Create and use a storage Map

Learn how to write into a storage map.

Goal

Learn how to use a StorageMap using FRAME V2's syntax.

Use cases

Declaring a Storage Map and using it in a pallet.

Overview

We will create 2 maps :

• WhenLastSomethingDone : to store the lastest block number when a function is called
• CallsCounterBySender : to store a counter by sender address for a function called

Steps

1. Define a StorageMap instance

Use FRAME's StorageMap to declare the struct as a new single item in storage:

#[pallet::storage]

#[pallet::getter(fn when_last_something_done)]

pub type WhenLastSomethingDone<T: Config> = StorageMap<_, Twox64Concat, T::AccountId, T::BlockNumber, ValueQuery>;

#[pallet::storage]

#[pallet::getter(fn call_counter_by_sender)]

pub type CallsCounterBySender<T: Config> = StorageMap<_, Twox64Concat, T::AccountId, u32, ValueQuery>;

2. Write to the WhenLastSomethingDone storage item

Example of using WhenLastSomethingDone inside a function using insert :

let who = ensure_signed(origin)?;

let now = frame_system::Pallet::<T>::block_number();

<WhenLastSomethingDone<T>>::insert(&who, now);

3. Write to the CallsCounterBySender storage item

Example of the CallsCounterBySender StorageMap being used inside a function:

let who = ensure_signed(origin)?;

if ! <CallsCounterBySender<T>>::contains_key(&who) {

<CallsCounterBySender<T>>::insert(&who, 0);

}

let counter = <CallsCounterBySender<T>>::get(&who).saturating_add(1);

<CallsCounterBySender<T>>::insert(&who, counter);

Examples

• Inside FRAME's pallet-balances
• StorageMaps in the archipel project

Resources

Rust docs

• StorageMap