MERN: Added a User object to another Model's array, prevented duplicate additions, but now it prevents OTHER users too

问题

My app has two Models. User and Tournament. User model has username and _id (mongodb). Tournament model has participants which is an Array.

I'm trying to make it so each and every user who signs up to a tournament can do so only once.

I wrote an Express route to POST to a tournament's participants array:

router.post('/:id', (req, res) => {
    Tournament.findById(req.params.id)
        .then(tournament => { 
            const userSignedUp = (participantsArray, user) => {
                return participantsArray.some(arr => arr.user === user);
            };
            // Check if User already signed up
            if (userSignedUp(tournament.participants, req.body.user._id) !== true && userSignedUp(tournament.participants, req.body.user.id) !== true) {
                tournament.participants.push(req.body.user);
            } else {
                return res.status(400).json({ msg: "This user is already signed up" });
            }
            return tournament.save();
        })
        .then(savedTournament => res.json(savedTournament))
        .catch(err => res.json(err));
});

After some experimentation I learned how to write that if() statement to prevent the User from signing up more than once:

Then I did my best to wire up Redux to send what I want to that route, via axios

REDUCER:

const initialState = {
  participant: null
};

export default function(state = initialState, action) {
  switch(action.type) {
    case USER_JOINS_TOURNAMENT:
      return {
        ...state,
        participant: action.payload
      }
    case TOURNAMENT_SIGN_UP_FAIL:
      return {
        ...state,
        participant: null
      }
    default:
      return state;
  };
};

ACTION:

export const addParticipant = (_id, user) => dispatch => {
  const config = {
    headers: {
      "Content-Type": "application/json"
    }
  };

  const body = JSON.stringify({ user });

  axios.post(`/tournaments/${_id}`, body, config)
    .then(() => dispatch({
      type: USER_JOINS_TOURNAMENT,
      payload: user
    }))
    .catch(err => {
      dispatch(returnErrors(err.response.data, err.response.status));
      dispatch({
        type: TOURNAMENT_SIGN_UP_FAIL
      });
    });
};

I wasn't sure if I needed config//body there, but it didn't work right without that.

With this in place, a User can log in, create a Tournament, and then Sign up once. If he signs up again, it'll block him with an error message.

However, when I log onto another User, it gives the same error message (even though the tournament.participants array doesn't have that user's ID in it)

AND in my terminal I get this error:

(node:9734) UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
[0]     at ServerResponse.setHeader (_http_outgoing.js:526:11)
[0]     at ServerResponse.header (/home/dustin/Desktop/Smash-Hosting/node_modules/express/lib/response.js:771:10)
[0]     at ServerResponse.send (/home/dustin/Desktop/Smash-Hosting/node_modules/express/lib/response.js:170:12)
[0]     at ServerResponse.json (/home/dustin/Desktop/Smash-Hosting/node_modules/express/lib/response.js:267:15)
[0]     at /home/dustin/Desktop/Smash-Hosting/routes/api/tournaments.js:109:21
[0]     at processTicksAndRejections (internal/process/task_queues.js:97:5)
[0] (node:9734) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
[0] (node:9734) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

I'm not really sure where to start. I read some docs/solutions to that error message but I didn't see something that stands out to me. Perhaps I coded myself into a corner. I hope this all makes sense.

来源：https://stackoverflow.com/questions/61351906/mern-added-a-user-object-to-another-models-array-prevented-duplicate-addition