How to mutate a `StorageDoubleMap` value?

Question

I have StorageDoubleMap for ERC1155 balances u32 -> AccountId -> u32;

How can I mutate this map like balances[1][SomeAccountId] -= 4?

Answer 1

Generally, you would use this syntax of try_mutate

similar to e.g.

<Balances<T>>::try_mutate(&asset_id, &owner, |balance| {
    *balance -= 4;
    Ok(())
})

Answer 2

To extend Lana's answer, you can just use mutate in the case that your operation is infallible:

<Balances<T>>::mutate(&asset_id, &owner, |balance| {
    *balance = balance.saturating_sub(4);
})

Answer 3

It's always good to search in the Substrate repository first.

---

/frame/im-online/src/lib.rs::mutate

AuthoredBlocks::<T>::mutate(&current_session, author, |authored| *authored += 1);

/frame/assets/src/lib.rs::try_mutate

Account::<T, I>::try_mutate(id, &who, |maybe_account| -> DispatchResult {
    maybe_account.as_mut().ok_or(Error::<T, I>::NoAccount)?.is_frozen = true;
    Ok(())
})?;

---

There are still a lot of examples. Check the Substrate repository.