Firestore Transaction Triggering Itself Recursively - google-cloud-firestore

Question

I have a booking feature in my app and I'm trying to use transactions to prevent multiple users from booking the same timeslot.
I have a collection called "bookedTimeslots" under each user. Each document in the collection represents a 15 minute timeslot with a specific start date. If a user books a one hour appointment, the system should create 4 "bookedTimeslot" records. First, though, I need to check if any of these records already exists and reject the operation if they do.
Below, I've pasted the code for the transaction in Node. The function keeps timing out, though. I'm guessing it's because the first "set" operation is updating the results of my bookedTimeslotQuery and triggering the transaction to re-evaluate. If that's the case, is there some way to batch all the sets together or prevent the transaction from triggering itself?
export const afs = admin.firestore();
const startDate = appointment.startDate;
const endDate = appointment.endDate;
const bookedTimeslotQuery = afs.collection("users").doc(userId).collection("bookedTimeslots")
.where("startDate", ">=", startDate)
.where("startDate", "<", endDate);
const timeslotIncrement = 15*60; // 15 minutes
await afs.runTransaction(async (t) => {
const docs = await t.get(bookedTimeslotQuery);
if (!docs.empty) {
throw new BookingError("timeslot not available", ErrorCode.TimeslotNotAvailable);
}
const tempStartDate = new Date(startDate);
while (tempStartDate < endDate) {
const bookingTimeslotId = uuidv4();
const bookingTimeslot = {
id: bookingTimeslotId,
startDate: tempStartDate,
duration: timeslotIncrement
};
t.set(afs.collection("users").doc(userId).collection("bookedTimeslots").doc(bookingTimeslotId), bookingTimeslot);
tempStartDate.setSeconds(tempStartDate.getSeconds() + timeslotIncrement);
}
});
UPDATE:
I found a workable solution for anyone who comes across this in the future. Instead of using a query, I used sequential individual document reads, then writes.
For my specific use case, instead of using a UUID for the document name, I use the ISOString of the document start date. Then, I sequentially call get then set inside the loop and individually check each document. Here's the code (it still needs to be cleaned up but this works).
export const afs = admin.firestore();
const startDate = appointment.startDate;
const endDate = appointment.endDate;
const timeslotIncrement = 15*60; // 15 minutes
await afs.runTransaction(async (t) => {
const tempStartDate1 = new Date(startDate);
while (startDate < endDate) {
const dateISOString = tempStartDate1.toISOString();
const docRef = afs.collection("users").doc(userId)
.collection("bookedTimeslots")
.doc(dateISOString);
const currentTimeslotDoc = await t.get(docRef);
if (currentTimeslotDoc.exists) {
throw new BookingError("timeslot not available", ErrorCode.TimeslotNotAvailable);
}
tempStartDate1.setSeconds(tempStartDate1.getSeconds() + timeslotIncrement);
}
const tempStartDate2 = new Date(startDate);
while (startDate < endDate) {
const dateISOString = tempStartDate2.toISOString();
const docRef = afs.collection("users").doc(userId)
.collection("bookedTimeslots")
.doc(dateISOString);
const bookingTimeslot = {
id: dateISOString,
startDate: new Date(tempStartDate2),
duration: timeslotIncrement
};
t.set(docRef, bookingTimeslot);
tempStartDate2.setSeconds(tempStartDate2.getSeconds() + timeslotIncrement);
}
});