Update only if key exist firebase cloud functions - node.js

Question

I have a function that counts all the members that are going to do a event.
I have the registered_people key to count how many users are on this event. This key is updated +1 or -1 when someone adds itself to the /registrations/approved link.
This works very well. See the method below.
exports.reservation = functions.database.ref('/agenda/activitys/{year}/{month}/{day}/{time}/{event}/registrations/approved/{key}').onWrite((event) => {
var collectionRef = event.data.adminRef.parent.parent;
var countRef = collectionRef.parent.child('registered_people');
console.log("Fired of reservation watcher");
return countRef.transaction(function(current) {
if (event.data.exists() && !event.data.previous.exists()) {
return (current || 0) + 1;
}
else if (!event.data.exists() && event.data.previous.exists()) {
return (current || 0) - 1;
}
});
});
But my problem is when the admin deletes an event. Url /agenda/activitys/{year}/{month}/{day}/{time}/{event} gets deleted, and the method defined above gets triggered and writes data again to the url. How can I prevent that this method writes anything when an admin deletes the event?
And this code is not working:
if (event.data.previous.exists()) {
return;
}
Because when a user wants to sign out from an event the registered_people must be updated. With the code defined above the delete does not work anymore. So I need to check if the event is deleted.

Answer 1

First, you are running and old version of functions & admin, update to be sure your firebase-functions & firebase-admin are updated:
In your functions folder run:
npm install firebase-functions#latest --save
npm install firebase-admin#latest --save
Then your code should look like this:
exports.reservation = functions.database.ref('/agenda/activitys/{year}/{month}/{day}/{time}/{event}/registrations/approved/{key}').onWrite((change, context) => {
var collectionRef = change.after.ref.parent.parent;
var countRef = collectionRef.parent.child('registered_people');
let increment;
if (change.after.exists() && !change.before.exists()) {
increment = 1;
} else if (!change.after.exists() && change.before.exists()) {
return null;
} else {
return null;
}
return countRef.transaction((current) => {
return (current || 0) + increment;
}).then(() => {
return console.log('Counter updated.');
});
});