CarReservation{

  String userName;
  String id;
  // in millis
  long startDate;
  long endDate;

  CarCategory carCategoryEnum;

   //--end common model
   // only vendor 1 supports discounts on referrals
   Referral referral;

   // only vendor 2 supports delivering and pick up from home
   Address homeDeliveryAddress;
    ...
 }

CarReservation{

  String userName;
  String id;
  // in millis
  long startDate;
  long endDate;

  CarCategory carCategoryEnum;

  //--end common model

  vendor_1_name : {
  // only vendor 1 supports discounts on referrals
    Referral referral;
  }

  vendor_2_name : {
    // only vendor 2 supports delivering and pick up from home
    Address homeDeliveryAddress;
  }
..
}