add mongo adapter via mongoose

db/index.ts

@@ -3,11 +3,11 @@ import BookingModel, { Booking, BookingDocument } from './booking'
 import BillModel, { Bill } from './bill'
 import { BOOKING_STATUS } from './enums'
 
-let connectedPromise: Promise<typeof mongoose>
+let connectedPromise: Promise<mongoose.Mongoose>
 
-function connect(): Promise<typeof mongoose> {
+export function connect(): Promise<mongoose.Mongoose> {
   if (connectedPromise) {
-    return
+    return connectedPromise
   }
 
   connectedPromise = mongoose.connect(process.env.MONGO_URI, {