Dynamic request model in FastAPI - fastapi

Question

I'm writing a class-based generic CRUD router/handler in FastAPI and need something similar to response_model for the request, to generate a Swagger UI to the input model without writing a separate function for the endpoint. Example for reading a user model and filtering it base on it's username:
class ModelReadHandler:
def __init__(self, base_url: str, response_model_cls: type, request_model_cls: type):
self.router = APIRouter()
self.response_model = response_model_cls
self.request_model = request_model_cls
self.add_routes()
def add_routes():
self.router.add_api_route(f'/{self.base_url}/', self.read, methods=['GET'],
response_model=self.response_model)
def read(self, input_model: self.request_model_cls # ** actually can't do this but looking the behaviour of this hypothetical code
session: Depends(get_session),
):
query = input_model.get_read_query()
return session.exec(query).all()
# -- the input_model would extend this abstract class
class InputModel(pydantic.BaseModel):
def get_read_query(self):
raise NotImplementedError()
# class example for InputModel
class UserInputModel(InputModel):
username: int
def get_read_query(self):
return select(UserModel).where(UserModel.username == self.username)
# and we instantiate the user handler like this
user_handler = ModelReadHandler(base_url='user',
request_model_cls=UserInputModel,
response_model_cls=UserModel)
app = FastAPI()
app.include_router(user_handler.router)
Now this doesn't work because there is no way to set the type of the input_model argumnent in ModelReadhandler.read(). Is there a way to achieve this behaviour? - the goal is to add dynamically generated endpoints with swagger uis indicating their request parameters.
Note
An alternative is to extend the class for each model and type hint according to the model. I do not want that.

Answer 1

FastAPI supports defining request models as Pydantic models and uses the defined models to automatically generate Swagger UI documentation for the request parameters. You can use the Body class from fastapi.params to define the request model for the endpoint.
class ModelReadHandler:
def __init__(self, base_url: str, response_model_cls: type, request_model_cls: type):
self.router = APIRouter()
self.response_model = response_model_cls
self.request_model = request_model_cls
self.add_routes()
def add_routes():
self.router.add_api_route(f'/{self.base_url}/', self.read, methods=['GET'],
response_model=self.response_model)
def read(self, input_model: Body(self.request_model)
session: Depends(get_session),
):
query = input_model.get_read_query()
return session.exec(query).all()
# -- the input_model would extend this abstract class
class InputModel(pydantic.BaseModel):
def get_read_query(self):
raise NotImplementedError()
# class example for InputModel
class UserInputModel(InputModel):
username: int
def get_read_query(self):
return select(UserModel).where(UserModel.username == self.username)
# and we instantiate the user handler like this
user_handler = ModelReadHandler(base_url='user',
request_model_cls=UserInputModel,
response_model_cls=UserModel)
app = FastAPI()
app.include_router(user_handler.router)