我有一个Mongo数据库的快速API，我试图实现我的一些功能，需要mongo连接测试这里我的集合用户及其字段：

from mongoengine import Document, EmailField, StringField, EnumField 
class User(Document):  
    meta = {"collection": "users"}
    email = EmailField(unique=True)
    hashed_password = StringField()
    role = EnumField(UserRole)

    def username(self):
        return self.email.split('@')[0]

    def __str__(self):
        return f'{self.email} ({self.role})

这里我的函数

class UserService:

    @staticmethod
    def existing_user(email: str) -> Optional[User]:

        try:
             user = User.objects(email=email).first()
        except ServerSelectionTimeoutError as e:
            raise e

        return user

测试这个函数的最好方法是什么（使用pytest）？我想嘲笑我的MongoDB，但我没有使用mongomock或直接使用monkeypatch管理
非常感谢
我的测试文件

from domain.entities.user import UserRole, User
 from mongomock import MongoClient
 class TestUserServiceMethods:

    email="john@doe.fr"

    # User example
    user = User(email=email,
                  role=UserRole.admin,
                  hashed_password="hashed_pwd")

    add_user = {
      "email": email,
      "role": UserRole.admin,
      "hashed_password": "hashed_pwd"
    }

    

    user_service = UserService()

    client = MongoClient()

    mock_db = client['ml-db']
    user_collection = mock_db['users']
    user_collection.insert_one(add_user)


    def test_existing_user(self, monkeypatch):
        with monkeypatch.context() as m:
            m.setattr('pymongo.MongoClient', lambda *args, **kwargs: self.client)

            result = self.user_service.existing_user(self.email)

             assert result == self.user

看起来模拟没有成功，因为我得到了错误：

mongoengine.connection.ConnectionFailure: You have not defined a default connection

我应该直接使用Fast app和test_client吗？