How to Retrieve Value by Single Key from StorageDoubleMap in Substrate?

Question

I'm developing a Substrate-based blockchain and using StorageDoubleMap to store label details. The StorageDoubleMap is defined as follows:

#[pallet::storage]
#[pallet::unbounded]
#[pallet::getter(fn labels)]
pub(super) type Labels<T: Config> = StorageDoubleMap<
    _,
    Twox64Concat,
    HashId<T>,
    Twox64Concat,
    AccountOf<T>,
    LabelsDetail<T>,
    OptionQuery,
>;

I need to retrieve LabelsDetail using just the label_id (of type HashId<T>), without knowing the AccountOf<T>.

What are the best practices for this scenario? Is it possible to find the LabelsDetail using only label_id, or do I need to change my storage structure or use additional indexing?

Answer 1

You can use iter_prefix() from the StorageDoubleMap to iterate over values that share the first key. You can find one such example here

Answer 2

Does the on-chain logic really need to find this value itself or is verifying a valid position enough?

The problem is that this could incur an unbound storage iteration and slow down the chain. If this runs on a parachain there would be the chance that it halts when there are lots of entries in the map.
Maybe it is possible to find the entry off-chain and provide the index as witness argument to the extrinsic.

If this is not a problem for you, then the iter_prefix mentioned above is indeed possible.