How to check if StorageMap is empty?

Question

I have created a simple storage map as follows and I just want to check if it's empty or not. I am comparing it with rust HashMaps, it has the is_empty() method to check. I intend to use this in the same pallet.

#[pallet::storage]
#[pallet::getter(fn check_storage_map)]
pub type SimpleStorageMap<T: Config> =
    StorageMap<_, Blake2_256, u32, u32, ValueQuery>;

I checked the storage map rust doc but could not find any function associated with it. I am new to substrate any advice or help is appreciated.

Answer 1

There exists the CountedStorageMap that keeps track of the count of elements in the map:

/// A wrapper around a `StorageMap` and a `StorageValue<Value=u32>` to keep track of how many items
/// are in a map, without needing to iterate all the values.
///
/// This storage item has additional storage read and write overhead when manipulating values
/// compared to a regular storage map.
///
/// For functions where we only add or remove a value, a single storage read is needed to check if
/// that value already exists. For mutate functions, two storage reads are used to check if the
/// value existed before and after the mutation.
///
/// Whenever the counter needs to be updated, an additional read and write occurs to update that
/// counter.
pub struct CountedStorageMap;

Answer 2

You have multiple options to achieve your requirement. Eventually it boils down to checking for an empty iterator IMO.

You can retrieve two iterators of your StorageMap:

1. Either the KeyPrefixIterator by calling SimpleStorageMap::<T>::iter_keys()
2. Or the PrefixIterator by calling SimpleStorageMap::<T>::iter_values()

Then, you could just check whether the first element of your Iterator is None or whether the collection to an arbitrary vector is empty:

assert!(SimpleStorageMap::<T>::iter_keys().next().is_none());

assert!(SimpleStorageMap::<T>::iter_keys().collect::<Vec<_>>().is_empty());