Can't read storage item from migration script, however can read from polkadot.js.app

Question

Here are the details, I need to do a storage migration for a pallet. I can read the storage item from the polkadot.js.app, as shown below:

however, I can not read it from migration script(returned None), code is:

let reg_info = RegistryInfoByIds::<T>::get(asset_id).ok_or(Error::<T>::AssetNotRegistered)?;

I assume the storage item must exist if we can query from the polkadot.js.app. Don't know what the problem is.

Answer 1

When updating types in a runtime migration, you must read the "old" storage using the old storage type.

Imagine you have an item originally stored as:

struct Item {
    id: u32,
    field1: Field,
}

Then imagine you update the item to be:

struct Item {
    id: u32,
    field1: Field,
    field2: Field,
}

If you were to try to read this storage item after updating the struct Item, you will get the "corrupt state" error because on your blockchain, there is no field2, and thus, when trying to read from state to create the new Item, it will fail.

Instead, a common pattern for migrations is to write a special type which acts as a way to read the old storage, and then re-write the storage with the new updated struct.

That pattern looks like this:

mod old {
    pub struct OldItem {
        id: u32,
        field1: Field,
    }

    #[frame_support::storage_alias]
    pub type ItemMap<T> = StorageMap<
        crate::Pallet<T>,
        Twox64Concat,
        u32,
        OldItem<T>,
    >;
}

fn migration() {
    // Note here we are reading from `old`, which has our old type.
    for (_key, old_item) in old::ListNodes::<T, I>::iter() {
        // Note here we are using the `crate::Item`, which has the updated object.
        let new_item = crate::Item {
             id: old_item.id,
             field1: old_item.field1,
             // Here we just duplicate field1, but you may have your own logic...
             field2: old_item.field1,
        };

        // Note that we write back to `crate::ItemMap` which uses the new type.
        crate::ItemMap::<T>::insert(old_item.id, new_item);
    }
}

There are plenty of examples in Substrate, but here is one:

https://github.com/paritytech/substrate/blob/master/frame/bags-list/src/migrations.rs