MongoError: Topology is closed, please connect

Actually there is one more thing that may be causing this as I also had the same exact error "Topology Closed, please connect".

The thing is if you have a dynamic IP address then in MongoDB atlas you should try allowing all IP addresses.

Add the IP address: 0.0.0.0/0All my problems were solved after whitelisting this IP address which allows all.

Image of dynamic IP 0.0.0.0/0:

Thanks to GitHub's "Used By" dropdown on the apollo-datasource-mongodb I was able to cheat off a few other repositories, and here's what I ended up with (with changes marked in comments):

const { MongoClient } = require('mongodb');const assert = require('assert');const { ApolloServer, gql } = require('apollo-server');const { MongoDataSource } = require('apollo-datasource-mongodb');// Isolated these for prominence and reuseconst dbURL = 'mongodb://localhost:27017';const dbName = 'projectdb';// Made each function async/await// Swapped the datasource's findOneById() for the collection itself & standard Mongo functionsclass Items extends MongoDataSource {  async getItem(id) {    return await this.collection.findOne({id: id});  }}const typeDefs = gql`  type Item {    id: Int!    title: String!  }  type Query {    item(id: Int!): Item  }`;// Made each query async/awaitconst resolvers = {  Query: {    item: async (_, { id }, { dataSources }) => {      return await dataSources.items.getItem(id);    },  }}// Move the ApolloServer constructor to its own function that takes the dbconst init = (db) = {  return new ApolloServer({    typeDefs,    resolvers,    dataSources: () => ({      items: new Items(db.collection('items')),    }),  });}// Use .connect() instead of new MongoClient// Pass the new db to the init function defined above once it's been defined// Call server.listen() from within MongoClientMongoClient.connect(dbURL, { useNewUrlParser: true, useUnifiedTopology: true }, (err, client) => {  assert.equal(null, err);  const db = client.db(dbName);  console.log(`Mongo database ${ dbName } at ${ dbURL }`);  const server = init(db);  server.listen().then(({ url }) => {    console.log(`Server ready at ${ url }`);  });});

With these changes, the Apollo Playground at localhost:4000 works great! Now to solve the 400 error I'm getting in my client app when I query...