Improve database module

lib/database.ts

@@ -1,14 +1,38 @@
 import mongoose from 'mongoose';
 
+const MONGODB_URI = process.env.MONGODB_URI;
+
+if (!MONGODB_URI) {
+  throw new Error('Please define the MONGODB_URI environment variable inside .env.local');
+}
+
+/**
+ * Global is used here to maintain a cached connection across hot reloads
+ * in development. This prevents connections growing exponentially
+ * during API Route usage.
+ */
+let cached = global.mongoose;
+
+if (!cached) {
+  cached = global.mongoose = { conn: null, promise: null };
+}
+
 export default async function dbConnect() {
-  // check if we have a connection to the database or if it's currently
-  // connecting or disconnecting (readyState 1, 2 and 3)
-  if (mongoose.connection.readyState >= 1) {
-    return;
+  if (cached.conn) {
+    return cached.conn;
   }
 
-  return mongoose.connect(process.env.MONGODB_URI, {
-    useNewUrlParser: true,
-    useUnifiedTopology: true
-  });
+  if (!cached.promise) {
+    const opts = {
+      bufferCommands: false
+    };
+
+    cached.promise = mongoose.connect(MONGODB_URI, opts).then((mongoose) => {
+      return mongoose;
+    });
+  }
+
+  cached.conn = await cached.promise;
+
+  return cached.conn;
 }