Cloud Functions for Firestore: accessing parent collection data - node.js

Question

Many blogs suggest to switch to Cloud Firestore because it's easy and well secured. Coming from Realtime Database and back when using Functions + RD it was easy to navigate through document triggers, like ref.parent
My setup is like this:
Users
{userid}
last_seen: "data"
{forms}
{formid}
However, i have added a document trigger with onCreate, and i want to get the value of last_seen:
exports.updateUser = functions.firestore.document('users/{userId}/forms/{formid}').onCreate((snap, context) => {
const newValue = snap.data();
console.log("test value : " + newValue.test); // works
console.log("form id: " + context.params.formid); // works
console.log("user last seen : " + newValue.last_seen); // doesn't work, can't access the parent collection data
});

Answer 1

I totally get the confusion with the switch to Firestore but it's almost the exact same way in this case.
In realtime, you have the snapshot:
exports.doStuff = functions.database.ref('/users/{userId}/forms/{formId}')
.onCreate((snapshot, context) => {
const ref = snapshot.ref;
const userRef = ref.parent.parent;
userRef.once('value').then(parentSnap => {
const user = parentSnap.val();
const lastSeen = user.last_seen;
});
});
In Firestore:
exports.doStuff = functions.firestore.document.onCreate('/users/{userId}/forms/{formId}')
.onCreate((snapshot, context) => {
const ref = snapshot.ref;
const userRef = ref.parent.parent;
userRef.get().then(parentSnap => {
const user = parentSnap.data();
const lastSeen = user.last_seen;
});
});
Another thing to consider is you are passing the userId in your params so you could just build your own DocumentReference (assuming you're also using firebaseAdmin)
functions.firestore.document.onCreate('/users/{userId}/forms/{formId}')
.onCreate((snapshot, context) => {
const userId = context.params.userId;
const userRef = firebaseAdmin.firestore().collection('users').doc(userId);
userRef.get().then(parentSnap => {
const user = parentSnap.data();
const lastSeen = user.last_seen;
});
});
It also allows you to decouple your logic for functions you may use often, consider it as a "helper" method: (NOTE, I switched to async/await on accident, it's a bit cleaner)
functions.firestore.document.onCreate('/users/{userId}/forms/{formId}')
.onCreate(async (snapshot, context) => {
const userId = context.params.userId;
const lastSeen = await getLastSeen(userId);
});
// == Helper Functions ==-------------------
export async getLastSeen(userId) {
if (!userId) return Promise.reject('no userId');
// User Ref
const userSnap = await firebaseAdmin.firestore().collection('users').doc(userId).get();
return userSnap.data().last_seen;
}
Now you can use getLastSeen() whenever you need it, and if you make a change you only have to adjust that one function. If it's not something you call often then don't worry about it, but I would consider maybe a getUser() helper...

Answer 2

In your code, snap is a DocumentSnapshot type object. As you can see from the linked API documentation, there is a ref property on that object that gets you a DocumentReference object pointing to the document that was added. That object has parent property that gives you a CollectionReference that points to the collection where the document exists, which also has a parent property. So, use these properties to navigate around your database as needed.

Answer 3

Get the reference where the change took place, move 2 levels up and capture data using ref.once() function:
exports.updateUser = functions.firestore.document('users/{userId}/forms/{formid}').onCreate( async (snap, context) => {
// Get the reference where the change took place
const changeRef = snap.after.ref;
// Move to grandad level (2 levels up)
const userIdRef = changeRef.parent.parent;
// Capture data
const snapshot = await userIdRef.once('value');
// Get variable
const lastSeen = snapshot.val().last_seen;
// Do your stuff...
return null;
});