Empty tuple is encoded as null

[ntn-x2]

After upgrading our parachain to use metadata v14, we now have no way to distinguish whether an element is present or not in a StorageMap<AccountId32 ()>, as the api.query.pallet.storage(key) returns null in both cases. I can see also from polkadotjs apps that the map is a map from AccountId32 to Option<Null>.
So far, the only workaround I found is to fetch ALL the keys via api.query.pallet.keys and then filter, but that can get quite expensive if the map has a lot of keys. Is there a more efficient way to check for existence in such a map or, if there is not, would it be possible to fix this issue by encoding empty tuples differently? I am not sure if this issue pertains the SDK side or the chain side.

I also opened an issue in the scale-info repo.

[jacogr]

The API will, when the storage key from the node returns null, apply the fallback as specified in the metadata. The fallback is generally the Default value for that type.

() has the same encoding as Null on the JS side (empty bytes), so () is treated as an alias for Null (since it has nothing inside it).

[ntn-x2]

Ok so there is no efficient way to understand whether a key exists in a given map or not? Assuming the default encoding is maintained.

[ntn-x2]

Ok I have found a workaround using raw keys that avoids the automatic encoding of () to Null.

  // Compute the raw key for the given storage element
  const rawKey = await api.query.pallet.storage.key(input)
  // Fetch the raw storage entry using the computed key
  const encodedStorageValue = await api.rpc.state.queryStorageAt<Codec[]>([rawKey])
  // Checks the retrieved encoding to check whether the value is actually present or not
  const isPresent = encodedStorageValue.pop()!.toU8a()[0] === 1

It might not be very beautiful, but it does what it needs to do without requiring to fetch the whole storage content and then filtering afterwards. I am still open to suggestions to make this better, but I also think this might help other people in the same situation.

[jacogr]

Closing.

[polkadot-js-bot]

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