Prevent Map Update if Key Exists-CodePudding

I'm trying to create a backend in firestore to manage availability. I have an employeeAvailability collection with documents for each employee that each contain a map where the keys are start times of the appointment booked for that employeee. The data is used to generate timeslots on the client side that a user can select to book (an appointment means the timeslot is unavailable).

employee12345 {
    appointments = { "10:00 AM" : true,
                     "11:00 AM" : true,
                     "12:00 PM" : true,
                     "1:00 PM" : true, 
                     "2:00 PM" : true
    }
}

I wanted to create a firestore rule that prevents users from double booking an appointment. When the user tries to book a timeslot for 1:00 PM, in the above example, I'd need to do the following update.

db.collection("employeeAvailability").document("employee12345").updateDate([
     "appointments.1:00 PM": true
]) { err in
    if let err = err {
        print("Error updating document: \(err)")
    } else {
        print("Document successfully updated")
    }
}

I'd like this transaction to be rejected and the firebase err to say something along the lines of "This appointment slot is already booked".

Obviously, there are client side rules preventing people from choosing booked timeslots, but it's possible I could have users simultaneously trying to book the same open spot and I'd like the database to handle these race conditions by accepting the first write and rejecting the second.

CodePudding user response：

I'd like the database to handle these race conditions by accepting the first write and rejecting the second.

You should then use a transaction, along the following lines:

  const db = firebase.firestore();

  const employeeDocRef = db
    .collection('employeeAvailability')
    .doc('employee12345');

  const timeSlot = '1:00 PM';

  db.runTransaction((transaction) => {
    return transaction.get(employeeDocRef).then((employeeDoc) => {
      const appointments = employeeDoc.get('appointments');

      if (appointments.hasOwnProperty(timeSlot)) {
        throw 'Slot already taken';
      }

      appointments[timeSlot] = true;
      transaction.update(employeeDocRef, { appointments });
    });
  })
    .then(() => {
      console.log('Transaction successfully committed!');
    })
    .catch((error) => {
      console.log('Transaction failed: ', error);
    });