How to change one of the two `StorageDoubleMap` keys?

Question

Say I have a StorageDoubleMap like this

pub type Users<T: Config> = StorageDoubleMap<_, Twox64Concat, Vec<u8>, Twox64Concat, Compact<u64>, Person<T::AccountId>,>;

where the key1 = Vec<u8>, key2 = Compact<u64>. For example I store it like <Users<T>>::insert(b"hello", 100, person);

What if I want to change key1 from hello to new_hello?

Answer 1

One thing you can do is first take a value from storage, removing it afterwards, and insert it again with the new key:

    let value = <Users<T>>::take(b"hello", 100).ok_or(Error::<T>::UserDoesntExist)?;
    /// Insert it
    <Users<T>>::insert(b"new_hello", 100, value);

Answer 2

https://paritytech.github.io/substrate/master/frame_support/storage/trait.IterableStorageDoubleMap.html#tymethod.drain_prefix

I think you could write something like this to mutate the entire prefix tree:

for (k, v) in X::drain_prefix(old_prefix) {
  X::insert(new_prefix, k, v);
}

But this might cost a lot of weight. Be careful with that.