How to get typed keys when querying a storage map via subxt

Question

The example here https://github.com/paritytech/subxt/blob/master/subxt/examples/storage_iterating.rs

use subxt::{OnlineClient, PolkadotConfig};

#[subxt::subxt(runtime_metadata_path = "../artifacts/polkadot_metadata_full.scale")]
pub mod polkadot {}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Create a new API client, configured to talk to Polkadot nodes.
    let api = OnlineClient::<PolkadotConfig>::new().await?;

    // Build a storage query to iterate over account information.
    let storage_query = polkadot::storage().system().account_iter();

    // Get back an iterator of results (here, we are fetching 10 items at
    // a time from the node, but we always iterate over one at a time).
    let mut results = api.storage().at_latest().await?.iter(storage_query).await?;

    while let Some(Ok((key, value))) = results.next().await {
        println!("Key: 0x{}", hex::encode(&key));
        println!("Value: {:?}", value);
    }

    Ok(())
}

Shows how to iterate over a storage map and get typed results. However, only the values are typed in this case and I didn't find a way to get a typed key value as well. Instead I have to decode into AccountId using a code like:

let account_bytes = key[40..].to_vec();
let array_u8: [u8; 32] = account_bytes.as_slice().try_into().unwrap();
let account = AccountId32::from(array_u8);

Is there a better way?

Answer 1

No, I don't think this is the perfect way to do it.

You need to manually cut the bytes based on the xxx_concat algorithm used by the storage.

As for the remaining part, if you don't want to specify a type every time, using scale value type might be another option.

For more information, please refer to: https://docs.rs/scale-value/latest/scale_value/struct.Value.html.