Efficent Subgroup Removal by Subkey: Double Maps

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For some runtimes, it may be necessary to remove a subset of values in a key-value mapping. If the subset maintain an associated identifier type, this can be done in a clean way with the double_map via the remove_prefix api.

pub type GroupIndex = u32; // this is Encode (which is necessary for double_map)

decl_storage! {
	trait Store for Module<T: Trait> as Dmap {
		/// Member score (double map)
		MemberScore get(fn member_score):
			double_map hasher(blake2_128_concat) GroupIndex, hasher(blake2_128_concat) T::AccountId => u32;
		/// Get group ID for member
		GroupMembership get(fn group_membership): map hasher(blake2_128_concat) T::AccountId => GroupIndex;
		/// For fast membership checks, see check-membership recipe for more details
		AllMembers get(fn all_members): Vec<T::AccountId>;
	}
}

For the purposes of this example, store the scores of each member in a map that associates this u32 value with two keys: (1) a GroupIndex identifier, and (2) the member's AccountId. This allows for efficient removal of all values associated with a specific GroupIndex identifier.

fn remove_group_score(origin, group: GroupIndex) -> DispatchResult {
	let member = ensure_signed(origin)?;

	let group_id = <GroupMembership<T>>::get(member);
	// check that the member is in the group
	ensure!(group_id == group, "member isn't in the group, can't remove it");

	// remove all group members from MemberScore at once
	<MemberScore<T>>::remove_prefix(&group_id);

	Self::deposit_event(RawEvent::RemoveGroup(group_id));
	Ok(())
}