Result in filter_map_async

[vtvz]

I have the filter for user what makes query to DB. I love that filter_map_async can provide some dependencies downstream to the leafs of graph. But it fails to handle any kinds of errors. My exact case is:

• Stop flow execution on error in filter
• Do something if user registered
• Do something if user not registered

My current approach looks like this:

Update::filter_message()
  .enter_dialogue::<Message, storage::SqliteStorage, DialogueState>()
  .branch(dptree::filter_map_async(filter_with_user).endpoint(with_user))
  .branch(dptree::filter_async(filter_without_user).endpoint(without_user))

• filter_with_user returns None on Error or lack of records in DB
• filter_with_user returns false on Error or record in DB

But it makes two queries in DB which twice more than I'd feel comfortable.

If I use following approach:

Update::filter_message()
  .enter_dialogue::<Message, storage::SqliteStorage, DialogueState>()
  .branch(dptree::filter_map_async(filter_with_user).endpoint(with_user))
  .endpoint(without_user)

It will lead a user to a wrong flow on casual error in db request.

It would be nice to have something like this:

Update::filter_message()
  .enter_dialogue::<Message, storage::SqliteStorage, DialogueState>()
  .branch(dptree::filter_map_result_async(filter_with_user).endpoint(with_user))
  .endpoint(without_user)

Where filter_with_user has Result<Option<T>> type and flow will be broken if error occurred.
Maybe I miss something, but I don't see a simple way to get around this requirement. Any help, please?

[WaffleLapkin]

I think this was already answered in telegram, but I'll answer it here too, so other users can find this too.

One way to handle these three cases (db error, some data, no data) is to first add it to the context with map and then filter the result. So it may look something like this:

type FetchRes = Result<Option<User>, Error>;

async fn fetch_user(/* args */) -> FetchRes { ... }

Update::filter_message()
    .enter_dialogue::<Message, storage::SqliteStorage, DialogueState>()
    .map_async(fetch_user)
        // Filter `Ok(Some(_))`
        .branch(filter_map(|r: FetchRes| r.ok().flatten()).endpoint(with_user))
        // Filter `Ok(None)`
        .branch(filter(|r: FetchRes| matches!(r, Ok(None)).endpoint(without_user))
        // Fallback for `Err(_)`
        .endpoint(db_error)

Hope that helps!

[vtvz]

Thanks! I planned to publish aswer from telegram channel to here, but completely forgot about it)