Provide example for balance `AccountStore` with instancing

[bkchr]

We should provide an example on how to use balance::AccountStore in the docs.

See https://github.com/paritytech/substrate/blob/master/frame/balances/src/tests_local.rs#L110-L115 as an example in the tests.

[herou]

@bkchr Is it a documentation ticket or coding ticket? I see that you have attached the Q2-easy label. Since it is easy, can I take it?

[gui1117]

I can mentor ppl on this one

[herou]

Please, write a short desc.

[gui1117]

the pallet balances store the balance of accounts in the generic account store which requires a stored map: https://substrate.dev/rustdocs/latest/pallet_balances/pallet/trait.Config.html#associatedtype.AccountStore

In polkadot and usual template nodes, we use frame_system::Pallet implementation of stored map https://substrate.dev/rustdocs/latest/frame_system/pallet/struct.Pallet.html#impl-StoredMap%3C%3CT%20as%20Config%3E%3A%3AAccountId%2C%20%3CT%20as%20Config%3E%3A%3AAccountData%3E
This stores the data alongside accountdata.

But people are usually confused about how to use this without storing in frame_system.
The most easy way is to use the storage: Account: https://substrate.dev/rustdocs/latest/pallet_balances/pallet/type.Account.html

One can see in the test tests_local and example of such configuration:

substrate/frame/balances/src/tests_local.rs

Lines 97 to 98 in 70a3bca

	type AccountStore =
	StorageMapShim<super::Account<Test>, system::Provider<Test>, u64, super::AccountData<u64>>;

To solve this issue, one must write in the root documentation of pallet_balances an example of the configuration of the pallet using the local storage Account similarly to how is configured the test runtime in tests_local.

[ECJ222]

Hi @thiolliere, Can I take on this issue?

[gui1117]

yes, nobody is working on it AFAIK, I put you assigned just for better discoverability