make mill optional

prisma/migrations/20221011095300_bill_optional/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "Booking" ALTER COLUMN "billId" DROP NOT NULL;

prisma/schema.prisma

@@ -45,7 +45,7 @@ model Booking {
   createdAt       DateTime      @default(now()) @db.Date
   updatedAt       DateTime      @updatedAt @db.Date
   bill            Bill?         @relation(fields: [billId], references: [id], onDelete: Cascade)
-  billId          Int           @unique
+  billId          Int?          @unique
   calendarEventId String?
   city            String
   destination     String