firestore cloud functions for getting aggregated values in collection - firebase

Question

lets say I have a app that stores users bills, user adds or deletes the bill. the structure of the data is as:
users/user_id/bills/bill_id
bill document structure as
{ bill_name: string, amount: number }
what I want to show the user aggregated values corresponding to bill name
lets say I have 2 entries in bills collection
{ bill_name: 'amazon', amount: 1000 }
{ bill_name: 'amazon', amount: 2000 }
my output should be
{ bill_name: 'amazon', amount: 3000 }
my question is what will be best to get the aggregated values
Create a cloud function that triggers on onWrite /user/user_id/bills/bill_id
and create a new entry in /users/user_id/aggregated_bills/ at the time user adds or deletes the bills and what this function does read the data from /users/user_id/aggregated_bills/bill_id/ where bill name is 'amazon' add do the math to this entry and store the new value in aggregated_bills collection.
also I want to know if we can add or read the database from cloud function other then it was referenced. in functions.firestore.document('/users/{user_id}/bills/{bill_id}').onWrite( ...
create a cloud function that triggers on HTTPS request, and read the data from /users/user_id/bills/ where bill name is 'amazon' and calculate the aggregated values there, and return the response.
may be any other solution for this problem
here I want to say that getting aggregated values will be not only for
one bill, but for multiple bills at the same time. lets say I want to
show the user dashboard where he is seeing aggregated values of his
top 20 bills

Answer 1

The onWrite trigger would make the most sense in this situation. Consider writing the aggregate results as an object/map on the parent document because then you will only need one read operation to consume the data - faster and cheaper.
Your cloud function would look something like this:
exports.aggregateBills = functions.firestore
.document('user/{user_id}/bills/{bill_id}')
.onWrite(event => {
const bill_id = event.params.bill_id;
const user_id = event.params.user_id;
const bill_name = event.data.data().bill_name;
// ref to the parent document
const docRef = admin.firestore().collection('users').doc(userId)
// get all bills and aggregate
return docRef.collection('bills')
.where('bill_name', '==', bill_name)
.get()
.then(querySnapshot => {
// get the total comment count
const bills = []
// loop over bills to create a plain JS array
querySnapshot.forEach(doc => {
bills.push( doc.data() )
});
const aggregateData = 'do your calculations here'
// run update
return docRef.update({ aggregateData })
})
});