Javascript – Firestore onDocumentCreated event handler for nested document – updating the parent document

Let’s say you’re counting the number of cities in a country, so you register an onDocumentCreated trigger for:

/countries/{countryId}/cities/{cityId}

In the trigger, you’ll want to increase the cityCount in the Country document at /countries/{countryId}. How should this be done?

The documentation states:

Collections and documents are created implicitly in Cloud Firestore.
Simply assign data to a document within a collection.
If either the collection or document does not exist, Cloud Firestore creates it.

My approach of running a transaction to update the count failed, because the parent document doesn’t exist at the moment the first city in that country was created.

const {onDocumentCreated} = require("firebase-functions/v2/firestore");
const {initializeApp, getApps} = require("firebase-admin/app");
const {getFirestore, DocumentReference} = require("firebase-admin/firestore");

if (!getApps().length) initializeApp();
const db = getFirestore();

exports.firestoreEventHander = onDocumentCreated(
    "countries/{countryId}/cities/{cityId}",
    async (event) => {
      const snapshot = event.data;
      console.log("snapshot exists:", snapshot.exists);  // true

      const countryRef = snapshot.ref.parent.parent;
      console.log("country path:", countryRef.path);  // countries/USA

      // Access snapshot through the db
      const cityDoc = await db.doc(`countries/${countryId}/cities/${cityId}`).get();
      console.log("cityDoc.exists:", cityDoc.exists);  // true

      // Check if the countryRef document exists
      const countryDoc = await countryRef.get();
      console.log("countryDoc:", countryDoc.data());  // ⚠️ undefined ⚠️
      console.log("countryDoc.exists:", countryDoc.exists);  // ⚠️ false ⚠️

      // Try to access the country via db.doc()
      const countryDoc2 = await db.doc(`countries/${countryId}`).get();
      console.log("countryDoc2:", countryDoc2.data());  // ⚠️ undefined ⚠️
      console.log("countryDoc2.exists:", countryDoc2.exists);  // ⚠️ false ⚠️
    },
);

The Trigger a function when a new document is created example doesn’t mention anything about the parent document of newly created nested documents not existing yet.