Firebase Multiple Collection Update - objective-c

Question

Is there a way to update two documents, each in a different
collection, in one request?
I know you can batch write using FIRWriteBatch - this seems to be limited to the same collection for any document updates. When trying to attach updates for documents in two different collections:
// Just for example
FIRWriteBatch *batch = FIRWriteBatch.new;
[batch updateData:#{#"posts" : #1} forDocument:[self.firebase.usersCollection documentWithPath:#"some_user_id"]];
[batch setData:#{#"test" : #"cool"} forDocument:[self.firebase.postsCollection documentWithPath:#"some_post_id"]];
[batch commitWithCompletion:^(NSError * _Nullable error) {
NSLog(#"error: %#", error.localizedDescription);
}];
It never executes - the app crashes and I get this:
Terminating app due to uncaught exception 'FIRInvalidArgumentException', reason:
'Provided document reference is from a different Firestore instance.'
Apparently the batch does not like updates in more than one collection.
Does anyone know how you can update two documents, each in a different collection, without one failing and the other succeeding?
I want to avoid, for example, successfully setting posts = 1 for a document in usersCollection, while failing to write a new document in postsCollection.
I understand it's pretty unlikely that one will write while the other fails, but in the case that it does happen, I obviously don't want inconsistent data.
NOTE:
For anyone who cares - I don't know if it ever will fail, but as of now I am running the transaction without reading the document before updating data... 🤷‍♂️ Cheers to -1 API call!

Answer 1

You should use a transaction, which is documented adjacent to batch writes:
Using the Cloud Firestore client libraries, you can group multiple
operations into a single transaction. Transactions are useful when you
want to update a field's value based on its current value, or the
value of some other field. You could increment a counter by creating a
transaction that reads the current value of the counter, increments
it, and writes the new value to Cloud Firestore.
You are not limited to a single collection when performing a transaction. You are just obliged to read the document before you write it:
A transaction consists of any number of get() operations followed by
any number of write operations such as set(), update(), or delete().
In the case of a concurrent edit, Cloud Firestore runs the entire
transaction again. For example, if a transaction reads documents and
another client modifies any of those documents, Cloud Firestore
retries the transaction. This feature ensures that the transaction
runs on up-to-date and consistent data.