NOW v2 Error: MongoClient must be connected before calling MongoClient.prototype.db

games.js

const { send } = require('micro');
const { handleErrors } = require('../../../lib/errors');
const cors = require('../../../lib/cors')();
const qs = require('micro-query');
const mongo = require('../../../lib/mongo');
const { ObjectId } = require('mongodb');

const handler = async (req, res) => {
  let { limit = 5, exclude = '' } = qs(req);

  limit = parseInt(limit);
  limit = limit > 10 ? 10 : limit;

  const db = await mongo();

  // fetches random games
  const games = await db
    .collection('games_v3')
    // this avoids showing the main quiz between the random results
    .aggregate([
      {
        $match: {
          _id: { $ne: exclude.length > 0 ? ObjectId(exclude) : null },
          removed: { $ne: true }
        }
      },
      { $sample: { size: limit } },
      { $project: { questions: 0, airtableId: 0, removed: 0 } }
    ])
    .toArray();

  send(res, 200, games);
};

module.exports = handleErrors(cors(handler));

mongo.js

// Based on: https://spectrum.chat/zeit/now/now-2-0-connect-to-database-on-every-function-invocation~e25b9e64-6271-4e15-822a-ddde047fa43d?m=MTU0NDkxODA3NDExMg==
const MongoClient = require('mongodb').MongoClient;

if (!process.env.MONGODB_URI) {
  throw new Error('Missing env MONGODB_URI');
}

let client = null;

module.exports = function getDb(fn) {
  if (client && !client.isConnected) {
    client = null;
    console.log('[mongo] client discard');
  }

  if (client === null) {
    client = new MongoClient(process.env.MONGODB_URI, {
      useNewUrlParser: true
    });
    console.log('[mongo] client init');
  } else if (client.isConnected) {
    console.log('[mongo] client connected, quick return');
    return client.db(process.env.MONGO_DB_NAME);
  }

  return new Promise((resolve, reject) => {
    client.connect(err => {
      if (err) {
        client = null;
        console.error('[mongo] client err', err);
        return reject(err);
      }

      console.log('[mongo] connected');
      resolve(client.db(process.env.MONGO_DB_NAME));
    });
  });
};

now.json

{
  "version": 2,
  "name": "kwis-app-api-v3",
  "alias": "api-v2.kwis.app",
  "builds": [{ "src": "api/**/index.js", "use": "@now/node" }],
  "routes": [
    {
      "src": "/games/random",
      "methods": ["GET", "OPTIONS"],
      "dest": "/api/games/random/index.js"
    },
    {
      // other routes
    }
  ],
  "env": {
    "...": "..."
  }
}

@Juanmaster that is what is happening I think (but with a Promise instead of async/await).