Fix with-mongodb hot-reload issue and race condition

examples/with-mongodb/util/mongodb.js

@@ -3,9 +3,6 @@ import { MongoClient } from 'mongodb'
 let uri = process.env.MONGODB_URI
 let dbName = process.env.MONGODB_DB
 
-let cachedClient = null
-let cachedDb = null
-
 if (!uri) {
   throw new Error(
     'Please define the MONGODB_URI environment variable inside .env.local'
@@ -18,20 +15,24 @@ if (!dbName) {
   )
 }
 
+// global is used here to ensure the connection is cached across
+// hot-reloads during development.
+let cached = global.mongo
+if (!cached) cached = global.mongo = {}
+
 export async function connectToDatabase() {
-  if (cachedClient && cachedDb) {
-    return { client: cachedClient, db: cachedDb }
+  if (cached.connection) return cached.connection
+  if (!cached.promise) {
+    cached.promise = new Promise(async (resolve) => {
+      const client = await MongoClient.connect(uri, {
+        useNewUrlParser: true,
+        useUnifiedTopology: true,
+      })
+      const db = await client.db(dbName)
+      cached.connection = { client, db }
+      resolve()
+    })
   }
-
-  const client = await MongoClient.connect(uri, {
-    useNewUrlParser: true,
-    useUnifiedTopology: true,
-  })
-
-  const db = await client.db(dbName)
-
-  cachedClient = client
-  cachedDb = db
-
-  return { client, db }
+  await cached.promise
+  return cached.connection
 }