Cloud Firestore - ensuring data consistency

Question

My database uses redundant data to speed up fetches and minimise the number of documents that need to be read for certain queries. For example I'd store the names of followed users in a map in a users document so I don't have to read another document to retrieve the names of each of the followed users.

User: (Collection) {
    userID: (Document) {

    //user state
    name: ...

    followingUsers: (Map) {
        followingUserID: nameOfUser,
        followingUserID: nameOfUser
    }
}

}

If a user was to change their name, what is the best way to propagate these changes to all places with the redundant data?

Answer 1

Good question! For starters, I'd recommend doing this kind of administrative task in a server SDK or cloud function, since you don't want a client to necessarily have the ability to start mucking with every single User doc.

The good news is that, once you start using the server SDKs, you can then put a query into a transaction. So let's say user_123 changes their name from "Jenny" to "Jen". Your transaction would look something like this in pseudo-code:

• Start Transaction
  • transaction.get(usersRef.where("followingUsers.user_123", ">=", ""))
  • Loop through query results. Grab the doc_id from each doc and use that to start building out the writes in your transaction.
    • transaction.update("/users/<doc_id>/", {"followingUsers.user_123" : "Jen"})
  • Also make sure you add transactions.update("/users/user_123", {"name": "Jen"})
• End transaction

This general approach would also work on the client-side, but you just wouldn't be able to do this in a transaction. (You could still put all of these changes into a batch write, though.)