How to trigger onCreate() when a new field is created in an existing Firestore document? - firebase

Question

I want to create a new node in firebase realtime database when a field is created in an existing Firestore document.
I have been trying this:
exports.addUserCredentials = functions.firestore
.document(`Users/{UserID}/{username}`)
.onCreate((snapshot, context) => {
const newUserData = snapshot.data()
const newUserUsername = newUserData
const newUserUidDoc = context.params.UserID
return admin.database().ref(`/userCredentials/${newUserUsername}`).set({"UID": newUserUidDoc})
})
I have searched around the web I saw the path must be directed towards a document only and not a collection. BUT username in the path is a FIELD in the document.
I am getting this error while deploying and I have seen all similar questions but those didn't perfectly answered mine:
! functions: failed to update function addUserCredentials
HTTP Error: 400, The request has errors
The reason is can't change my path to Users/{UserID} which will make my code run perfectly is the fields of document are not added at once.
Here is screenshot of my firestore structure:
The 4 fields of document are updated in 2 batches.
The EMAIL and timeCreated fields are added first and those create the document.
While on the other hand, phoneData and username are fields are CREATED [not updated] after 5 seconds of Email and timeCreated.
So if I use onCreate() on the path Users/{UserID}, it will return UNDEFINED to my realtime database as the username field is ABSENT at that instant.
Is there any way to apply onCreate() on a specific field of the document?
[I am doing this to create a separate node which contains username and UID, this is to check if an username exists when a new user is trying to sign up]
So if the node is created with value undefined it will be an issue.
It will be like this:

Answer 1

The EMAIL and timeCreated fields are added first and those create the document. While on the other hand, phoneData and username are fields are CREATED [not updated] after 5 seconds of Email and timeCreated.
No matter what fields are adding once you created a document,it will be considered as an update operation against that document.As you mentioned in the question,there will be no field with field name called username with a document while you creating document.So it is not possible to get the value of username while you creating the document.
According to your explanation the field username will be only available with the onUpdate trigger.
So the code should be something like below
exports.addUserCredentials = functions.firestore
.document(`Users/{UserID}`)
.onUpdate((snapshot, context) => {
const beforeData = snapshot.before.data()
const afterData = snapshot.after.data()
if(!beforeData.username && afterData.username){
return admin.database().ref(`/userCredentials/${newUserUsername}`).set({"UID": newUserUidDoc})
}
})