, in __call__
    await self.middleware_stack(scope, receive, send)
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\starlette\middleware\errors.py", line 184, in __call__
    raise exc
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\starlette\middleware\errors.py", line 162, in __call__
    await self.app(scope, receive, _send)
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\starlette\middleware\exceptions.py", line 75, in __call__
    raise exc
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\starlette\middleware\exceptions.py", line 64, in __call__
    await self.app(scope, receive, sender)
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\fastapi\middleware\asyncexitstack.py", line 21, in __call__
    raise e
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\fastapi\middleware\asyncexitstack.py", line 18, in __call__
    await self.app(scope, receive, send)
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\starlette\routing.py", line 680, in __call__
    await route.handle(scope, receive, send)
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\starlette\routing.py", line 275, in handle
    await self.app(scope, receive, send)
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\starlette\routing.py", line 65, in app
    response = await func(request)
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\fastapi\routing.py", line 253, in app
    content = await serialize_response(
  File "C:\Users\45291029\Documents\venv\ml\lib\site-packages\fastapi\routing.py", line 139, in serialize_response
    raise ValidationError(errors, field.type_)
pydantic.error_wrappers.ValidationError: 200 validation errors for EvergreenOutput
response -> 0 -> product_id
  field required (type=value_error.missing)

无类型注解时的响应结构

[
  {
    "Evergreen": {
      "id_seq": 344,
      "phase_start": "2011-07-22",
      "product_name": "HTTP Server (IHS)",
      "software_product_version_id": 359483,
      "phase_end": "2018-04-30",
      "product_id": 359496,
      "phase_type": "Generally Available",
      "software_product_version_name": "IBM HTTP Server v8.0"
    },
    "Mapping": {
      "eim_product_id": 359496,
      "mem_product_id": 19,
      "name": "HTTP Server (IHS)",
      "product_alias": "IHS",
      "manufacturer": "IBM",
      "id_seq": 0,
      "vendor": "IBM",
      "eim_name": "HTTP Server (IHS)",
      "lev": 100
    }
  }
]

解决方案

问题根源

响应模型写法错误：list[EvergreenOutput, MappingOutput]不是合法的类型注解，无法表示“列表元素同时包含两种模型”的结构。FastAPI会尝试把每个返回元素适配成EvergreenOutput，但实际返回的是包含Evergreen和Mapping的组合对象，导致字段匹配失败。
查询返回结构不匹配：select(Evergreen, Mapping)返回的是包含两个模型实例的组合对象（或元组），和定义的单个输出模型结构完全不符。

修复步骤

定义组合响应模型：在schemas.py中新增一个包含两个输出模型的组合类：

class CombinedLicenceOutput(SQLModel):
    Evergreen: EvergreenOutput
    Mapping: MappingOutput

修改接口的响应模型和返回类型：更新main.py中的接口定义，使用组合模型作为列表元素：

from schemas import CombinedLicenceOutput

@app.get(
    "/licence_ol/",
    summary="Query Licence info by product id",
    response_description="Successful Query",
    response_model=list[CombinedLicenceOutput],
    tags=[Tags.items]
)
def get_product_id(session: Session = Depends(get_session))-> list[CombinedLicenceOutput]: 
    query = select(Evergreen, Mapping).where(Evergreen.product_id == Mapping.eim_product_id).limit(200)
    # 将查询结果转换为符合CombinedLicenceOutput的结构
    results = []
    for evergreen, mapping in session.exec(query).fetchall():
        results.append({
            "Evergreen": evergreen,
            "Mapping": mapping
        })
    return results

部分版本的SQLModel会自动将查询返回的元组转换为带键的字典，此时可以直接返回session.exec(query).fetchall()，但显式转换结构更稳妥。

修改后，响应模型与实际返回结构完全匹配，可通过Pydantic验证，接口即可正常返回数据。

内容的提问来源于stack exchange，提问作者moth

FastAPI同时使用两个Pydantic响应模型报错问题

初始可运行的main.py代码 #

schemas.py代码 #

修改后报错的main.py代码 #

错误信息 #

无类型注解时的响应结构 #

解决方案 #

问题根源 #

修复步骤 #