FastAPI无法返回MongoDB对象的问题求助及报错排查

我尝试向以下路由发起请求：
http://127.0.0.1:8000/testadmins
使用如下函数：

@app.get("/testadmins/")
async def get_admins():
    return await get_database()

async def get_database():
    uri = "(my mongo uri)"
    client = MongoClient(uri, server_api=ServerApi('1'))
    adms = client.test['admins'].find()
    print(adms)
    
    return adms

该函数应返回集合中的所有管理员数据。控制台打印admins数据正常，但路由无法返回，出现错误：TypeError('vars() argument must have __dict__ attribute')。我尝试添加async await、用FastAPI的json_encodable()包装函数，但都无效。在Postman中请求返回500（内部服务器错误）。

（更新：我推测问题与日期时间类型有关，因为逐个测试键值时，仅datetime.datetime(...)格式的字段无法正常返回。）
（__v和_id字段也无法正常返回，但不确定原因）

错误追踪信息：

Traceback (most recent call last):
  File "/opt/homebrew/lib/python3.11/site-packages/fastapi/encoders.py", line 235, in jsonable_encoder
    data = vars(obj)
           ^^^^^^^^^
TypeError: vars() argument must have __dict__ attribute

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/opt/homebrew/lib/python3.11/site-packages/uvicorn/protocols/http/httptools_impl.py", line 426, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/uvicorn/middleware/proxy_headers.py", line 84, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/fastapi/applications.py", line 289, in __call__
    await super().__call__(scope, receive, send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/applications.py", line 122, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/middleware/errors.py", line 184, in __call__
    raise exc
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/middleware/errors.py", line 162, in __call__
    await self.app(scope, receive, _send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/middleware/exceptions.py", line 79, in __call__
    raise exc
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/middleware/exceptions.py", line 68, in __call__
    await self.app(scope, receive, sender)
  File "/opt/homebrew/lib/python3.11/site-packages/fastapi/middleware/asyncexitstack.py", line 20, in __call__
    raise e
  File "/opt/homebrew/lib/python3.11/site-packages/fastapi/middleware/asyncexitstack.py", line 17, in __call__
    await self.app(scope, receive, send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/routing.py", line 718, in __call__
    await route.handle(scope, receive, send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/routing.py", line 276, in handle
    await self.app(scope, receive, send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/routing.py", line 66, in app
    response = await func(request)
               ^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/fastapi/routing.py", line 291, in app
    content = await serialize_response(
              ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/fastapi/routing.py", line 179, in serialize_response
    return jsonable_encoder(response_content)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/fastapi/encoders.py", line 209, in jsonable_encoder
    jsonable_encoder(
  File "/opt/homebrew/lib/python3.11/site-packages/fastapi/encoders.py", line 238, in jsonable_encoder
    raise ValueError(errors) from e
ValueError: [TypeError("'ObjectId' object is not iterable"), TypeError('vars() argument must have __dict__ attribute')]

问题核心是MongoDB返回的游标、ObjectId、datetime类型无法被FastAPI默认的JSON编码器序列化，具体解决步骤如下：

1. 转换游标为Python列表 #

find()返回的是MongoDB游标对象，不是可直接序列化的列表，需要先转为列表：

adms = list(client.test['admins'].find())

2. 手动处理特殊字段 #

MongoDB的_id是ObjectId类型，datetime字段也无法直接转为JSON，可遍历数据逐个处理：

async def get_database():
    uri = "(my mongo uri)"
    client = MongoClient(uri, server_api=ServerApi('1'))
    cursor = client.test['admins'].find()
    adms = []
    for doc in cursor:
        # 将ObjectId转为字符串
        doc['_id'] = str(doc['_id'])
        # 将datetime字段转为ISO格式字符串（替换为你实际的日期字段名）
        if 'create_time' in doc:
            doc['create_time'] = doc['create_time'].isoformat()
        # 不需要__v字段可直接删除
        if '__v' in doc:
            del doc['__v']
        adms.append(doc)
    return adms

3. 使用MongoDB自带的JSON编码器 #

用bson.json_util工具一键处理所有MongoDB特殊类型：

from bson import json_util
import json

async def get_database():
    uri = "(my mongo uri)"
    client = MongoClient(uri, server_api=ServerApi('1'))
    cursor = client.test['admins'].find()
    adms = list(cursor)
    # 序列化后转回字典，让FastAPI可正常返回
    json_str = json_util.dumps(adms)
    return json.loads(json_str)

4. 全局自定义FastAPI编码器 #

如果需要全局处理MongoDB类型，可自定义编码器并绑定到FastAPI应用：

from fastapi import FastAPI
import json
from bson import ObjectId
from datetime import datetime

class CustomJSONEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, ObjectId):
            return str(obj)
        if isinstance(obj, datetime):
            return obj.isoformat()
        return super().default(obj)

app = FastAPI()

@app.get("/testadmins/")
async def get_admins():
    data = await get_database()
    return json.loads(json.dumps(data, cls=CustomJSONEncoder))

以上任意一种方式都能解决序列化错误，让路由正常返回MongoDB数据。

内容的提问来源于stack exchange，提问作者Tiron Raul