2

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?

1 Answer 1

2

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.

2
  • You can also use ValueQuery, and then define what default value you want with OnEmpty.
    – Shawn Tabrizi
    Jun 16 at 17:40
  • This worked perfectly thank you! Jun 16 at 17:46

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