How to store hashes in the Substrate Chain? - rust

Question

My objective is to store hash values in the substrate chain. I have declared the storage and the module for it in the following code:
use support::{decl_module, decl_storage, dispatch::Result, ensure, StorageMap};
use system::ensure_signed;
pub trait Trait: balances::Trait {}
decl_storage! {
trait Store for Module<T: Trait> as KittyStorage {
Value: map str => Option<T::AccountId>;
}
}
decl_module! {
pub struct Module<T: Trait> for enum Call where origin: T::Origin {
fn set_value(origin, value: u64) -> Result {
let sender = ensure_signed(origin)?;
ensure!(!<Value<T>>::exists(value), "key already exists");
<Value<T>>::insert(value, sender);
Ok(())
}
}
}
The code was working fine as expected as long as I was using u64 in the line but I received an error when I changed it to str:
Value: map str => Option<T::AccountId>;
The error is:
error[E0277]: the trait bound `str: _::_parity_scale_codec::EncodeLike` is not satisfied
--> /Users/aviralsrivastava/dev/substrate-package/substrate-node-template/runtime/src/substratekitties.rs:6:1
|
6 | / decl_storage! {
7 | | trait Store for Module<T: Trait> as KittyStorage {
8 | | Value: map str => Option<T::AccountId>;
9 | | }
10 | | }
| |_^ the trait `_::_parity_scale_codec::EncodeLike` is not implemented for `str`
|
I tried reading about it but could not find any other method of storing a string. Although, my string will be of fixed size as it will always be SHA256.

Answer 1

You should be using a hash of known size, so do something like:
type MyHash = [u8; 32];
This would be a 256-bit hash.
Then you can create a storage item:
Value: map MyHash => Option<T::AccountId>;
You can also use the Hash primitive defined in your runtime with T::Hash, which makes it compatible with the default hashing primitives in your runtime. That would look like:
Value: map T::Hash => Option<T::AccountId>;
In Substrate, it is H256 by default (which is basically the same thing as above, but more general as it can change and be redefined by the runtime).