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Creating storage maps

info

For this section, you should be in: pallets/connect/lib.rs to follow along.

This section will focus on adding a new FRAME storage item to pallet_connect. With our events and errors defined, we have two items left on our action list:

  1. Implement events and errors
  2. Implement storage items
  3. Implement dispatchable functions (extrinsics)

Defining a Storage Item​

While we have errors to define user as out of bounds and events that emit when someone successfully registers, there is currently no place to store the data about the user in underlying storage.

There are several types of storage items in FRAME. We will use a StorageMap, which works similarly to a HashMap in principle. Navigate to the struct called UserMetadata, and right under it, add this type alias:

    /// Registered users mapped by address
    #[pallet::storage]
    #[pallet::getter(fn registered_users)]
    pub type RegisteredUsers<T: Config> =
        StorageMap<_, Blake2_128Concat, T::AccountId, UserMetadata<T>, OptionQuery>;

This storage item maps the AccountId from Config (hence T::AccountId) to the struct UserMetadata. The #[pallet::storage] defines and expands this type alias for us.

A few notes about this storage item:

  • Blake2_128Concat refers to how the keys are hashed, referring to the Blake2_128 hashing algorithm. This is a secure hashing algorithm.
  • T::AccountId is the key to access the value, UserMetadata<T>.
  • The generic T must implement the Config trait. This gives it access to overarching types, such as core primitives (i.e., AccountId).
  • OptionQuery refers to how to handle when the value is retrieved. OptionQuery works by returning an Option<T>, meaning a user can either be Some(UserMetadata) or None.