Rollback database session in dependency using custom route class

[finswimmer]

Hello,

what I try to achieve is to rollback a database session on an unhanded exception and customize the response in that case.

The first one can be done, handling the database connection in its own method with a try, except, finally block and use it as a dependency. The second one can be done using a custom route class.

But combining both will not work as I wish, because the dependency would be executed after leaving the custom route class.

I've found similar issues/questions about this, but no satisfying answer.

Here's my minimal example which of course doesn't work as a like to have. (Of course the get_db() method returns a database session in real life)

What must I change or how does the pattern must look like to reach out what I want?

#!usr/bin/env python
# -*- coding: utf-8 -*-
import logging
from typing import Callable

import uvicorn
from fastapi import Depends, FastAPI, Request, Response
from fastapi.exceptions import HTTPException
from fastapi.routing import APIRoute

logging.basicConfig(level=logging.DEBUG)


class ExceptionRouter(APIRoute):
    def get_route_handler(self) -> Callable:
        original_route_handler = super().get_route_handler()

        async def custom_route_handler(request: Request) -> Response:
            try:
                return await original_route_handler(request)
            except Exception as exc:
                logging.exception("Unexpected error")
                body = await request.body()
                detail = {"errors": str(exc), "body": body.decode()}
                raise HTTPException(status_code=500, detail=detail)

        return custom_route_handler


def get_db() -> str:
    db = "mydb"
    try:
        yield db
    except Exception:
        logging.debug("rolling back db")
        raise
    finally:
        logging.debug("closing db")


app = FastAPI()
app.router.route_class = ExceptionRouter


@app.get("/exception",)
async def raise_exception(db: str = Depends(get_db)):
    raise Exception("Unexpected error!")


if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8000)

fin swimmer

[tiangolo]

This is because dependencies with yield execute the finishing code after any possible background task is done.

That way, a background task could use the same DB session from the request, even though it's run after the request is finished. You can read more about it here: https://fastapi.tiangolo.com/tutorial/dependencies/dependencies-with-yield/#dependencies-with-yield-and-httpexception

The way to do it would be to commit your DB changes and anything else you need to do in your path operation function so that any exception that needs to be raised is raised there. And then you would have a custom exception handler: https://fastapi.tiangolo.com/tutorial/handling-errors/#install-custom-exception-handlers

[github-actions]

Assuming the original issue was solved, it will be automatically closed now. But feel free to add more comments or create new issues.