Update/delete preconditions

The library supports the preconditions:

use firestore::*;
use serde::{Deserialize, Serialize};
#[derive(Debug, Clone, Deserialize, Serialize)]
struct MyTestStructure {
    some_num: u64,
}
const TEST_COLLECTION_NAME: &str = "test";
async fn example(db: FirestoreDb) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
let object_updated: MyTestStructure = db
    .fluent()
    .update()
    .fields(paths!(MyTestStructure::{some_num}))
    .in_col(TEST_COLLECTION_NAME)
    .precondition(FirestoreWritePrecondition::Exists(true))
    .document_id("test-1")
    .object(&MyTestStructure { some_num: 1 })
    .execute()
    .await?;
let _ = object_updated;
Ok(())
}

Concurrent writes to one document

Single writes to the same document are applied one after another, and the last one wins. There is no conflict error between them: an update() overwrites the fields an earlier write stored, whether or not its sender saw that write. A document that gets more writes per second than it can take fails them with ABORTED, see When a write fails.

FirestoreWritePrecondition::UpdateTime makes a write conditional on the document not having changed since it was read. A write that finds a different update time fails with FAILED_PRECONDITION, as a DatabaseError with the code FailedPrecondition. The library does not retry it, and sending the same write again fails the same way. The error also comes back when the library sent the write again after a first attempt that was applied, see Single writes.

To read a document and write a value computed from it, use run_transaction, which runs the whole function again when the document changed, see Transactions.