How to store hashes in the Substrate Chain?

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.

As explained [in this answer](https://stackoverflow.com/questions/24158114/what-are-the-differences-between-rusts-string-and-str) `str` has unknown length and cannot be instantiated AFAIK. — apopiak, Mar 17 '20 at 17:26
[Here](https://docs.rs/parity-scale-codec/1.2.0/parity_scale_codec/trait.Encode.html) is a list of blanket implementations for `Encode` which you can peruse for a type to use. — apopiak, Mar 17 '20 at 17:28

score 1 · Accepted Answer · answered Mar 17 '20 at 18:13

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).

How to store hashes in the Substrate Chain?

1 Answers1