another try to fix types for models

db/booking.ts

@@ -1,14 +1,14 @@
 import * as mongoose from 'mongoose'
 import { v4 as uuidv4 } from 'uuid'
 import { dateFormatBackend, getDays } from '../helpers/date'
-import { Booker } from './booker'
+import { BookerDocument } from './booker'
 import { BOOKING_STATUS } from './bookingStatus'
 
-export interface Booking
+export interface BookingDocument
   extends mongoose.Document,
     mongoose.SchemaTimestampsConfig {
   uuid: string
-  booker: Booker
+  booker: BookerDocument
   startDate: Date
   endDate: Date
   status: BOOKING_STATUS
@@ -18,7 +18,11 @@ export interface Booking
   days?: string[]
 }
 
-const BookingSchema = new mongoose.Schema<Booking>(
+export interface BookingModel extends mongoose.Model<BookingDocument> {
+  findBookedDays(): Promise<string[]>
+}
+
+const BookingSchema = new mongoose.Schema<BookingDocument>(
   {
     // need a seperate uuid to be able to target a booking anonimously
     uuid: {
@@ -78,12 +82,14 @@ BookingSchema.static('findBookedDays', async function (): Promise<string[]> {
   ).exec()
 
   return bookings
-    .map((booking: Booking) => booking.days)
+    .map((booking: BookingDocument) => booking.days)
     .flat()
     .sort()
 })
 
-const Model: mongoose.Model<Booking> =
-  mongoose.models.Booking || mongoose.model('Booking', BookingSchema)
+const Model: BookingModel = mongoose.model<BookingDocument, BookingModel>(
+  'Booking',
+  BookingSchema
+)
 
 export default Model