Unclear what type to assign to AccountStore in balances Trait of a runtime module

[ltfschoen]

In our blockchain's runtime/src/lib.rs, I have the following (which is consistent with what's shown in https://github.com/paritytech/substrate/blob/master/bin/node-template/runtime/src/lib.rs#L219)

impl balances::Trait for Runtime {
    type AccountStore = System;
    ...

then in a specific runtime module where I want to use balances, I have

impl balances::Trait for Test {
        type AccountStore = ();
        ...
}
...

type System = system::Module<Test>;
pub type Balances = balances::Module<Test>;

But when I run the tests for that runtime module with cargo test -p roaming-accounting-policies, I get error:

error[E0277]: the trait bound `(): sp_api_hidden_includes_decl_storage::hidden_include::traits::StoredMap<u64, pallet_balances::AccountData<u64>>` is not satisfied
  --> pallets/roaming/roaming-accounting-policies/src/mock.rs:55:2
   |
54 | impl balances::Trait for Test {
   | ----------------------------- in this `impl` item
55 |     type AccountStore = ();
   |     ^^^^^^^^^^^^^^^^^^^^^^^ the trait `sp_api_hidden_includes_decl_storage::hidden_include::traits::StoredMap<u64, pallet_balances::AccountData<u64>>` is not implemented for `()`

Unfortunately the mock of the node-template's "template" runtime module https://github.com/paritytech/substrate/blob/master/bin/node-template/pallets/template/src/mock.rs doesn't include impl balances::Trait for Test { to show an example of what type should be included.

[ondratra]

I found this issue while searching for the same error message. I was able to resolve the issue by doing the following:

impl system::Trait for Test {    
    type AccountData = pallet_balances::AccountData<u64>;
    ...
}

impl pallet_balances::Trait for Test {
    type AccountStore = system::Module<Self>;
    ...
}