Firestore Cloud Function Trigger only when path updates with new entire - function

Question

I have a firestore database that looks like this
/entries/ ....
/users/{userid}...
a bunch of documents is being sent into ... of entries and userid contains on 8 docs of user profile information.
my problem is that the entries doc contains field hours and no relation to the user doc which contains the field weekly_capacity
I need to aggregate this the two fields hours/weekly capacity setting them to Full-time equivalency variable
But the Full-time equivalency needs to be accurate and this company FTE can change so it would need to calculate the FTE over various date even if the user changed their FTE status x number of times.
And the current app only fetched the entries when the user logins into the app, which can be whenever.
None of the API requests that I am using will give me a json that holds both weekly_capacity and hours on the same fetch. If every time a user logs into the app firestore calls the http to fetch all entries then how can I compare the hours field on the collection's entries to the weekly_capacity field
Just a little context: FTE = Full-time equivalency and is used to measure as a standard to see if an employee compares to there core commit hours they signed up for which is 40. SO if I agreed to work 40 and I work actually work 40 hours then I would be 1 whole FTE. If I worked 20 and I suppose to work 40 I am .5 FTE. The math is really simple it's just that in my situation the variable FTE can change any time and the app will allow the user to enter a range of dates fetching the total actual hours they worked and FTE letting them know how many hours they were supposed to work vs how many hours they actually worked. Since the variable changes, I need some way in firestore to track the change and aggregate correctly against the hours actually worked. To give an error example: let's say I changed my FTE from 1 to .7 on March 20th, I then want to generate a report of March 1 to March 30th stating my hours worked and FTE status meaning did I reach my goal. The kicker is that I can't fetch or merge the entries which hold the var hours and /users/ which hold the var weekly_capacity.
I don't even think a cloud function would solve the problem since entries are only fetched when the user logins in right?

Answer 1

I'm assuming the following for answering your question.
Requirement: To calculate FTE for a user when user's weekly_capacity is updated or user logs in.
Problems:
Some way in firestore to track the change.
Calculate FTE correctly according to the change.
Here's what I think will solve the problems.
Google Cloud Firestore supports listeners for the collections in which you store the data. So you can listen for any change in users collection and entries collection. This is how you can track the change.
To calculate FTE, when a change is made to weekly_capacity of user document or a new entry is made to entries collection you need to query both collections separately to get the records corresponding to the user affected. You can also use a collections-group query for this purpose but that depends on your database design.
Hope that helps.