Firebase Transactions Reservations System

Question

I am building a reservation system using firebase. The user can select a number of seats and i want to write them in my document. The selected seats are inside an array and they are objects, like so:

    {
      reservationId: 'id', 
      selectedSeats: [{"x": 0, "y": 0,},
                      {"x": 1, "y": 0,},
                      {"x": 2, "y": 0,}]
    }

I want to ensure that no other user could write to the database with at least 1 same seat with transactions.

let transaction = db.runTransaction(t => {
  return t.get(venueId)
    .then(doc => {
      //I need to get all the objects inside the document and check their array of reservations.
      t.set(venueId, {reservation});
    });
}).then(result => {
  console.log('Transaction success!');
}).catch(err => {
  console.log('Transaction failure:', err);
});

How can i make the transaction fail if a seat is found in the array of each object inside the document?

Answer 1

You could use array-contains-any for checking if there are any of your seats reserved on the db already and return a sucessful or failing promise depending on the results, also you need to create a reference to an empty document outside the transaction before you can create the document inside it, here is what it could look like:

//reference to a document that still does not exists
var docRef = db.collection(reservation).doc()
let transaction = db.runTransaction(t => {
  return t.get(venueId)
    .then(doc => {
      db.collection('reservation')
        .where('selectedSeats', 'array-contains-any', reservation.selectedSeats)
        .get()
        .then(snapshot =>{
            if (snapshot.empty) {
                t.set(docRef, {
                    uid: venueId,
                    reservation: reservation
                });
                return Promise.resolve('Reservation Complete');
            }
            return Promise.reject('Seats already taken');
        })
    });
}).then(result => {
  console.log('Transaction success!');
}).catch(err => {
  console.log('Transaction failure:', err);
});

NOTE : As you can see on the documentation for array-contains-any , this where clause will be limited to 10 values on your reservation.selectedSeats array. Also I am not sure if you have to use venueId to get the empty document or not, since you want to use that ID, so I encourage you to try both ways.