Firestore rules - Get specific field from document [duplicate] - firebase

Question

This question already has answers here:
Cloud Firestore: Enforcing Unique User Names
(8 answers)
I want to make unique usernames in firebase/firestore
(1 answer)
Closed 7 days ago.
My data of usercollection looks like this:
username: "johndoe"
email: "john.doe#test.com"
firstName: "John"
lastName: "Doe"
loginUid: "...."
and this is my rule:
rules_version = '2';
service cloud.firestore {
match /databases/{database}/documents {
match /{document=**} {
allow read, write: if request.auth != null;
}
}
}
I just want to read usernames from firestore, if not authenticated to check before registration, if username is already taken.
Is this possible. If not, is there another way, like creating a view of all usernames, which i could read from?

Answer 1

I understand that you want to check if a user document already exists before creating a new one.
A straightforward approach is to maintain a collection of documents with the user name as Document ID and set a security rule like the following one:
match /usersList/{userName} {
allow create: if !exists(/databases/$(database)/documents/usersList/$(userName));
allow update, delete: if false;
}
This collection (usersList) can be the collection where you store the users. But it could also be a collection just dedicated to the verification and the users are contained in another collection.
In this last case you would use a batched write to create the two docs, since batched writes are atomic: If the doc in the usersListRef collection already exists, the new user doc will not be created.
const userName = '....';
// Get a new write batch
const batch = writeBatch(db);
const usersListRef = doc(db, 'usersList', userName);
batch.set(usersListRef, { ... });
const userRef = doc(db, 'users', userName);
batch.set(userRef, { ... });
// Commit the batch
await batch.commit();