Count grandchild nodes in a cloud function - node.js

Question

I have the following data structure:
/users
/{user_uid}
/lists
/{list_uid}
Using a cloud function, i'd like to be able to have a /list_count reference on the root of my database, to be able to easily track list count without having to do a fat client-side call to count them.
at the moment I have this implementation, which I find a bit ugly:
exports.countlists = functions.database.ref('/users/{uuid}/lists').onWrite(event => {
const ref = event.data.ref.parent.parent.parent.child('list_count');
return ref.transaction(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;
}
});
});
The issue being that I get an error in the firebase console:
Error serializing return value: TypeError: Converting circular structure to JSON

Answer 1

as Inlined said here:
The problem you're running into is that the promised value that ref.transaction returns isn't serializeable as JSON. The easiest way to fix this (before we fix it in the Firebase SDK) is to transform the value to something like null.
I think to fix your problem, do this:
exports.countlists = functions.database.ref('/users/{uuid}/lists').onWrite(event => {
let root = admin.database().ref(`users/${event.params.uuid}/lists/list_count`)
return root.transaction(function(current){
if (event.data.exists() && !event.data.previous.exists()) {
return (current || 0) + 1;
}
else{
return (current || 0) - 1;
}
}.then(() => null));
});