put models in file not schema

db/booking.js

@@ -0,0 +1,41 @@
+import * as mongoose from 'mongoose'
+
+import { getDays, dateFormat } from '../helpers/date'
+
+const BookingSchema = new mongoose.Schema(
+  {
+    booker: {
+      type: mongoose.Schema.Types.ObjectId,
+      ref: 'Booker',
+      required: true,
+    },
+    startDate: {
+      type: Date,
+      required: true,
+      get: dateFormat,
+      min: new Date(),
+    },
+    endDate: { type: Date, required: false, get: dateFormat, min: new Date() },
+    status: {
+      type: String,
+      enum: ['requested', 'confirmed', 'rejected'],
+      required: true,
+      default: 'requested',
+    },
+    purpose: { type: String, required: false },
+    org: { type: String, required: false },
+    destination: { type: String, required: false },
+  },
+  {
+    timestamps: true,
+    toJSON: { virtuals: true, getters: true },
+    toObject: { virtuals: true, getters: true },
+  }
+)
+
+BookingSchema.virtual('days').get(function () {
+  return getDays({ startDate: this.startDate, endDate: this.endDate })
+})
+
+export default mongoose.models.Booking ||
+  mongoose.model('Booking', BookingSchema)