How can I update or create the parent document when a nested city document is created in Firestore?

I have a Firestore trigger that fires when a new city document is created under a country at the path:

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

The goal is to increment the cityCount field in the corresponding country document at /countries/{countryId}. However, my approach using a transaction with update() fails for the very first city created because the parent country document doesn’t exist at that moment.

The Firestore documentation explains that collections and documents are created implicitly, but it doesn’t mention that a nested document creation may occur before its parent document exists. Below is the code I’m using which illustrates the issue:

JAVASCRIPTCopied
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 ⚠️
    },
);

I’m looking for a solution that reliably updates (or creates) the parent document when the first nested document is added. What is the best way to achieve this?