Returning dynamic default values from StorageMap

Question

I'm working with a StorageMap and want to return a dynamic default value based on the queried key when the key isn't found. Here's my attempt:

#[pallet::storage]
#[pallet::getter(fn asset)]
pub(super) type Asset<T: Config> =
    StorageMap<_, Blake2_128Concat, U256, T::AccountId, ValueQuery, GetDefault<T::AccountId>>;

pub struct GetDefault<T>(sp_std::marker::PhantomData<T>);

impl<AccountId> Get<AccountId> for GetDefault<AccountId> {
    fn get() -> AccountId {
        // logic to derive dynamic default value based on asset_id
    }
}

Is there a way to achieve this?

Answer 1

The FRAME syntax does not allow for this directly since the key is not passed to the Get default impl.

One way to achieve this would be to use an OptionQuery and create a custom getter function like this:

#[pallet::storage]
pub(super) type Asset<T: Config> =
    StorageMap<_, Blake2_128Concat, U256, T::AccountId, OptionQuery>;

fn asset<T: Config>(key: U256) -> <T as frame_system::Config>::AccountId {
    Asset::<T>::get(key).unwrap_or_else(|| {
        todo!("Custom default logic goes here")
    })
}

This replaces the auto-generated asset getter that you formerly created with
#[pallet::getter(fn asset)] and offers a custom default per key.