UnhandledPromiseRejectionWarning: MongooseServerSelectionError

Question

I am following these tutorials as I wanted to start with MongoDB and the MERN stack:

• https://medium.com/@beaucarnes/learn-the-mern-stack-by-building-an-exercise-tracker-mern-tutorial-59c13c1237a1
• https://www.youtube.com/watch?v=7CqJlxBYj-M

Everything was fine until my connection with the database failed. I checked the username/password million times, I copied and pasted the original in case I had some syntax error. This is the error:

(node:10156) UnhandledPromiseRejectionWarning: MongooseServerSelectionError: connection <monitor> to 34.249.129.6:27017 closed
    at new MongooseServerSelectionError (C:\Users\Mario\Desktop\ReactProject\MongoDB\mern-tracker\node_modules\mongoose\lib\error\serverSelection.js:22:11)
    at NativeConnection.Connection.openUri (C:\Users\Mario\Desktop\ReactProject\MongoDB\mern-tracker\node_modules\mongoose\lib\connection.js:808:32)
    at Mongoose.connect (C:\Users\Mario\Desktop\ReactProject\MongoDB\mern-tracker\node_modules\mongoose\lib\index.js:333:15)
    at Object.<anonymous> (C:\Users\Mario\Desktop\ReactProject\MongoDB\mern-tracker\server\server.js:51:10)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
    at startup (internal/bootstrap/node.js:283:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:622:3)
(node:10156) 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(). (rejection id: 1)
(node:10156) [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.

and this is my server.js:

const express = require("express");
const mongoose = require("mongoose");
require("dotenv").config();
const app = express();

const port = process.env.PORT || 5000;

app.use(cors());
app.use(express.json());

const uri = process.env.ATLAS_URI;



mongoose.connect(uri, {
  useNewUrlParser: true,
  useCreateIndex: true,
  useUnifiedTopology: true
});

const connection = mongoose.connection;
// connection.on("error", error => console.error(error));
connection.once("open", uri => {
  console.log("MongoDB database connections established");
});

const exercisesRoute = require("./routes/exercises");
const usersRoute = require("./routes/users");

app.use("./exercises", exercisesRoute);
app.use("./users", usersRoute);

app.listen(port, () => {
  console.log(`Server is running on port ${port}`);
});

Since the error is saying about Unhandled promise, what I also tried in my mongoose connection is:

  mongoose
    .connect(uri, {
      useNewUrlParser: true,
      useCreateIndex: true,
      useUnifiedTopology: true
    })
    .catch(err => {
      console.log(Error, err.message);
    })
    .then(() => console.log("DB Connected!"))

and that gives me the next error:

{ [Function: Error] stackTraceLimit: 16, prepareStackTrace: undefined } 'connection <monitor> to 52.212.0.151:27017 closed'
DB Connected!

and yet, is not connected.

Sometimes in the error, the connection <monitor> to 52.212.0.151:27017 closed' changes to 34.249.129.6:27017

Another thing that I tried was deleting the Cluster and creating a new one but the same thing happens I am using windows and node v10.16.3

Hope you guys can help me or advice me.

Answer 1

Try this

mongoose.connect(uri, {
      useNewUrlParser: true,
      useCreateIndex: true,
      useUnifiedTopology: true
    }).then(res=>{
            console.log("DB Connected!")
    }).catch(err => {
      console.log(Error, err.message);
    })

Answer 2

Try this It seems like you are using MongoDB atlas

mongoose
     .connect( uri, { useNewUrlParser: true, useCreateIndex: true, useUnifiedTopology: true })
     .then(() => console.log( 'Database Connected' ))
     .catch(err => console.log( err ));

and go to your MongoDB Atlas -> NetworkAccess-> Edit-> and add Current IP address it works!!