Using T::AccountId in a custom map

Question

I would like to map T::AccountId to a custom struct I have Amendments <AccountId:

    #[pallet::storage]
    #[pallet::getter(fn declaration_map)]
    pub(super) type Declarations<T: Config> = StorageMap
    <
    _,
    Blake2_128Concat,
    T::AccountId,
    Amendment<T::AccountId>,
    ValueQuery
    >;

However when I try to use T::AccountId in this context I get this error:

error[E0277]: the trait bound `<T as frame_system::Config>::AccountId: std::default::Default` is not satisfied
   --> pallets/template/src/lib.rs:243:12
    |
243 |     #[pallet::storage]
    |               ^^^^^^^ the trait `std::default::Default` is not implemented for `<T as frame_system::Config>::AccountId`

It makes sense that T::AccountId doesn't have a default since it wouldn't make sense for it to have one. However I understand that a map in this context probably can't take a AccountId without having a default value for it. Is there a way for me to get around this so I can still map the AccountIds?

Answer 1

As you specify to it to use a ValueQuery, a default has to exist.
The solution is to use an OptionQuery instead which then returns None:

#[pallet::storage]
#[pallet::getter(fn declaration_map)]
pub(super) type Declarations<T: Config> = StorageMap
    <
        _,
        Blake2_128Concat,
        T::AccountId,
        Amendment<T::AccountId>,
        OptionQuery // <- HERE
    >;

A ResultQuery should probably also work.