Implement StorageNMap (#8635)

* Implement StorageNMap * Change copyright date to 2021 * Rewrite keys to use impl_for_tuples instead of recursion * Implement prefix iteration on StorageNMap * Implement EncodeLike for key arguments * Rename KeyGenerator::Arg to KeyGenerator::KArg * Support StorageNMap in decl_storage and #[pallet::storage] macros * Use StorageNMap in assets pallet * Support migrate_keys in StorageNMap * Reduce line characters on select files * Refactor crate imports in decl_storage macros * Some more line char reductions and doc comment update * Update UI test expectations * Revert whitespace changes to untouched files * Generate Key struct instead of a 1-tuple when only 1 pair of key and hasher is provided * Revert formatting changes to unrelated files * Introduce KeyGeneratorInner * Add tests for StorageNMap in FRAMEv2 pallet macro * Small fixes to unit tests for StorageNMap * Bump runtime metadata version * Remove unused import * Update tests to use runtime metadata v13 * Introduce and use EncodeLikeTuple as a trait bound for KArg * Add some rustdocs * Revert usage of StorageNMap in assets pallet * Make use of ext::PunctuatedTrailing * Add rustdoc for final_hash * Fix StorageNMap proc macro expansions for single key cases * Create associated const in KeyGenerator for hasher metadata * Refactor code according to comments from Basti * Add module docs for generator/nmap.rs * Re-export storage::Key as NMapKey in pallet prelude * Seal the EncodeLikeTuple trait * Extract sealing code out of key.rs Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
2026-04-27 08:07:58 +00:00 · 2021-05-14 02:44:29 -07:00
parent c6b1240e51
commit 033d8289f0
26 changed files with 3210 additions and 50 deletions
@@ -25,10 +25,12 @@
 //! This is internal api and is subject to change.

 mod map;
+mod nmap;
 mod double_map;
 mod value;

 pub use map::StorageMap;
+pub use nmap::StorageNMap;
 pub use double_map::StorageDoubleMap;
 pub use value::StorageValue;

@@ -0,0 +1,541 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Generator for `StorageNMap` used by `decl_storage` and storage types.
+//!
+//! By default each key value is stored at:
+//! ```nocompile
+//! Twox128(pallet_prefix) ++ Twox128(storage_prefix)
+//!     ++ Hasher1(encode(key1)) ++ Hasher2(encode(key2)) ++ ... ++ HasherN(encode(keyN))
+//! ```
+//!
+//! # Warning
+//!
+//! If the keys are not trusted (e.g. can be set by a user), a cryptographic `hasher` such as
+//! `blake2_256` must be used.  Otherwise, other values in storage with the same prefix can
+//! be compromised.
+
+use crate::{
+	hash::{StorageHasher, Twox128},
+	storage::{
+		self,
+		types::{
+			EncodeLikeTuple, HasKeyPrefix, HasReversibleKeyPrefix, KeyGenerator,
+			ReversibleKeyGenerator, TupleToEncodedIter,
+		},
+		unhashed, PrefixIterator, StorageAppend,
+	},
+	Never,
+};
+use codec::{Decode, Encode, EncodeLike, FullCodec};
+#[cfg(not(feature = "std"))]
+use sp_std::prelude::*;
+
+/// Generator for `StorageNMap` used by `decl_storage` and storage types.
+///
+/// By default each key value is stored at:
+/// ```nocompile
+/// Twox128(pallet_prefix) ++ Twox128(storage_prefix)
+///     ++ Hasher1(encode(key1)) ++ Hasher2(encode(key2)) ++ ... ++ HasherN(encode(keyN))
+/// ```
+///
+/// # Warning
+///
+/// If the keys are not trusted (e.g. can be set by a user), a cryptographic `hasher` such as
+/// `blake2_256` must be used.  Otherwise, other values in storage with the same prefix can
+/// be compromised.
+pub trait StorageNMap<K: KeyGenerator, V: FullCodec> {
+	/// The type that get/take returns.
+	type Query;
+
+	/// Module prefix. Used for generating final key.
+	fn module_prefix() -> &'static [u8];
+
+	/// Storage prefix. Used for generating final key.
+	fn storage_prefix() -> &'static [u8];
+
+	/// The full prefix; just the hash of `module_prefix` concatenated to the hash of
+	/// `storage_prefix`.
+	fn prefix_hash() -> Vec<u8> {
+		let module_prefix_hashed = Twox128::hash(Self::module_prefix());
+		let storage_prefix_hashed = Twox128::hash(Self::storage_prefix());
+
+		let mut result =
+			Vec::with_capacity(module_prefix_hashed.len() + storage_prefix_hashed.len());
+
+		result.extend_from_slice(&module_prefix_hashed[..]);
+		result.extend_from_slice(&storage_prefix_hashed[..]);
+
+		result
+	}
+
+	/// Convert an optional value retrieved from storage to the type queried.
+	fn from_optional_value_to_query(v: Option<V>) -> Self::Query;
+
+	/// Convert a query to an optional value into storage.
+	fn from_query_to_optional_value(v: Self::Query) -> Option<V>;
+
+	/// Generate a partial key used in top storage.
+	fn storage_n_map_partial_key<KP>(key: KP) -> Vec<u8>
+	where
+		K: HasKeyPrefix<KP>,
+	{
+		let module_prefix_hashed = Twox128::hash(Self::module_prefix());
+		let storage_prefix_hashed = Twox128::hash(Self::storage_prefix());
+		let key_hashed = <K as HasKeyPrefix<KP>>::partial_key(key);
+
+		let mut final_key = Vec::with_capacity(
+			module_prefix_hashed.len() + storage_prefix_hashed.len() + key_hashed.len(),
+		);
+
+		final_key.extend_from_slice(&module_prefix_hashed[..]);
+		final_key.extend_from_slice(&storage_prefix_hashed[..]);
+		final_key.extend_from_slice(key_hashed.as_ref());
+
+		final_key
+	}
+
+	/// Generate the full key used in top storage.
+	fn storage_n_map_final_key<KG, KArg>(key: KArg) -> Vec<u8>
+	where
+		KG: KeyGenerator,
+		KArg: EncodeLikeTuple<KG::KArg> + TupleToEncodedIter,
+	{
+		let module_prefix_hashed = Twox128::hash(Self::module_prefix());
+		let storage_prefix_hashed = Twox128::hash(Self::storage_prefix());
+		let key_hashed = KG::final_key(key);
+
+		let mut final_key = Vec::with_capacity(
+			module_prefix_hashed.len() + storage_prefix_hashed.len() + key_hashed.len(),
+		);
+
+		final_key.extend_from_slice(&module_prefix_hashed[..]);
+		final_key.extend_from_slice(&storage_prefix_hashed[..]);
+		final_key.extend_from_slice(key_hashed.as_ref());
+
+		final_key
+	}
+}
+
+impl<K, V, G> storage::StorageNMap<K, V> for G
+where
+	K: KeyGenerator,
+	V: FullCodec,
+	G: StorageNMap<K, V>,
+{
+	type Query = G::Query;
+
+	fn hashed_key_for<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg) -> Vec<u8> {
+		Self::storage_n_map_final_key::<K, _>(key)
+	}
+
+	fn contains_key<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg) -> bool {
+		unhashed::exists(&Self::storage_n_map_final_key::<K, _>(key))
+	}
+
+	fn get<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg) -> Self::Query {
+		G::from_optional_value_to_query(unhashed::get(&Self::storage_n_map_final_key::<K, _>(key)))
+	}
+
+	fn try_get<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg) -> Result<V, ()> {
+		unhashed::get(&Self::storage_n_map_final_key::<K, _>(key)).ok_or(())
+	}
+
+	fn take<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg) -> Self::Query {
+		let final_key = Self::storage_n_map_final_key::<K, _>(key);
+
+		let value = unhashed::take(&final_key);
+		G::from_optional_value_to_query(value)
+	}
+
+	fn swap<KOther, KArg1, KArg2>(key1: KArg1, key2: KArg2)
+	where
+		KOther: KeyGenerator,
+		KArg1: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		KArg2: EncodeLikeTuple<KOther::KArg> + TupleToEncodedIter,
+	{
+		let final_x_key = Self::storage_n_map_final_key::<K, _>(key1);
+		let final_y_key = Self::storage_n_map_final_key::<KOther, _>(key2);
+
+		let v1 = unhashed::get_raw(&final_x_key);
+		if let Some(val) = unhashed::get_raw(&final_y_key) {
+			unhashed::put_raw(&final_x_key, &val);
+		} else {
+			unhashed::kill(&final_x_key);
+		}
+		if let Some(val) = v1 {
+			unhashed::put_raw(&final_y_key, &val);
+		} else {
+			unhashed::kill(&final_y_key);
+		}
+	}
+
+	fn insert<KArg, VArg>(key: KArg, val: VArg)
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		VArg: EncodeLike<V>,
+	{
+		unhashed::put(&Self::storage_n_map_final_key::<K, _>(key), &val);
+	}
+
+	fn remove<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg) {
+		unhashed::kill(&Self::storage_n_map_final_key::<K, _>(key));
+	}
+
+	fn remove_prefix<KP>(partial_key: KP)
+	where
+		K: HasKeyPrefix<KP>,
+	{
+		unhashed::kill_prefix(&Self::storage_n_map_partial_key(partial_key));
+	}
+
+	fn iter_prefix_values<KP>(partial_key: KP) -> PrefixIterator<V>
+	where
+		K: HasKeyPrefix<KP>,
+	{
+		let prefix = Self::storage_n_map_partial_key(partial_key);
+		PrefixIterator {
+			prefix: prefix.clone(),
+			previous_key: prefix,
+			drain: false,
+			closure: |_raw_key, mut raw_value| V::decode(&mut raw_value),
+		}
+	}
+
+	fn mutate<KArg, R, F>(key: KArg, f: F) -> R
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		F: FnOnce(&mut Self::Query) -> R,
+	{
+		Self::try_mutate(key, |v| Ok::<R, Never>(f(v)))
+			.expect("`Never` can not be constructed; qed")
+	}
+
+	fn try_mutate<KArg, R, E, F>(key: KArg, f: F) -> Result<R, E>
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		F: FnOnce(&mut Self::Query) -> Result<R, E>
+	{
+		let final_key = Self::storage_n_map_final_key::<K, _>(key);
+		let mut val = G::from_optional_value_to_query(unhashed::get(final_key.as_ref()));
+
+		let ret = f(&mut val);
+		if ret.is_ok() {
+			match G::from_query_to_optional_value(val) {
+				Some(ref val) => unhashed::put(final_key.as_ref(), val),
+				None => unhashed::kill(final_key.as_ref()),
+			}
+		}
+		ret
+	}
+
+	fn mutate_exists<KArg, R, F>(key: KArg, f: F) -> R
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		F: FnOnce(&mut Option<V>) -> R,
+	{
+		Self::try_mutate_exists(key, |v| Ok::<R, Never>(f(v)))
+			.expect("`Never` can not be constructed; qed")
+	}
+
+	fn try_mutate_exists<KArg, R, E, F>(key: KArg, f: F) -> Result<R, E>
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		F: FnOnce(&mut Option<V>) -> Result<R, E>,
+	{
+		let final_key = Self::storage_n_map_final_key::<K, _>(key);
+		let mut val = unhashed::get(final_key.as_ref());
+
+		let ret = f(&mut val);
+		if ret.is_ok() {
+			match val {
+				Some(ref val) => unhashed::put(final_key.as_ref(), val),
+				None => unhashed::kill(final_key.as_ref()),
+			}
+		}
+		ret
+	}
+
+	fn append<Item, EncodeLikeItem, KArg>(key: KArg, item: EncodeLikeItem)
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		Item: Encode,
+		EncodeLikeItem: EncodeLike<Item>,
+		V: StorageAppend<Item>,
+	{
+		let final_key = Self::storage_n_map_final_key::<K, _>(key);
+		sp_io::storage::append(&final_key, item.encode());
+	}
+
+	fn migrate_keys<KArg>(key: KArg, hash_fns: K::HArg) -> Option<V>
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+	{
+		let old_key = {
+			let module_prefix_hashed = Twox128::hash(Self::module_prefix());
+			let storage_prefix_hashed = Twox128::hash(Self::storage_prefix());
+			let key_hashed = K::migrate_key(&key, hash_fns);
+
+			let mut final_key = Vec::with_capacity(
+				module_prefix_hashed.len() + storage_prefix_hashed.len() + key_hashed.len(),
+			);
+
+			final_key.extend_from_slice(&module_prefix_hashed[..]);
+			final_key.extend_from_slice(&storage_prefix_hashed[..]);
+			final_key.extend_from_slice(key_hashed.as_ref());
+
+			final_key
+		};
+		unhashed::take(old_key.as_ref()).map(|value| {
+			unhashed::put(Self::storage_n_map_final_key::<K, _>(key).as_ref(), &value);
+			value
+		})
+	}
+}
+
+impl<K: ReversibleKeyGenerator, V: FullCodec, G: StorageNMap<K, V>>
+	storage::IterableStorageNMap<K, V> for G
+{
+	type Iterator = PrefixIterator<(K::Key, V)>;
+
+	fn iter_prefix<KP>(kp: KP) -> PrefixIterator<(<K as HasKeyPrefix<KP>>::Suffix, V)>
+	where
+		K: HasReversibleKeyPrefix<KP>,
+	{
+		let prefix = G::storage_n_map_partial_key(kp);
+		PrefixIterator {
+			prefix: prefix.clone(),
+			previous_key: prefix,
+			drain: false,
+			closure: |raw_key_without_prefix, mut raw_value| {
+				let partial_key = K::decode_partial_key(raw_key_without_prefix)?;
+				Ok((partial_key, V::decode(&mut raw_value)?))
+			},
+		}
+	}
+
+	fn drain_prefix<KP>(kp: KP) -> PrefixIterator<(<K as HasKeyPrefix<KP>>::Suffix, V)>
+	where
+		K: HasReversibleKeyPrefix<KP>,
+	{
+		let mut iter = Self::iter_prefix(kp);
+		iter.drain = true;
+		iter
+	}
+
+	fn iter() -> Self::Iterator {
+		let prefix = G::prefix_hash();
+		Self::Iterator {
+			prefix: prefix.clone(),
+			previous_key: prefix,
+			drain: false,
+			closure: |raw_key_without_prefix, mut raw_value| {
+				let (final_key, _) = K::decode_final_key(raw_key_without_prefix)?;
+				Ok((final_key, V::decode(&mut raw_value)?))
+			},
+		}
+	}
+
+	fn drain() -> Self::Iterator {
+		let mut iterator = Self::iter();
+		iterator.drain = true;
+		iterator
+	}
+
+	fn translate<O: Decode, F: FnMut(K::Key, O) -> Option<V>>(mut f: F) {
+		let prefix = G::prefix_hash();
+		let mut previous_key = prefix.clone();
+		while let Some(next) =
+			sp_io::storage::next_key(&previous_key).filter(|n| n.starts_with(&prefix))
+		{
+			previous_key = next;
+			let value = match unhashed::get::<O>(&previous_key) {
+				Some(value) => value,
+				None => {
+					log::error!("Invalid translate: fail to decode old value");
+					continue;
+				}
+			};
+
+			let final_key = match K::decode_final_key(&previous_key[prefix.len()..]) {
+				Ok((final_key, _)) => final_key,
+				Err(_) => {
+					log::error!("Invalid translate: fail to decode key");
+					continue;
+				}
+			};
+
+			match f(final_key, value) {
+				Some(new) => unhashed::put::<V>(&previous_key, &new),
+				None => unhashed::kill(&previous_key),
+			}
+		}
+	}
+}
+
+/// Test iterators for StorageNMap
+#[cfg(test)]
+mod test_iterators {
+	use crate::{
+		hash::StorageHasher,
+		storage::{generator::StorageNMap, unhashed, IterableStorageNMap},
+	};
+	use codec::{Decode, Encode};
+
+	pub trait Config: 'static {
+		type Origin;
+		type BlockNumber;
+		type PalletInfo: crate::traits::PalletInfo;
+		type DbWeight: crate::traits::Get<crate::weights::RuntimeDbWeight>;
+	}
+
+	crate::decl_module! {
+		pub struct Module<T: Config> for enum Call where origin: T::Origin, system=self {}
+	}
+
+	#[derive(PartialEq, Eq, Clone, Encode, Decode)]
+	struct NoDef(u32);
+
+	crate::decl_storage! {
+		trait Store for Module<T: Config> as Test {
+			NMap: nmap hasher(blake2_128_concat) u16, hasher(twox_64_concat) u32 => u64;
+		}
+	}
+
+	fn key_before_prefix(mut prefix: Vec<u8>) -> Vec<u8> {
+		let last = prefix.iter_mut().last().unwrap();
+		assert!(*last != 0, "mock function not implemented for this prefix");
+		*last -= 1;
+		prefix
+	}
+
+	fn key_after_prefix(mut prefix: Vec<u8>) -> Vec<u8> {
+		let last = prefix.iter_mut().last().unwrap();
+		assert!(
+			*last != 255,
+			"mock function not implemented for this prefix"
+		);
+		*last += 1;
+		prefix
+	}
+
+	#[test]
+	fn n_map_reversible_reversible_iteration() {
+		sp_io::TestExternalities::default().execute_with(|| {
+			// All map iterator
+			let prefix = NMap::prefix_hash();
+
+			unhashed::put(&key_before_prefix(prefix.clone()), &1u64);
+			unhashed::put(&key_after_prefix(prefix.clone()), &1u64);
+
+			for i in 0..4 {
+				NMap::insert((i as u16, i as u32), i as u64);
+			}
+
+			assert_eq!(
+				NMap::iter().collect::<Vec<_>>(),
+				vec![((3, 3), 3), ((0, 0), 0), ((2, 2), 2), ((1, 1), 1)],
+			);
+
+			assert_eq!(NMap::iter_values().collect::<Vec<_>>(), vec![3, 0, 2, 1],);
+
+			assert_eq!(
+				NMap::drain().collect::<Vec<_>>(),
+				vec![((3, 3), 3), ((0, 0), 0), ((2, 2), 2), ((1, 1), 1)],
+			);
+
+			assert_eq!(NMap::iter().collect::<Vec<_>>(), vec![]);
+			assert_eq!(
+				unhashed::get(&key_before_prefix(prefix.clone())),
+				Some(1u64)
+			);
+			assert_eq!(unhashed::get(&key_after_prefix(prefix.clone())), Some(1u64));
+
+			// Prefix iterator
+			let k1 = 3 << 8;
+			let prefix = NMap::storage_n_map_partial_key((k1,));
+
+			unhashed::put(&key_before_prefix(prefix.clone()), &1u64);
+			unhashed::put(&key_after_prefix(prefix.clone()), &1u64);
+
+			for i in 0..4 {
+				NMap::insert((k1, i as u32), i as u64);
+			}
+
+			assert_eq!(
+				NMap::iter_prefix((k1,)).collect::<Vec<_>>(),
+				vec![(1, 1), (2, 2), (0, 0), (3, 3)],
+			);
+
+			assert_eq!(
+				NMap::iter_prefix_values((k1,)).collect::<Vec<_>>(),
+				vec![1, 2, 0, 3],
+			);
+
+			assert_eq!(
+				NMap::drain_prefix((k1,)).collect::<Vec<_>>(),
+				vec![(1, 1), (2, 2), (0, 0), (3, 3)],
+			);
+
+			assert_eq!(NMap::iter_prefix((k1,)).collect::<Vec<_>>(), vec![]);
+			assert_eq!(
+				unhashed::get(&key_before_prefix(prefix.clone())),
+				Some(1u64)
+			);
+			assert_eq!(unhashed::get(&key_after_prefix(prefix.clone())), Some(1u64));
+
+			// Translate
+			let prefix = NMap::prefix_hash();
+
+			unhashed::put(&key_before_prefix(prefix.clone()), &1u64);
+			unhashed::put(&key_after_prefix(prefix.clone()), &1u64);
+			for i in 0..4 {
+				NMap::insert((i as u16, i as u32), i as u64);
+			}
+
+			// Wrong key1
+			unhashed::put(&[prefix.clone(), vec![1, 2, 3]].concat(), &3u64.encode());
+
+			// Wrong key2
+			unhashed::put(
+				&[
+					prefix.clone(),
+					crate::Blake2_128Concat::hash(&1u16.encode()),
+				]
+				.concat(),
+				&3u64.encode(),
+			);
+
+			// Wrong value
+			unhashed::put(
+				&[
+					prefix.clone(),
+					crate::Blake2_128Concat::hash(&1u16.encode()),
+					crate::Twox64Concat::hash(&2u32.encode()),
+				]
+				.concat(),
+				&vec![1],
+			);
+
+			NMap::translate(|(_k1, _k2), v: u64| Some(v * 2));
+			assert_eq!(
+				NMap::iter().collect::<Vec<_>>(),
+				vec![((3, 3), 6), ((0, 0), 0), ((2, 2), 4), ((1, 1), 2)],
+			);
+		})
+	}
+}
@@ -20,9 +20,16 @@
 use sp_core::storage::ChildInfo;
 use sp_std::prelude::*;
 use codec::{FullCodec, FullEncode, Encode, EncodeLike, Decode};
-use crate::hash::{Twox128, StorageHasher, ReversibleStorageHasher};
+use crate::{
+	hash::{Twox128, StorageHasher, ReversibleStorageHasher},
+	storage::types::{
+		EncodeLikeTuple, HasKeyPrefix, HasReversibleKeyPrefix, KeyGenerator,
+		ReversibleKeyGenerator, TupleToEncodedIter,
+	},
+};
 use sp_runtime::generic::{Digest, DigestItem};
 pub use sp_runtime::TransactionOutcome;
+pub use types::Key;

 pub mod unhashed;
 pub mod hashed;
@@ -359,6 +366,39 @@ pub trait IterableStorageDoubleMap<
 	fn translate<O: Decode, F: FnMut(K1, K2, O) -> Option<V>>(f: F);
 }

+/// A strongly-typed map with arbitrary number of keys in storage whose keys and values can be
+/// iterated over.
+pub trait IterableStorageNMap<K: ReversibleKeyGenerator, V: FullCodec>: StorageNMap<K, V> {
+	/// The type that iterates over all `(key1, (key2, (key3, ... (keyN, ()))), value)` tuples
+	type Iterator: Iterator<Item = (K::Key, V)>;
+
+	/// Enumerate all elements in the map with prefix key `kp` in no particular order. If you add or
+	/// remove values whose prefix is `kp` to the map while doing this, you'll get undefined
+	/// results.
+	fn iter_prefix<KP>(kp: KP) -> PrefixIterator<(<K as HasKeyPrefix<KP>>::Suffix, V)>
+	where K: HasReversibleKeyPrefix<KP>;
+
+	/// Remove all elements from the map with prefix key `kp` and iterate through them in no
+	/// particular order. If you add elements with prefix key `kp` to the map while doing this,
+	/// you'll get undefined results.
+	fn drain_prefix<KP>(kp: KP) -> PrefixIterator<(<K as HasKeyPrefix<KP>>::Suffix, V)>
+	where K: HasReversibleKeyPrefix<KP>;
+
+	/// Enumerate all elements in the map in no particular order. If you add or remove values to
+	/// the map while doing this, you'll get undefined results.
+	fn iter() -> Self::Iterator;
+
+	/// Remove all elements from the map and iterate through them in no particular order. If you
+	/// add elements to the map while doing this, you'll get undefined results.
+	fn drain() -> Self::Iterator;
+
+	/// Translate the values of all elements by a function `f`, in the map in no particular order.
+	/// By returning `None` from `f` for an element, you'll remove it from the map.
+	///
+	/// NOTE: If a value fail to decode because storage is corrupted then it is skipped.
+	fn translate<O: Decode, F: FnMut(K::Key, O) -> Option<V>>(f: F);
+}
+
 /// An implementation of a map with a two keys.
 ///
 /// It provides an important ability to efficiently remove all entries
@@ -510,6 +550,121 @@ pub trait StorageDoubleMap<K1: FullEncode, K2: FullEncode, V: FullCodec> {
 	>(key1: KeyArg1, key2: KeyArg2) -> Option<V>;
 }

+/// An implementation of a map with an arbitrary number of keys.
+///
+/// Details of implementation can be found at [`generator::StorageNMap`].
+pub trait StorageNMap<K: KeyGenerator, V: FullCodec> {
+	/// The type that get/take returns.
+	type Query;
+
+	/// Get the storage key used to fetch a value corresponding to a specific key.
+	fn hashed_key_for<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg) -> Vec<u8>;
+
+	/// Does the value (explicitly) exist in storage?
+	fn contains_key<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg) -> bool;
+
+	/// Load the value associated with the given key from the map.
+	fn get<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg) -> Self::Query;
+
+	/// Try to get the value for the given key from the map.
+	///
+	/// Returns `Ok` if it exists, `Err` if not.
+	fn try_get<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg) -> Result<V, ()>;
+
+	/// Swap the values of two keys.
+	fn swap<KOther, KArg1, KArg2>(key1: KArg1, key2: KArg2)
+	where
+		KOther: KeyGenerator,
+		KArg1: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		KArg2: EncodeLikeTuple<KOther::KArg> + TupleToEncodedIter;
+
+	/// Store a value to be associated with the given key from the map.
+	fn insert<KArg, VArg>(key: KArg, val: VArg)
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		VArg: EncodeLike<V>;
+
+	/// Remove the value under a key.
+	fn remove<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg);
+
+	/// Remove all values under the partial prefix key.
+	fn remove_prefix<KP>(partial_key: KP) where K: HasKeyPrefix<KP>;
+
+	/// Iterate over values that share the partial prefix key.
+	fn iter_prefix_values<KP>(partial_key: KP) -> PrefixIterator<V> where K: HasKeyPrefix<KP>;
+
+	/// Mutate the value under a key.
+	fn mutate<KArg, R, F>(key: KArg, f: F) -> R
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		F: FnOnce(&mut Self::Query) -> R;
+
+	/// Mutate the item, only if an `Ok` value is returned.
+	fn try_mutate<KArg, R, E, F>(key: KArg, f: F) -> Result<R, E>
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		F: FnOnce(&mut Self::Query) -> Result<R, E>;
+
+	/// Mutate the value under a key.
+	///
+	/// Deletes the item if mutated to a `None`.
+	fn mutate_exists<KArg, R, F>(key: KArg, f: F) -> R
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		F: FnOnce(&mut Option<V>) -> R;
+
+	/// Mutate the item, only if an `Ok` value is returned. Deletes the item if mutated to a `None`.
+	fn try_mutate_exists<KArg, R, E, F>(key: KArg, f: F) -> Result<R, E>
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		F: FnOnce(&mut Option<V>) -> Result<R, E>;
+
+	/// Take the value under a key.
+	fn take<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg) -> Self::Query;
+
+	/// Append the given items to the value in the storage.
+	///
+	/// `V` is required to implement `codec::EncodeAppend`.
+	///
+	/// # Warning
+	///
+	/// If the storage item is not encoded properly, the storage will be overwritten
+	/// and set to `[item]`. Any default value set for the storage item will be ignored
+	/// on overwrite.
+	fn append<Item, EncodeLikeItem, KArg>(key: KArg, item: EncodeLikeItem)
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter,
+		Item: Encode,
+		EncodeLikeItem: EncodeLike<Item>,
+		V: StorageAppend<Item>;
+
+	/// Read the length of the storage value without decoding the entire value under the
+	/// given `key`.
+	///
+	/// `V` is required to implement [`StorageDecodeLength`].
+	///
+	/// If the value does not exists or it fails to decode the length, `None` is returned.
+	/// Otherwise `Some(len)` is returned.
+	///
+	/// # Warning
+	///
+	/// `None` does not mean that `get()` does not return a value. The default value is completly
+	/// ignored by this function.
+	fn decode_len<KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter>(key: KArg) -> Option<usize>
+	where
+		V: StorageDecodeLength,
+	{
+		V::decode_len(&Self::hashed_key_for(key))
+	}
+
+	/// Migrate an item with the given `key` from defunct `hash_fns` to the current hashers.
+	///
+	/// If the key doesn't exist, then it's a no-op. If it does, then it returns its value.
+	fn migrate_keys<KArg>(key: KArg, hash_fns: K::HArg) -> Option<V>
+	where
+		KArg: EncodeLikeTuple<K::KArg> + TupleToEncodedIter;
+}
+
 /// Iterate over a prefix and decode raw_key and raw_value into `T`.
 ///
 /// If any decoding fails it skips it and continues to the next key.
@@ -806,7 +961,7 @@ pub trait StorageDecodeLength: private::Sealed + codec::DecodeLength {
 }

 /// Provides `Sealed` trait to prevent implementing trait `StorageAppend` & `StorageDecodeLength`
-/// outside of this crate.
+/// & `EncodeLikeTuple` outside of this crate.
 mod private {
 	use super::*;
 	use bounded_vec::BoundedVec;
@@ -818,6 +973,33 @@ mod private {
 	impl<T, S> Sealed for BoundedVec<T, S> {}
 	impl<K, V, S> Sealed for bounded_btree_map::BoundedBTreeMap<K, V, S> {}
 	impl<T, S> Sealed for bounded_btree_set::BoundedBTreeSet<T, S> {}
+
+	macro_rules! impl_sealed_for_tuple {
+		($($elem:ident),+) => {
+			paste::paste! {
+				impl<$($elem: Encode,)+> Sealed for ($($elem,)+) {}
+				impl<$($elem: Encode,)+> Sealed for &($($elem,)+) {}
+			}
+		};
+	}
+
+	impl_sealed_for_tuple!(A);
+	impl_sealed_for_tuple!(A, B);
+	impl_sealed_for_tuple!(A, B, C);
+	impl_sealed_for_tuple!(A, B, C, D);
+	impl_sealed_for_tuple!(A, B, C, D, E);
+	impl_sealed_for_tuple!(A, B, C, D, E, F);
+	impl_sealed_for_tuple!(A, B, C, D, E, F, G);
+	impl_sealed_for_tuple!(A, B, C, D, E, F, G, H);
+	impl_sealed_for_tuple!(A, B, C, D, E, F, G, H, I);
+	impl_sealed_for_tuple!(A, B, C, D, E, F, G, H, I, J);
+	impl_sealed_for_tuple!(A, B, C, D, E, F, G, H, I, J, K);
+	impl_sealed_for_tuple!(A, B, C, D, E, F, G, H, I, J, K, L);
+	impl_sealed_for_tuple!(A, B, C, D, E, F, G, H, I, J, K, L, M);
+	impl_sealed_for_tuple!(A, B, C, D, E, F, G, H, I, J, K, L, M, O);
+	impl_sealed_for_tuple!(A, B, C, D, E, F, G, H, I, J, K, L, M, O, P);
+	impl_sealed_for_tuple!(A, B, C, D, E, F, G, H, I, J, K, L, M, O, P, Q);
+	impl_sealed_for_tuple!(A, B, C, D, E, F, G, H, I, J, K, L, M, O, P, Q, R);
 }

 impl<T: Encode> StorageAppend<T> for Vec<T> {}
@@ -0,0 +1,957 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Storage key type.
+
+use crate::hash::{ReversibleStorageHasher, StorageHasher};
+use codec::{Encode, EncodeLike, FullCodec};
+use paste::paste;
+use sp_std::prelude::*;
+
+/// A type used exclusively by storage maps as their key type.
+///
+/// The final key generated has the following form:
+/// ```nocompile
+/// Hasher1(encode(key1))
+///     ++ Hasher2(encode(key2))
+///     ++ ...
+///     ++ HasherN(encode(keyN))
+/// ```
+pub struct Key<Hasher, KeyType>(core::marker::PhantomData<(Hasher, KeyType)>);
+
+/// A trait that contains the current key as an associated type.
+pub trait KeyGenerator {
+	type Key: EncodeLike<Self::Key>;
+	type KArg: Encode;
+	type HashFn: FnOnce(&[u8]) -> Vec<u8>;
+	type HArg;
+
+	const HASHER_METADATA: &'static [frame_metadata::StorageHasher];
+
+	/// Given a `key` tuple, calculate the final key by encoding each element individuallly and
+	/// hashing them using the corresponding hasher in the `KeyGenerator`.
+	fn final_key<KArg: EncodeLikeTuple<Self::KArg> + TupleToEncodedIter>(key: KArg) -> Vec<u8>;
+	/// Given a `key` tuple, migrate the keys from using the old hashers as given by `hash_fns`
+	/// to using the newer hashers as specified by this `KeyGenerator`.
+	fn migrate_key<KArg: EncodeLikeTuple<Self::KArg> + TupleToEncodedIter>(
+		key: &KArg,
+		hash_fns: Self::HArg,
+	) -> Vec<u8>;
+}
+
+/// A trait containing methods that are only implemented on the Key struct instead of the entire tuple.
+pub trait KeyGeneratorInner: KeyGenerator {
+	type Hasher: StorageHasher;
+
+	/// Hash a given `encoded` byte slice using the `KeyGenerator`'s associated `StorageHasher`.
+	fn final_hash(encoded: &[u8]) -> Vec<u8>;
+}
+
+impl<H: StorageHasher, K: FullCodec> KeyGenerator for Key<H, K> {
+	type Key = K;
+	type KArg = (K,);
+	type HashFn = Box<dyn FnOnce(&[u8]) -> Vec<u8>>;
+	type HArg = (Self::HashFn,);
+
+	const HASHER_METADATA: &'static [frame_metadata::StorageHasher] = &[H::METADATA];
+
+	fn final_key<KArg: EncodeLikeTuple<Self::KArg> + TupleToEncodedIter>(key: KArg) -> Vec<u8> {
+		H::hash(
+			&key.to_encoded_iter()
+				.next()
+				.expect("should have at least one element!"),
+		)
+		.as_ref()
+		.to_vec()
+	}
+
+	fn migrate_key<KArg: EncodeLikeTuple<Self::KArg> + TupleToEncodedIter>(
+		key: &KArg,
+		hash_fns: Self::HArg,
+	) -> Vec<u8> {
+		(hash_fns.0)(
+			&key.to_encoded_iter()
+				.next()
+				.expect("should have at least one element!"),
+		)
+	}
+}
+
+impl<H: StorageHasher, K: FullCodec> KeyGeneratorInner for Key<H, K> {
+	type Hasher = H;
+
+	fn final_hash(encoded: &[u8]) -> Vec<u8> {
+		H::hash(encoded).as_ref().to_vec()
+	}
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(2, 18)]
+#[tuple_types_custom_trait_bound(KeyGeneratorInner)]
+impl KeyGenerator for Tuple {
+	for_tuples!( type Key = ( #(Tuple::Key),* ); );
+	for_tuples!( type KArg = ( #(Tuple::Key),* ); );
+	for_tuples!( type HArg = ( #(Tuple::HashFn),* ); );
+	type HashFn = Box<dyn FnOnce(&[u8]) -> Vec<u8>>;
+
+	const HASHER_METADATA: &'static [frame_metadata::StorageHasher] = &[
+		for_tuples!( #(Tuple::Hasher::METADATA),* )
+	];
+
+	fn final_key<KArg: EncodeLikeTuple<Self::KArg> + TupleToEncodedIter>(key: KArg) -> Vec<u8> {
+		let mut final_key = Vec::new();
+		let mut iter = key.to_encoded_iter();
+		for_tuples!(
+			#(
+				let next_encoded = iter.next().expect("KArg number should be equal to Key number");
+				final_key.extend_from_slice(&Tuple::final_hash(&next_encoded));
+			)*
+		);
+		final_key
+	}
+
+	fn migrate_key<KArg: EncodeLikeTuple<Self::KArg> + TupleToEncodedIter>(
+		key: &KArg,
+		hash_fns: Self::HArg,
+	) -> Vec<u8> {
+		let mut migrated_key = Vec::new();
+		let mut iter = key.to_encoded_iter();
+		for_tuples!(
+			#(
+				let next_encoded = iter.next().expect("KArg number should be equal to Key number");
+				migrated_key.extend_from_slice(&(hash_fns.Tuple)(&next_encoded));
+			)*
+		);
+		migrated_key
+	}
+}
+
+/// Marker trait to indicate that each element in the tuple encodes like the corresponding element
+/// in another tuple.
+///
+/// This trait is sealed.
+pub trait EncodeLikeTuple<T>: crate::storage::private::Sealed {}
+
+macro_rules! impl_encode_like_tuples {
+	($($elem:ident),+) => {
+		paste! {
+			impl<$($elem: Encode,)+ $([<$elem $elem>]: Encode + EncodeLike<$elem>,)+>
+				EncodeLikeTuple<($($elem,)+)> for
+				($([<$elem $elem>],)+) {}
+			impl<$($elem: Encode,)+ $([<$elem $elem>]: Encode + EncodeLike<$elem>,)+>
+				EncodeLikeTuple<($($elem,)+)> for
+				&($([<$elem $elem>],)+) {}
+		}
+	};
+}
+
+impl_encode_like_tuples!(A);
+impl_encode_like_tuples!(A, B);
+impl_encode_like_tuples!(A, B, C);
+impl_encode_like_tuples!(A, B, C, D);
+impl_encode_like_tuples!(A, B, C, D, E);
+impl_encode_like_tuples!(A, B, C, D, E, F);
+impl_encode_like_tuples!(A, B, C, D, E, F, G);
+impl_encode_like_tuples!(A, B, C, D, E, F, G, H);
+impl_encode_like_tuples!(A, B, C, D, E, F, G, H, I);
+impl_encode_like_tuples!(A, B, C, D, E, F, G, H, I, J);
+impl_encode_like_tuples!(A, B, C, D, E, F, G, H, I, J, K);
+impl_encode_like_tuples!(A, B, C, D, E, F, G, H, I, J, K, L);
+impl_encode_like_tuples!(A, B, C, D, E, F, G, H, I, J, K, L, M);
+impl_encode_like_tuples!(A, B, C, D, E, F, G, H, I, J, K, L, M, O);
+impl_encode_like_tuples!(A, B, C, D, E, F, G, H, I, J, K, L, M, O, P);
+impl_encode_like_tuples!(A, B, C, D, E, F, G, H, I, J, K, L, M, O, P, Q);
+impl_encode_like_tuples!(A, B, C, D, E, F, G, H, I, J, K, L, M, O, P, Q, R);
+
+/// Trait to indicate that a tuple can be converted into an iterator of a vector of encoded bytes.
+pub trait TupleToEncodedIter {
+	fn to_encoded_iter(&self) -> sp_std::vec::IntoIter<Vec<u8>>;
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(1, 18)]
+#[tuple_types_custom_trait_bound(Encode)]
+impl TupleToEncodedIter for Tuple {
+	fn to_encoded_iter(&self) -> sp_std::vec::IntoIter<Vec<u8>> {
+		[for_tuples!( #(self.Tuple.encode()),* )]
+			.to_vec()
+			.into_iter()
+	}
+}
+
+impl<T: TupleToEncodedIter> TupleToEncodedIter for &T {
+	fn to_encoded_iter(&self) -> sp_std::vec::IntoIter<Vec<u8>> {
+		(*self).to_encoded_iter()
+	}
+}
+
+/// A trait that indicates the hashers for the keys generated are all reversible.
+pub trait ReversibleKeyGenerator: KeyGenerator {
+	type ReversibleHasher;
+	fn decode_final_key(key_material: &[u8]) -> Result<(Self::Key, &[u8]), codec::Error>;
+}
+
+impl<H: ReversibleStorageHasher, K: FullCodec> ReversibleKeyGenerator for Key<H, K> {
+	type ReversibleHasher = H;
+
+	fn decode_final_key(key_material: &[u8]) -> Result<(Self::Key, &[u8]), codec::Error> {
+		let mut current_key_material = Self::ReversibleHasher::reverse(key_material);
+		let key = K::decode(&mut current_key_material)?;
+		Ok((key, current_key_material))
+	}
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(2, 18)]
+#[tuple_types_custom_trait_bound(ReversibleKeyGenerator + KeyGeneratorInner)]
+impl ReversibleKeyGenerator for Tuple {
+	for_tuples!( type ReversibleHasher = ( #(Tuple::ReversibleHasher),* ); );
+
+	fn decode_final_key(key_material: &[u8]) -> Result<(Self::Key, &[u8]), codec::Error> {
+		let mut current_key_material = key_material;
+		Ok((
+			(for_tuples! {
+				#({
+					let (key, material) = Tuple::decode_final_key(current_key_material)?;
+					current_key_material = material;
+					key
+				}),*
+			}),
+			current_key_material,
+		))
+	}
+}
+
+/// Trait indicating whether a KeyGenerator has the prefix P.
+pub trait HasKeyPrefix<P>: KeyGenerator {
+	type Suffix;
+
+	fn partial_key(prefix: P) -> Vec<u8>;
+}
+
+/// Trait indicating whether a ReversibleKeyGenerator has the prefix P.
+pub trait HasReversibleKeyPrefix<P>: ReversibleKeyGenerator + HasKeyPrefix<P> {
+	fn decode_partial_key(key_material: &[u8]) -> Result<Self::Suffix, codec::Error>;
+}
+
+macro_rules! impl_key_prefix_for {
+	(($($keygen:ident),+), ($($prefix:ident),+), ($($suffix:ident),+)) => {
+		paste! {
+			impl<$($keygen: FullCodec,)+ $( [<$keygen $keygen>]: StorageHasher),+>
+				HasKeyPrefix<($($prefix),+)> for
+				($(Key<[<$keygen $keygen>], $keygen>),+)
+			{
+				type Suffix = ($($suffix),+);
+
+				fn partial_key(prefix: ($($prefix),+)) -> Vec<u8> {
+					<($(Key<[<$prefix $prefix>], $prefix>),+)>::final_key(prefix)
+				}
+			}
+
+			impl<$($keygen: FullCodec,)+ $( [<$keygen $keygen>]: ReversibleStorageHasher),+>
+				HasReversibleKeyPrefix<($($prefix),+)> for
+				($(Key<[<$keygen $keygen>], $keygen>),+)
+			{
+				fn decode_partial_key(key_material: &[u8]) -> Result<Self::Suffix, codec::Error> {
+					<($(Key<[<$suffix $suffix>], $suffix>),+)>::decode_final_key(key_material).map(|k| k.0)
+				}
+			}
+		}
+	};
+	(($($keygen:ident),+), $prefix:ident, ($($suffix:ident),+)) => {
+		paste! {
+			impl<$($keygen: FullCodec,)+ $( [<$keygen $keygen>]: StorageHasher),+>
+				HasKeyPrefix<($prefix,)> for
+				($(Key<[<$keygen $keygen>], $keygen>),+)
+			{
+				type Suffix = ($($suffix),+);
+
+				fn partial_key(prefix: ($prefix,)) -> Vec<u8> {
+					<Key<[<$prefix $prefix>], $prefix>>::final_key(prefix)
+				}
+			}
+
+			impl<$($keygen: FullCodec,)+ $( [<$keygen $keygen>]: ReversibleStorageHasher),+>
+				HasReversibleKeyPrefix<($prefix,)> for
+				($(Key<[<$keygen $keygen>], $keygen>),+)
+			{
+				fn decode_partial_key(key_material: &[u8]) -> Result<Self::Suffix, codec::Error> {
+					<($(Key<[<$suffix $suffix>], $suffix>),+)>::decode_final_key(key_material).map(|k| k.0)
+				}
+			}
+		}
+	};
+	(($($keygen:ident),+), ($($prefix:ident),+), $suffix:ident) => {
+		paste! {
+			impl<$($keygen: FullCodec,)+ $( [<$keygen $keygen>]: StorageHasher),+>
+				HasKeyPrefix<($($prefix),+)> for
+				($(Key<[<$keygen $keygen>], $keygen>),+)
+			{
+				type Suffix = $suffix;
+
+				fn partial_key(prefix: ($($prefix),+)) -> Vec<u8> {
+					<($(Key<[<$prefix $prefix>], $prefix>),+)>::final_key(prefix)
+				}
+			}
+
+			impl<$($keygen: FullCodec,)+ $( [<$keygen $keygen>]: ReversibleStorageHasher),+>
+				HasReversibleKeyPrefix<($($prefix),+)> for
+				($(Key<[<$keygen $keygen>], $keygen>),+)
+			{
+				fn decode_partial_key(key_material: &[u8]) -> Result<Self::Suffix, codec::Error> {
+					<Key<[<$suffix $suffix>], $suffix>>::decode_final_key(key_material).map(|k| k.0)
+				}
+			}
+		}
+	};
+}
+
+impl<A: FullCodec, B: FullCodec, X: StorageHasher, Y: StorageHasher> HasKeyPrefix<(A,)>
+	for (Key<X, A>, Key<Y, B>)
+{
+	type Suffix = B;
+
+	fn partial_key(prefix: (A,)) -> Vec<u8> {
+		<Key<X, A>>::final_key(prefix)
+	}
+}
+
+impl<A: FullCodec, B: FullCodec, X: ReversibleStorageHasher, Y: ReversibleStorageHasher>
+	HasReversibleKeyPrefix<(A,)> for (Key<X, A>, Key<Y, B>)
+{
+	fn decode_partial_key(key_material: &[u8]) -> Result<B, codec::Error> {
+		<Key<Y, B>>::decode_final_key(key_material).map(|k| k.0)
+	}
+}
+
+impl_key_prefix_for!((A, B, C), (A, B), C);
+impl_key_prefix_for!((A, B, C), A, (B, C));
+impl_key_prefix_for!((A, B, C, D), (A, B, C), D);
+impl_key_prefix_for!((A, B, C, D), (A, B), (C, D));
+impl_key_prefix_for!((A, B, C, D), A, (B, C, D));
+impl_key_prefix_for!((A, B, C, D, E), (A, B, C, D), E);
+impl_key_prefix_for!((A, B, C, D, E), (A, B, C), (D, E));
+impl_key_prefix_for!((A, B, C, D, E), (A, B), (C, D, E));
+impl_key_prefix_for!((A, B, C, D, E), A, (B, C, D, E));
+impl_key_prefix_for!((A, B, C, D, E, F), (A, B, C, D, E), F);
+impl_key_prefix_for!((A, B, C, D, E, F), (A, B, C, D), (E, F));
+impl_key_prefix_for!((A, B, C, D, E, F), (A, B, C), (D, E, F));
+impl_key_prefix_for!((A, B, C, D, E, F), (A, B), (C, D, E, F));
+impl_key_prefix_for!((A, B, C, D, E, F), A, (B, C, D, E, F));
+impl_key_prefix_for!((A, B, C, D, E, F, G), (A, B, C, D, E, F), G);
+impl_key_prefix_for!((A, B, C, D, E, F, G), (A, B, C, D, E), (F, G));
+impl_key_prefix_for!((A, B, C, D, E, F, G), (A, B, C, D), (E, F, G));
+impl_key_prefix_for!((A, B, C, D, E, F, G), (A, B, C), (D, E, F, G));
+impl_key_prefix_for!((A, B, C, D, E, F, G), (A, B), (C, D, E, F, G));
+impl_key_prefix_for!((A, B, C, D, E, F, G), A, (B, C, D, E, F, G));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H), (A, B, C, D, E, F, G), H);
+impl_key_prefix_for!((A, B, C, D, E, F, G, H), (A, B, C, D, E, F), (G, H));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H), (A, B, C, D, E), (F, G, H));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H), (A, B, C, D), (E, F, G, H));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H), (A, B, C), (D, E, F, G, H));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H), (A, B), (C, D, E, F, G, H));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H), A, (B, C, D, E, F, G, H));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H, I), (A, B, C, D, E, F, G, H), I);
+impl_key_prefix_for!((A, B, C, D, E, F, G, H, I), (A, B, C, D, E, F, G), (H, I));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H, I), (A, B, C, D, E, F), (G, H, I));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H, I), (A, B, C, D, E), (F, G, H, I));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H, I), (A, B, C, D), (E, F, G, H, I));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H, I), (A, B, C), (D, E, F, G, H, I));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H, I), (A, B), (C, D, E, F, G, H, I));
+impl_key_prefix_for!((A, B, C, D, E, F, G, H, I), A, (B, C, D, E, F, G, H, I));
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J),
+	(A, B, C, D, E, F, G, H, I),
+	J
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J),
+	(A, B, C, D, E, F, G, H),
+	(I, J)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J),
+	(A, B, C, D, E, F, G),
+	(H, I, J)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J),
+	(A, B, C, D, E, F),
+	(G, H, I, J)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J),
+	(A, B, C, D, E),
+	(F, G, H, I, J)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J),
+	(A, B, C, D),
+	(E, F, G, H, I, J)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J),
+	(A, B, C),
+	(D, E, F, G, H, I, J)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J),
+	(A, B),
+	(C, D, E, F, G, H, I, J)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J),
+	A,
+	(B, C, D, E, F, G, H, I, J)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(A, B, C, D, E, F, G, H, I, J),
+	K
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(A, B, C, D, E, F, G, H, I),
+	(J, K)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(A, B, C, D, E, F, G, H),
+	(I, J, K)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(A, B, C, D, E, F, G),
+	(H, I, J, K)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(A, B, C, D, E, F),
+	(G, H, I, J, K)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(A, B, C, D, E),
+	(F, G, H, I, J, K)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(A, B, C, D),
+	(E, F, G, H, I, J, K)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(A, B, C),
+	(D, E, F, G, H, I, J, K)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(A, B),
+	(C, D, E, F, G, H, I, J, K)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K),
+	A,
+	(B, C, D, E, F, G, H, I, J, K)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(A, B, C, D, E, F, G, H, I, J, K),
+	L
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(A, B, C, D, E, F, G, H, I, J),
+	(K, L)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(A, B, C, D, E, F, G, H, I),
+	(J, K, L)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(A, B, C, D, E, F, G, H),
+	(I, J, K, L)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(A, B, C, D, E, F, G),
+	(H, I, J, K, L)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(A, B, C, D, E, F),
+	(G, H, I, J, K, L)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(A, B, C, D, E),
+	(F, G, H, I, J, K, L)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(A, B, C, D),
+	(E, F, G, H, I, J, K, L)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(A, B, C),
+	(D, E, F, G, H, I, J, K, L)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(A, B),
+	(C, D, E, F, G, H, I, J, K, L)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	A,
+	(B, C, D, E, F, G, H, I, J, K, L)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	M
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(L, M)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(A, B, C, D, E, F, G, H, I, J),
+	(K, L, M)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(A, B, C, D, E, F, G, H, I),
+	(J, K, L, M)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(A, B, C, D, E, F, G, H),
+	(I, J, K, L, M)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(A, B, C, D, E, F, G),
+	(H, I, J, K, L, M)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(A, B, C, D, E, F),
+	(G, H, I, J, K, L, M)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(A, B, C, D, E),
+	(F, G, H, I, J, K, L, M)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(A, B, C, D),
+	(E, F, G, H, I, J, K, L, M)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(A, B, C),
+	(D, E, F, G, H, I, J, K, L, M)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(A, B),
+	(C, D, E, F, G, H, I, J, K, L, M)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	A,
+	(B, C, D, E, F, G, H, I, J, K, L, M)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	N
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(M, N)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(L, M, N)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(A, B, C, D, E, F, G, H, I, J),
+	(K, L, M, N)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(A, B, C, D, E, F, G, H, I),
+	(J, K, L, M, N)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(A, B, C, D, E, F, G, H),
+	(I, J, K, L, M, N)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(A, B, C, D, E, F, G),
+	(H, I, J, K, L, M, N)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(A, B, C, D, E, F),
+	(G, H, I, J, K, L, M, N)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(A, B, C, D, E),
+	(F, G, H, I, J, K, L, M, N)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(A, B, C, D),
+	(E, F, G, H, I, J, K, L, M, N)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(A, B, C),
+	(D, E, F, G, H, I, J, K, L, M, N)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(A, B),
+	(C, D, E, F, G, H, I, J, K, L, M, N)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	A,
+	(B, C, D, E, F, G, H, I, J, K, L, M, N)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	O
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(M, N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(L, M, N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B, C, D, E, F, G, H, I, J),
+	(K, L, M, N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B, C, D, E, F, G, H, I),
+	(J, K, L, M, N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B, C, D, E, F, G, H),
+	(I, J, K, L, M, N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B, C, D, E, F, G),
+	(H, I, J, K, L, M, N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B, C, D, E, F),
+	(G, H, I, J, K, L, M, N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B, C, D, E),
+	(F, G, H, I, J, K, L, M, N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B, C, D),
+	(E, F, G, H, I, J, K, L, M, N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B, C),
+	(D, E, F, G, H, I, J, K, L, M, N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(A, B),
+	(C, D, E, F, G, H, I, J, K, L, M, N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	A,
+	(B, C, D, E, F, G, H, I, J, K, L, M, N, O)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	P
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(M, N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(L, M, N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C, D, E, F, G, H, I, J),
+	(K, L, M, N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C, D, E, F, G, H, I),
+	(J, K, L, M, N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C, D, E, F, G, H),
+	(I, J, K, L, M, N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C, D, E, F, G),
+	(H, I, J, K, L, M, N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C, D, E, F),
+	(G, H, I, J, K, L, M, N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C, D, E),
+	(F, G, H, I, J, K, L, M, N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C, D),
+	(E, F, G, H, I, J, K, L, M, N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B, C),
+	(D, E, F, G, H, I, J, K, L, M, N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(A, B),
+	(C, D, E, F, G, H, I, J, K, L, M, N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	A,
+	(B, C, D, E, F, G, H, I, J, K, L, M, N, O, P)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	Q
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(M, N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(L, M, N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D, E, F, G, H, I, J),
+	(K, L, M, N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D, E, F, G, H, I),
+	(J, K, L, M, N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D, E, F, G, H),
+	(I, J, K, L, M, N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D, E, F, G),
+	(H, I, J, K, L, M, N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D, E, F),
+	(G, H, I, J, K, L, M, N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D, E),
+	(F, G, H, I, J, K, L, M, N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C, D),
+	(E, F, G, H, I, J, K, L, M, N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B, C),
+	(D, E, F, G, H, I, J, K, L, M, N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	(A, B),
+	(C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	A,
+	(B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q),
+	R
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P),
+	(Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O),
+	(P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N),
+	(O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E, F, G, H, I, J, K, L, M),
+	(N, O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E, F, G, H, I, J, K, L),
+	(M, N, O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E, F, G, H, I, J, K),
+	(L, M, N, O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E, F, G, H, I, J),
+	(K, L, M, N, O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E, F, G, H, I),
+	(J, K, L, M, N, O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E, F, G, H),
+	(I, J, K, L, M, N, O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E, F, G),
+	(H, I, J, K, L, M, N, O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E, F),
+	(G, H, I, J, K, L, M, N, O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D, E),
+	(F, G, H, I, J, K, L, M, N, O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C, D),
+	(E, F, G, H, I, J, K, L, M, N, O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B, C),
+	(D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	(A, B),
+	(C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R)
+);
+impl_key_prefix_for!(
+	(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R),
+	A,
+	(B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R)
+);
@@ -21,13 +21,20 @@
 use codec::FullCodec;
 use frame_metadata::{DefaultByte, StorageEntryModifier};

-mod value;
-mod map;
 mod double_map;
+mod key;
+mod map;
+mod nmap;
+mod value;

-pub use value::{StorageValue, StorageValueMetadata};
-pub use map::{StorageMap, StorageMapMetadata};
 pub use double_map::{StorageDoubleMap, StorageDoubleMapMetadata};
+pub use key::{
+	EncodeLikeTuple, HasKeyPrefix, HasReversibleKeyPrefix, Key, KeyGenerator,
+	ReversibleKeyGenerator, TupleToEncodedIter,
+};
+pub use map::{StorageMap, StorageMapMetadata};
+pub use nmap::{StorageNMap, StorageNMapMetadata};
+pub use value::{StorageValue, StorageValueMetadata};

 /// Trait implementing how the storage optional value is converted into the queried type.
 ///
@@ -104,5 +111,5 @@ impl<Value: FullCodec, OnEmpty: crate::traits::Get<Value>> DefaultByte
 		OnEmpty::get().encode()
 	}
 }
-unsafe impl <Value, OnEmpty: crate::traits::Get<Value>> Send for OnEmptyGetter<Value, OnEmpty> {}
-unsafe impl <Value, OnEmpty: crate::traits::Get<Value>> Sync for OnEmptyGetter<Value, OnEmpty> {}
+unsafe impl<Value, OnEmpty: crate::traits::Get<Value>> Send for OnEmptyGetter<Value, OnEmpty> {}
+unsafe impl<Value, OnEmpty: crate::traits::Get<Value>> Sync for OnEmptyGetter<Value, OnEmpty> {}
@@ -0,0 +1,995 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Storage map type. Implements StorageDoubleMap, StorageIterableDoubleMap,
+//! StoragePrefixedDoubleMap traits and their methods directly.
+
+use crate::{
+	storage::{
+		types::{
+			EncodeLikeTuple, HasKeyPrefix, HasReversibleKeyPrefix, OnEmptyGetter,
+			OptionQuery, QueryKindTrait, TupleToEncodedIter,
+		},
+		KeyGenerator, PrefixIterator, StorageAppend, StorageDecodeLength,
+	},
+	traits::{GetDefault, StorageInstance},
+};
+use codec::{Decode, Encode, EncodeLike, FullCodec};
+use frame_metadata::{DefaultByteGetter, StorageEntryModifier};
+use sp_std::prelude::*;
+
+/// A type that allow to store values for an arbitrary number of keys in the form of
+/// `(Key<Hasher1, key1>, Key<Hasher2, key2>, ..., Key<HasherN, keyN>)`.
+///
+/// Each value is stored at:
+/// ```nocompile
+/// Twox128(Prefix::pallet_prefix())
+///		++ Twox128(Prefix::STORAGE_PREFIX)
+///		++ Hasher1(encode(key1))
+///		++ Hasher2(encode(key2))
+/// 	++ ...
+/// 	++ HasherN(encode(keyN))
+/// ```
+///
+/// # Warning
+///
+/// If the keys are not trusted (e.g. can be set by a user), a cryptographic `hasher`
+/// such as `blake2_128_concat` must be used for the key hashers. Otherwise, other values
+/// in storage can be compromised.
+pub struct StorageNMap<Prefix, Key, Value, QueryKind = OptionQuery, OnEmpty = GetDefault>(
+	core::marker::PhantomData<(Prefix, Key, Value, QueryKind, OnEmpty)>,
+);
+
+impl<Prefix, Key, Value, QueryKind, OnEmpty> crate::storage::generator::StorageNMap<Key, Value>
+	for StorageNMap<Prefix, Key, Value, QueryKind, OnEmpty>
+where
+	Prefix: StorageInstance,
+	Key: super::key::KeyGenerator,
+	Value: FullCodec,
+	QueryKind: QueryKindTrait<Value, OnEmpty>,
+	OnEmpty: crate::traits::Get<QueryKind::Query> + 'static,
+{
+	type Query = QueryKind::Query;
+	fn module_prefix() -> &'static [u8] {
+		Prefix::pallet_prefix().as_bytes()
+	}
+	fn storage_prefix() -> &'static [u8] {
+		Prefix::STORAGE_PREFIX.as_bytes()
+	}
+	fn from_optional_value_to_query(v: Option<Value>) -> Self::Query {
+		QueryKind::from_optional_value_to_query(v)
+	}
+	fn from_query_to_optional_value(v: Self::Query) -> Option<Value> {
+		QueryKind::from_query_to_optional_value(v)
+	}
+}
+
+impl<Prefix, Key, Value, QueryKind, OnEmpty> crate::storage::StoragePrefixedMap<Value>
+	for StorageNMap<Prefix, Key, Value, QueryKind, OnEmpty>
+where
+	Prefix: StorageInstance,
+	Key: super::key::KeyGenerator,
+	Value: FullCodec,
+	QueryKind: QueryKindTrait<Value, OnEmpty>,
+	OnEmpty: crate::traits::Get<QueryKind::Query> + 'static,
+{
+	fn module_prefix() -> &'static [u8] {
+		<Self as crate::storage::generator::StorageNMap<Key, Value>>::module_prefix()
+	}
+	fn storage_prefix() -> &'static [u8] {
+		<Self as crate::storage::generator::StorageNMap<Key, Value>>::storage_prefix()
+	}
+}
+
+impl<Prefix, Key, Value, QueryKind, OnEmpty> StorageNMap<Prefix, Key, Value, QueryKind, OnEmpty>
+where
+	Prefix: StorageInstance,
+	Key: super::key::KeyGenerator,
+	Value: FullCodec,
+	QueryKind: QueryKindTrait<Value, OnEmpty>,
+	OnEmpty: crate::traits::Get<QueryKind::Query> + 'static,
+{
+	/// Get the storage key used to fetch a value corresponding to a specific key.
+	pub fn hashed_key_for<KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter>(key: KArg) -> Vec<u8> {
+		<Self as crate::storage::StorageNMap<Key, Value>>::hashed_key_for(key)
+	}
+
+	/// Does the value (explicitly) exist in storage?
+	pub fn contains_key<KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter>(key: KArg) -> bool {
+		<Self as crate::storage::StorageNMap<Key, Value>>::contains_key(key)
+	}
+
+	/// Load the value associated with the given key from the map.
+	pub fn get<KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter>(key: KArg) -> QueryKind::Query {
+		<Self as crate::storage::StorageNMap<Key, Value>>::get(key)
+	}
+
+	/// Try to get the value for the given key from the map.
+	///
+	/// Returns `Ok` if it exists, `Err` if not.
+	pub fn try_get<KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter>(
+		key: KArg,
+	) -> Result<Value, ()> {
+		<Self as crate::storage::StorageNMap<Key, Value>>::try_get(key)
+	}
+
+	/// Take a value from storage, removing it afterwards.
+	pub fn take<KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter>(key: KArg) -> QueryKind::Query {
+		<Self as crate::storage::StorageNMap<Key, Value>>::take(key)
+	}
+
+	/// Swap the values of two key-pairs.
+	pub fn swap<KOther, KArg1, KArg2>(key1: KArg1, key2: KArg2)
+	where
+		KOther: KeyGenerator,
+		KArg1: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter,
+		KArg2: EncodeLikeTuple<KOther::KArg> + TupleToEncodedIter,
+	{
+		<Self as crate::storage::StorageNMap<Key, Value>>::swap::<KOther, _, _>(key1, key2)
+	}
+
+	/// Store a value to be associated with the given keys from the map.
+	pub fn insert<KArg, VArg>(key: KArg, val: VArg)
+	where
+		KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter,
+		VArg: EncodeLike<Value>,
+	{
+		<Self as crate::storage::StorageNMap<Key, Value>>::insert(key, val)
+	}
+
+	/// Remove the value under the given keys.
+	pub fn remove<KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter>(key: KArg) {
+		<Self as crate::storage::StorageNMap<Key, Value>>::remove(key)
+	}
+
+	/// Remove all values under the first key.
+	pub fn remove_prefix<KP>(partial_key: KP)
+	where
+		Key: HasKeyPrefix<KP>,
+	{
+		<Self as crate::storage::StorageNMap<Key, Value>>::remove_prefix(partial_key)
+	}
+
+	/// Iterate over values that share the first key.
+	pub fn iter_prefix_values<KP>(partial_key: KP) -> PrefixIterator<Value>
+	where
+		Key: HasKeyPrefix<KP>,
+	{
+		<Self as crate::storage::StorageNMap<Key, Value>>::iter_prefix_values(partial_key)
+	}
+
+	/// Mutate the value under the given keys.
+	pub fn mutate<KArg, R, F>(key: KArg, f: F) -> R
+	where
+		KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter,
+		F: FnOnce(&mut QueryKind::Query) -> R,
+	{
+		<Self as crate::storage::StorageNMap<Key, Value>>::mutate(key, f)
+	}
+
+	/// Mutate the value under the given keys when the closure returns `Ok`.
+	pub fn try_mutate<KArg, R, E, F>(key: KArg, f: F) -> Result<R, E>
+	where
+		KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter,
+		F: FnOnce(&mut QueryKind::Query) -> Result<R, E>,
+	{
+		<Self as crate::storage::StorageNMap<Key, Value>>::try_mutate(key, f)
+	}
+
+	/// Mutate the value under the given keys. Deletes the item if mutated to a `None`.
+	pub fn mutate_exists<KArg, R, F>(key: KArg, f: F) -> R
+	where
+		KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter,
+		F: FnOnce(&mut Option<Value>) -> R,
+	{
+		<Self as crate::storage::StorageNMap<Key, Value>>::mutate_exists(key, f)
+	}
+
+	/// Mutate the item, only if an `Ok` value is returned. Deletes the item if mutated to a `None`.
+	pub fn try_mutate_exists<KArg, R, E, F>(key: KArg, f: F) -> Result<R, E>
+	where
+		KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter,
+		F: FnOnce(&mut Option<Value>) -> Result<R, E>,
+	{
+		<Self as crate::storage::StorageNMap<Key, Value>>::try_mutate_exists(key, f)
+	}
+
+	/// Append the given item to the value in the storage.
+	///
+	/// `Value` is required to implement [`StorageAppend`].
+	///
+	/// # Warning
+	///
+	/// If the storage item is not encoded properly, the storage will be overwritten
+	/// and set to `[item]`. Any default value set for the storage item will be ignored
+	/// on overwrite.
+	pub fn append<Item, EncodeLikeItem, KArg>(key: KArg, item: EncodeLikeItem)
+	where
+		KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter,
+		Item: Encode,
+		EncodeLikeItem: EncodeLike<Item>,
+		Value: StorageAppend<Item>,
+	{
+		<Self as crate::storage::StorageNMap<Key, Value>>::append(key, item)
+	}
+
+	/// Read the length of the storage value without decoding the entire value under the
+	/// given `key1` and `key2`.
+	///
+	/// `Value` is required to implement [`StorageDecodeLength`].
+	///
+	/// If the value does not exists or it fails to decode the length, `None` is returned.
+	/// Otherwise `Some(len)` is returned.
+	///
+	/// # Warning
+	///
+	/// `None` does not mean that `get()` does not return a value. The default value is completly
+	/// ignored by this function.
+	pub fn decode_len<KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter>(key: KArg) -> Option<usize>
+	where
+		Value: StorageDecodeLength,
+	{
+		<Self as crate::storage::StorageNMap<Key, Value>>::decode_len(key)
+	}
+
+	/// Migrate an item with the given `key` from defunct `hash_fns` to the current hashers.
+	///
+	/// If the key doesn't exist, then it's a no-op. If it does, then it returns its value.
+	pub fn migrate_keys<KArg>(key: KArg, hash_fns: Key::HArg) -> Option<Value>
+	where
+		KArg: EncodeLikeTuple<Key::KArg> + TupleToEncodedIter
+	{
+		<Self as crate::storage::StorageNMap<Key, Value>>::migrate_keys::<_>(key, hash_fns)
+	}
+
+	/// Remove all value of the storage.
+	pub fn remove_all() {
+		<Self as crate::storage::StoragePrefixedMap<Value>>::remove_all()
+	}
+
+	/// Iter over all value of the storage.
+	///
+	/// NOTE: If a value failed to decode becaues storage is corrupted then it is skipped.
+	pub fn iter_values() -> crate::storage::PrefixIterator<Value> {
+		<Self as crate::storage::StoragePrefixedMap<Value>>::iter_values()
+	}
+
+	/// Translate the values of all elements by a function `f`, in the map in no particular order.
+	/// By returning `None` from `f` for an element, you'll remove it from the map.
+	///
+	/// NOTE: If a value fail to decode because storage is corrupted then it is skipped.
+	///
+	/// # Warning
+	///
+	/// This function must be used with care, before being updated the storage still contains the
+	/// old type, thus other calls (such as `get`) will fail at decoding it.
+	///
+	/// # Usage
+	///
+	/// This would typically be called inside the module implementation of on_runtime_upgrade.
+	pub fn translate_values<OldValue: Decode, F: FnMut(OldValue) -> Option<Value>>(f: F) {
+		<Self as crate::storage::StoragePrefixedMap<Value>>::translate_values(f)
+	}
+}
+
+impl<Prefix, Key, Value, QueryKind, OnEmpty> StorageNMap<Prefix, Key, Value, QueryKind, OnEmpty>
+where
+	Prefix: StorageInstance,
+	Key: super::key::ReversibleKeyGenerator,
+	Value: FullCodec,
+	QueryKind: QueryKindTrait<Value, OnEmpty>,
+	OnEmpty: crate::traits::Get<QueryKind::Query> + 'static,
+{
+	/// Enumerate all elements in the map with prefix key `kp` in no particular order.
+	///
+	/// If you add or remove values whose prefix key is `kp` to the map while doing this, you'll get
+	/// undefined results.
+	pub fn iter_prefix<KP>(
+		kp: KP,
+	) -> crate::storage::PrefixIterator<(<Key as HasKeyPrefix<KP>>::Suffix, Value)>
+	where
+		Key: HasReversibleKeyPrefix<KP>,
+	{
+		<Self as crate::storage::IterableStorageNMap<Key, Value>>::iter_prefix(kp)
+	}
+
+	/// Remove all elements from the map with prefix key `kp` and iterate through them in no
+	/// particular order.
+	///
+	/// If you add elements with prefix key `k1` to the map while doing this, you'll get undefined
+	/// results.
+	pub fn drain_prefix<KP>(
+		kp: KP,
+	) -> crate::storage::PrefixIterator<(<Key as HasKeyPrefix<KP>>::Suffix, Value)>
+	where
+		Key: HasReversibleKeyPrefix<KP>,
+	{
+		<Self as crate::storage::IterableStorageNMap<Key, Value>>::drain_prefix(kp)
+	}
+
+	/// Enumerate all elements in the map in no particular order.
+	///
+	/// If you add or remove values to the map while doing this, you'll get undefined results.
+	pub fn iter() -> crate::storage::PrefixIterator<(Key::Key, Value)> {
+		<Self as crate::storage::IterableStorageNMap<Key, Value>>::iter()
+	}
+
+	/// Remove all elements from the map and iterate through them in no particular order.
+	///
+	/// If you add elements to the map while doing this, you'll get undefined results.
+	pub fn drain() -> crate::storage::PrefixIterator<(Key::Key, Value)> {
+		<Self as crate::storage::IterableStorageNMap<Key, Value>>::drain()
+	}
+
+	/// Translate the values of all elements by a function `f`, in the map in no particular order.
+	///
+	/// By returning `None` from `f` for an element, you'll remove it from the map.
+	///
+	/// NOTE: If a value fail to decode because storage is corrupted then it is skipped.
+	pub fn translate<O: Decode, F: FnMut(Key::Key, O) -> Option<Value>>(f: F) {
+		<Self as crate::storage::IterableStorageNMap<Key, Value>>::translate(f)
+	}
+}
+
+/// Part of storage metadata for a storage n map.
+///
+/// NOTE: Generic hashers is supported.
+pub trait StorageNMapMetadata {
+	const MODIFIER: StorageEntryModifier;
+	const NAME: &'static str;
+	const DEFAULT: DefaultByteGetter;
+	const HASHERS: &'static [frame_metadata::StorageHasher];
+}
+
+impl<Prefix, Key, Value, QueryKind, OnEmpty> StorageNMapMetadata
+	for StorageNMap<Prefix, Key, Value, QueryKind, OnEmpty>
+where
+	Prefix: StorageInstance,
+	Key: super::key::KeyGenerator,
+	Value: FullCodec,
+	QueryKind: QueryKindTrait<Value, OnEmpty>,
+	OnEmpty: crate::traits::Get<QueryKind::Query> + 'static,
+{
+	const MODIFIER: StorageEntryModifier = QueryKind::METADATA;
+	const NAME: &'static str = Prefix::STORAGE_PREFIX;
+	const DEFAULT: DefaultByteGetter = DefaultByteGetter(
+		&OnEmptyGetter::<QueryKind::Query, OnEmpty>(core::marker::PhantomData),
+	);
+	const HASHERS: &'static [frame_metadata::StorageHasher] = Key::HASHER_METADATA;
+}
+
+#[cfg(test)]
+mod test {
+	use super::*;
+	use crate::hash::*;
+	use crate::storage::types::{Key, ValueQuery};
+	use frame_metadata::StorageEntryModifier;
+	use sp_io::{hashing::twox_128, TestExternalities};
+
+	struct Prefix;
+	impl StorageInstance for Prefix {
+		fn pallet_prefix() -> &'static str {
+			"test"
+		}
+		const STORAGE_PREFIX: &'static str = "foo";
+	}
+
+	struct ADefault;
+	impl crate::traits::Get<u32> for ADefault {
+		fn get() -> u32 {
+			98
+		}
+	}
+
+	#[test]
+	fn test_1_key() {
+		type A = StorageNMap<Prefix, Key<Blake2_128Concat, u16>, u32, OptionQuery>;
+		type AValueQueryWithAnOnEmpty =
+			StorageNMap<Prefix, Key<Blake2_128Concat, u16>, u32, ValueQuery, ADefault>;
+		type B = StorageNMap<Prefix, Key<Blake2_256, u16>, u32, ValueQuery>;
+		type C = StorageNMap<Prefix, Key<Blake2_128Concat, u16>, u8, ValueQuery>;
+		type WithLen = StorageNMap<Prefix, Key<Blake2_128Concat, u16>, Vec<u32>>;
+
+		TestExternalities::default().execute_with(|| {
+			let mut k: Vec<u8> = vec![];
+			k.extend(&twox_128(b"test"));
+			k.extend(&twox_128(b"foo"));
+			k.extend(&3u16.blake2_128_concat());
+			assert_eq!(A::hashed_key_for((&3,)).to_vec(), k);
+
+			assert_eq!(A::contains_key((3,)), false);
+			assert_eq!(A::get((3,)), None);
+			assert_eq!(AValueQueryWithAnOnEmpty::get((3,)), 98);
+
+			A::insert((3,), 10);
+			assert_eq!(A::contains_key((3,)), true);
+			assert_eq!(A::get((3,)), Some(10));
+			assert_eq!(AValueQueryWithAnOnEmpty::get((3,)), 10);
+
+			A::swap::<Key<Blake2_128Concat, u16>, _, _>((3,), (2,));
+			assert_eq!(A::contains_key((3,)), false);
+			assert_eq!(A::contains_key((2,)), true);
+			assert_eq!(A::get((3,)), None);
+			assert_eq!(AValueQueryWithAnOnEmpty::get((3,)), 98);
+			assert_eq!(A::get((2,)), Some(10));
+			assert_eq!(AValueQueryWithAnOnEmpty::get((2,)), 10);
+
+			A::remove((2,));
+			assert_eq!(A::contains_key((2,)), false);
+			assert_eq!(A::get((2,)), None);
+
+			AValueQueryWithAnOnEmpty::mutate((2,), |v| *v = *v * 2);
+			AValueQueryWithAnOnEmpty::mutate((2,), |v| *v = *v * 2);
+			assert_eq!(A::contains_key((2,)), true);
+			assert_eq!(A::get((2,)), Some(98 * 4));
+
+			A::remove((2,));
+			let _: Result<(), ()> = AValueQueryWithAnOnEmpty::try_mutate((2,), |v| {
+				*v = *v * 2;
+				Ok(())
+			});
+			let _: Result<(), ()> = AValueQueryWithAnOnEmpty::try_mutate((2,), |v| {
+				*v = *v * 2;
+				Ok(())
+			});
+			assert_eq!(A::contains_key((2,)), true);
+			assert_eq!(A::get((2,)), Some(98 * 4));
+
+			A::remove((2,));
+			let _: Result<(), ()> = AValueQueryWithAnOnEmpty::try_mutate((2,), |v| {
+				*v = *v * 2;
+				Err(())
+			});
+			assert_eq!(A::contains_key((2,)), false);
+
+			A::remove((2,));
+			AValueQueryWithAnOnEmpty::mutate_exists((2,), |v| {
+				assert!(v.is_none());
+				*v = Some(10);
+			});
+			assert_eq!(A::contains_key((2,)), true);
+			assert_eq!(A::get((2,)), Some(10));
+			AValueQueryWithAnOnEmpty::mutate_exists((2,), |v| {
+				*v = Some(v.unwrap() * 10);
+			});
+			assert_eq!(A::contains_key((2,)), true);
+			assert_eq!(A::get((2,)), Some(100));
+
+			A::remove((2,));
+			let _: Result<(), ()> = AValueQueryWithAnOnEmpty::try_mutate_exists((2,), |v| {
+				assert!(v.is_none());
+				*v = Some(10);
+				Ok(())
+			});
+			assert_eq!(A::contains_key((2,)), true);
+			assert_eq!(A::get((2,)), Some(10));
+			let _: Result<(), ()> = AValueQueryWithAnOnEmpty::try_mutate_exists((2,), |v| {
+				*v = Some(v.unwrap() * 10);
+				Ok(())
+			});
+			assert_eq!(A::contains_key((2,)), true);
+			assert_eq!(A::get((2,)), Some(100));
+			assert_eq!(A::try_get((2,)), Ok(100));
+			let _: Result<(), ()> = AValueQueryWithAnOnEmpty::try_mutate_exists((2,), |v| {
+				*v = Some(v.unwrap() * 10);
+				Err(())
+			});
+			assert_eq!(A::contains_key((2,)), true);
+			assert_eq!(A::get((2,)), Some(100));
+
+			A::insert((2,), 10);
+			assert_eq!(A::take((2,)), Some(10));
+			assert_eq!(A::contains_key((2,)), false);
+			assert_eq!(AValueQueryWithAnOnEmpty::take((2,)), 98);
+			assert_eq!(A::contains_key((2,)), false);
+			assert_eq!(A::try_get((2,)), Err(()));
+
+			B::insert((2,), 10);
+			assert_eq!(
+				A::migrate_keys((2,), (Box::new(|key| Blake2_256::hash(key).to_vec()),),),
+				Some(10)
+			);
+			assert_eq!(A::contains_key((2,)), true);
+			assert_eq!(A::get((2,)), Some(10));
+
+			A::insert((3,), 10);
+			A::insert((4,), 10);
+			A::remove_all();
+			assert_eq!(A::contains_key((3,)), false);
+			assert_eq!(A::contains_key((4,)), false);
+
+			A::insert((3,), 10);
+			A::insert((4,), 10);
+			assert_eq!(A::iter_values().collect::<Vec<_>>(), vec![10, 10]);
+
+			C::insert((3,), 10);
+			C::insert((4,), 10);
+			A::translate_values::<u8, _>(|v| Some((v * 2).into()));
+			assert_eq!(A::iter().collect::<Vec<_>>(), vec![(4, 20), (3, 20)]);
+
+			A::insert((3,), 10);
+			A::insert((4,), 10);
+			assert_eq!(A::iter().collect::<Vec<_>>(), vec![(4, 10), (3, 10)]);
+			assert_eq!(A::drain().collect::<Vec<_>>(), vec![(4, 10), (3, 10)]);
+			assert_eq!(A::iter().collect::<Vec<_>>(), vec![]);
+
+			C::insert((3,), 10);
+			C::insert((4,), 10);
+			A::translate::<u8, _>(|k1, v| Some((k1 as u16 * v as u16).into()));
+			assert_eq!(A::iter().collect::<Vec<_>>(), vec![(4, 40), (3, 30)]);
+
+			assert_eq!(A::MODIFIER, StorageEntryModifier::Optional);
+			assert_eq!(
+				AValueQueryWithAnOnEmpty::MODIFIER,
+				StorageEntryModifier::Default
+			);
+			assert_eq!(A::NAME, "foo");
+			assert_eq!(
+				AValueQueryWithAnOnEmpty::DEFAULT.0.default_byte(),
+				98u32.encode()
+			);
+			assert_eq!(A::DEFAULT.0.default_byte(), Option::<u32>::None.encode());
+
+			WithLen::remove_all();
+			assert_eq!(WithLen::decode_len((3,)), None);
+			WithLen::append((0,), 10);
+			assert_eq!(WithLen::decode_len((0,)), Some(1));
+		});
+	}
+
+	#[test]
+	fn test_2_keys() {
+		type A = StorageNMap<
+			Prefix,
+			(Key<Blake2_128Concat, u16>, Key<Twox64Concat, u8>),
+			u32,
+			OptionQuery,
+		>;
+		type AValueQueryWithAnOnEmpty = StorageNMap<
+			Prefix,
+			(Key<Blake2_128Concat, u16>, Key<Twox64Concat, u8>),
+			u32,
+			ValueQuery,
+			ADefault,
+		>;
+		type B = StorageNMap<Prefix, (Key<Blake2_256, u16>, Key<Twox128, u8>), u32, ValueQuery>;
+		type C = StorageNMap<
+			Prefix,
+			(Key<Blake2_128Concat, u16>, Key<Twox64Concat, u8>),
+			u8,
+			ValueQuery,
+		>;
+		type WithLen =
+			StorageNMap<Prefix, (Key<Blake2_128Concat, u16>, Key<Twox64Concat, u8>), Vec<u32>>;
+
+		TestExternalities::default().execute_with(|| {
+			let mut k: Vec<u8> = vec![];
+			k.extend(&twox_128(b"test"));
+			k.extend(&twox_128(b"foo"));
+			k.extend(&3u16.blake2_128_concat());
+			k.extend(&30u8.twox_64_concat());
+			assert_eq!(A::hashed_key_for((3, 30)).to_vec(), k);
+
+			assert_eq!(A::contains_key((3, 30)), false);
+			assert_eq!(A::get((3, 30)), None);
+			assert_eq!(AValueQueryWithAnOnEmpty::get((3, 30)), 98);
+
+			A::insert((3, 30), 10);
+			assert_eq!(A::contains_key((3, 30)), true);
+			assert_eq!(A::get((3, 30)), Some(10));
+			assert_eq!(AValueQueryWithAnOnEmpty::get((3, 30)), 10);
+
+			A::swap::<(Key<Blake2_128Concat, u16>, Key<Twox64Concat, u8>), _, _>((3, 30), (2, 20));
+			assert_eq!(A::contains_key((3, 30)), false);
+			assert_eq!(A::contains_key((2, 20)), true);
+			assert_eq!(A::get((3, 30)), None);
+			assert_eq!(AValueQueryWithAnOnEmpty::get((3, 30)), 98);
+			assert_eq!(A::get((2, 20)), Some(10));
+			assert_eq!(AValueQueryWithAnOnEmpty::get((2, 20)), 10);
+
+			A::remove((2, 20));
+			assert_eq!(A::contains_key((2, 20)), false);
+			assert_eq!(A::get((2, 20)), None);
+
+			AValueQueryWithAnOnEmpty::mutate((2, 20), |v| *v = *v * 2);
+			AValueQueryWithAnOnEmpty::mutate((2, 20), |v| *v = *v * 2);
+			assert_eq!(A::contains_key((2, 20)), true);
+			assert_eq!(A::get((2, 20)), Some(98 * 4));
+
+			A::remove((2, 20));
+			let _: Result<(), ()> = AValueQueryWithAnOnEmpty::try_mutate((2, 20), |v| {
+				*v = *v * 2;
+				Err(())
+			});
+			assert_eq!(A::contains_key((2, 20)), false);
+
+			A::remove((2, 20));
+			let _: Result<(), ()> = AValueQueryWithAnOnEmpty::try_mutate((2, 20), |v| {
+				*v = *v * 2;
+				Err(())
+			});
+			assert_eq!(A::contains_key((2, 20)), false);
+
+			A::remove((2, 20));
+			AValueQueryWithAnOnEmpty::mutate_exists((2, 20), |v| {
+				assert!(v.is_none());
+				*v = Some(10);
+			});
+			assert_eq!(A::contains_key((2, 20)), true);
+			assert_eq!(A::get((2, 20)), Some(10));
+			AValueQueryWithAnOnEmpty::mutate_exists((2, 20), |v| {
+				*v = Some(v.unwrap() * 10);
+			});
+			assert_eq!(A::contains_key((2, 20)), true);
+			assert_eq!(A::get((2, 20)), Some(100));
+
+			A::remove((2, 20));
+			let _: Result<(), ()> = AValueQueryWithAnOnEmpty::try_mutate_exists((2, 20), |v| {
+				assert!(v.is_none());
+				*v = Some(10);
+				Ok(())
+			});
+			assert_eq!(A::contains_key((2, 20)), true);
+			assert_eq!(A::get((2, 20)), Some(10));
+			let _: Result<(), ()> = AValueQueryWithAnOnEmpty::try_mutate_exists((2, 20), |v| {
+				*v = Some(v.unwrap() * 10);
+				Ok(())
+			});
+			assert_eq!(A::contains_key((2, 20)), true);
+			assert_eq!(A::get((2, 20)), Some(100));
+			assert_eq!(A::try_get((2, 20)), Ok(100));
+			let _: Result<(), ()> = AValueQueryWithAnOnEmpty::try_mutate_exists((2, 20), |v| {
+				*v = Some(v.unwrap() * 10);
+				Err(())
+			});
+			assert_eq!(A::contains_key((2, 20)), true);
+			assert_eq!(A::get((2, 20)), Some(100));
+
+			A::insert((2, 20), 10);
+			assert_eq!(A::take((2, 20)), Some(10));
+			assert_eq!(A::contains_key((2, 20)), false);
+			assert_eq!(AValueQueryWithAnOnEmpty::take((2, 20)), 98);
+			assert_eq!(A::contains_key((2, 20)), false);
+			assert_eq!(A::try_get((2, 20)), Err(()));
+
+			B::insert((2, 20), 10);
+			assert_eq!(
+				A::migrate_keys(
+					(2, 20),
+					(
+						Box::new(|key| Blake2_256::hash(key).to_vec()),
+						Box::new(|key| Twox128::hash(key).to_vec()),
+					),
+				),
+				Some(10)
+			);
+			assert_eq!(A::contains_key((2, 20)), true);
+			assert_eq!(A::get((2, 20)), Some(10));
+
+			A::insert((3, 30), 10);
+			A::insert((4, 40), 10);
+			A::remove_all();
+			assert_eq!(A::contains_key((3, 30)), false);
+			assert_eq!(A::contains_key((4, 40)), false);
+
+			A::insert((3, 30), 10);
+			A::insert((4, 40), 10);
+			assert_eq!(A::iter_values().collect::<Vec<_>>(), vec![10, 10]);
+
+			C::insert((3, 30), 10);
+			C::insert((4, 40), 10);
+			A::translate_values::<u8, _>(|v| Some((v * 2).into()));
+			assert_eq!(
+				A::iter().collect::<Vec<_>>(),
+				vec![((4, 40), 20), ((3, 30), 20)]
+			);
+
+			A::insert((3, 30), 10);
+			A::insert((4, 40), 10);
+			assert_eq!(
+				A::iter().collect::<Vec<_>>(),
+				vec![((4, 40), 10), ((3, 30), 10)]
+			);
+			assert_eq!(
+				A::drain().collect::<Vec<_>>(),
+				vec![((4, 40), 10), ((3, 30), 10)]
+			);
+			assert_eq!(A::iter().collect::<Vec<_>>(), vec![]);
+
+			C::insert((3, 30), 10);
+			C::insert((4, 40), 10);
+			A::translate::<u8, _>(|(k1, k2), v| Some((k1 * k2 as u16 * v as u16).into()));
+			assert_eq!(
+				A::iter().collect::<Vec<_>>(),
+				vec![((4, 40), 1600), ((3, 30), 900)]
+			);
+
+			assert_eq!(A::MODIFIER, StorageEntryModifier::Optional);
+			assert_eq!(
+				AValueQueryWithAnOnEmpty::MODIFIER,
+				StorageEntryModifier::Default
+			);
+			assert_eq!(A::NAME, "foo");
+			assert_eq!(
+				AValueQueryWithAnOnEmpty::DEFAULT.0.default_byte(),
+				98u32.encode()
+			);
+			assert_eq!(A::DEFAULT.0.default_byte(), Option::<u32>::None.encode());
+
+			WithLen::remove_all();
+			assert_eq!(WithLen::decode_len((3, 30)), None);
+			WithLen::append((0, 100), 10);
+			assert_eq!(WithLen::decode_len((0, 100)), Some(1));
+
+			A::insert((3, 30), 11);
+			A::insert((3, 31), 12);
+			A::insert((4, 40), 13);
+			A::insert((4, 41), 14);
+			assert_eq!(
+				A::iter_prefix_values((3,)).collect::<Vec<_>>(),
+				vec![12, 11]
+			);
+			assert_eq!(
+				A::iter_prefix_values((4,)).collect::<Vec<_>>(),
+				vec![13, 14]
+			);
+		});
+	}
+
+	#[test]
+	fn test_3_keys() {
+		type A = StorageNMap<
+			Prefix,
+			(
+				Key<Blake2_128Concat, u16>,
+				Key<Blake2_128Concat, u16>,
+				Key<Twox64Concat, u16>,
+			),
+			u32,
+			OptionQuery,
+		>;
+		type AValueQueryWithAnOnEmpty = StorageNMap<
+			Prefix,
+			(
+				Key<Blake2_128Concat, u16>,
+				Key<Blake2_128Concat, u16>,
+				Key<Twox64Concat, u16>,
+			),
+			u32,
+			ValueQuery,
+			ADefault,
+		>;
+		type B = StorageNMap<
+			Prefix,
+			(
+				Key<Blake2_256, u16>,
+				Key<Blake2_256, u16>,
+				Key<Twox128, u16>,
+			),
+			u32,
+			ValueQuery,
+		>;
+		type C = StorageNMap<
+			Prefix,
+			(
+				Key<Blake2_128Concat, u16>,
+				Key<Blake2_128Concat, u16>,
+				Key<Twox64Concat, u16>,
+			),
+			u8,
+			ValueQuery,
+		>;
+		type WithLen = StorageNMap<
+			Prefix,
+			(
+				Key<Blake2_128Concat, u16>,
+				Key<Blake2_128Concat, u16>,
+				Key<Twox64Concat, u16>,
+			),
+			Vec<u32>,
+		>;
+
+		TestExternalities::default().execute_with(|| {
+			let mut k: Vec<u8> = vec![];
+			k.extend(&twox_128(b"test"));
+			k.extend(&twox_128(b"foo"));
+			k.extend(&1u16.blake2_128_concat());
+			k.extend(&10u16.blake2_128_concat());
+			k.extend(&100u16.twox_64_concat());
+			assert_eq!(A::hashed_key_for((1, 10, 100)).to_vec(), k);
+
+			assert_eq!(A::contains_key((1, 10, 100)), false);
+			assert_eq!(A::get((1, 10, 100)), None);
+			assert_eq!(AValueQueryWithAnOnEmpty::get((1, 10, 100)), 98);
+
+			A::insert((1, 10, 100), 30);
+			assert_eq!(A::contains_key((1, 10, 100)), true);
+			assert_eq!(A::get((1, 10, 100)), Some(30));
+			assert_eq!(AValueQueryWithAnOnEmpty::get((1, 10, 100)), 30);
+
+			A::swap::<
+				(
+					Key<Blake2_128Concat, u16>,
+					Key<Blake2_128Concat, u16>,
+					Key<Twox64Concat, u16>,
+				),
+				_,
+				_,
+			>((1, 10, 100), (2, 20, 200));
+			assert_eq!(A::contains_key((1, 10, 100)), false);
+			assert_eq!(A::contains_key((2, 20, 200)), true);
+			assert_eq!(A::get((1, 10, 100)), None);
+			assert_eq!(AValueQueryWithAnOnEmpty::get((1, 10, 100)), 98);
+			assert_eq!(A::get((2, 20, 200)), Some(30));
+			assert_eq!(AValueQueryWithAnOnEmpty::get((2, 20, 200)), 30);
+
+			A::remove((2, 20, 200));
+			assert_eq!(A::contains_key((2, 20, 200)), false);
+			assert_eq!(A::get((2, 20, 200)), None);
+
+			AValueQueryWithAnOnEmpty::mutate((2, 20, 200), |v| *v = *v * 2);
+			AValueQueryWithAnOnEmpty::mutate((2, 20, 200), |v| *v = *v * 2);
+			assert_eq!(A::contains_key((2, 20, 200)), true);
+			assert_eq!(A::get((2, 20, 200)), Some(98 * 4));
+
+			A::remove((2, 20, 200));
+			let _: Result<(), ()> = AValueQueryWithAnOnEmpty::try_mutate((2, 20, 200), |v| {
+				*v = *v * 2;
+				Err(())
+			});
+			assert_eq!(A::contains_key((2, 20, 200)), false);
+
+			A::remove((2, 20, 200));
+			AValueQueryWithAnOnEmpty::mutate_exists((2, 20, 200), |v| {
+				assert!(v.is_none());
+				*v = Some(10);
+			});
+			assert_eq!(A::contains_key((2, 20, 200)), true);
+			assert_eq!(A::get((2, 20, 200)), Some(10));
+			AValueQueryWithAnOnEmpty::mutate_exists((2, 20, 200), |v| {
+				*v = Some(v.unwrap() * 10);
+			});
+			assert_eq!(A::contains_key((2, 20, 200)), true);
+			assert_eq!(A::get((2, 20, 200)), Some(100));
+
+			A::remove((2, 20, 200));
+			let _: Result<(), ()> =
+				AValueQueryWithAnOnEmpty::try_mutate_exists((2, 20, 200), |v| {
+					assert!(v.is_none());
+					*v = Some(10);
+					Ok(())
+				});
+			assert_eq!(A::contains_key((2, 20, 200)), true);
+			assert_eq!(A::get((2, 20, 200)), Some(10));
+			let _: Result<(), ()> =
+				AValueQueryWithAnOnEmpty::try_mutate_exists((2, 20, 200), |v| {
+					*v = Some(v.unwrap() * 10);
+					Ok(())
+				});
+			assert_eq!(A::contains_key((2, 20, 200)), true);
+			assert_eq!(A::get((2, 20, 200)), Some(100));
+			assert_eq!(A::try_get((2, 20, 200)), Ok(100));
+			let _: Result<(), ()> =
+				AValueQueryWithAnOnEmpty::try_mutate_exists((2, 20, 200), |v| {
+					*v = Some(v.unwrap() * 10);
+					Err(())
+				});
+			assert_eq!(A::contains_key((2, 20, 200)), true);
+			assert_eq!(A::get((2, 20, 200)), Some(100));
+
+			A::insert((2, 20, 200), 10);
+			assert_eq!(A::take((2, 20, 200)), Some(10));
+			assert_eq!(A::contains_key((2, 20, 200)), false);
+			assert_eq!(AValueQueryWithAnOnEmpty::take((2, 20, 200)), 98);
+			assert_eq!(A::contains_key((2, 20, 200)), false);
+			assert_eq!(A::try_get((2, 20, 200)), Err(()));
+
+			B::insert((2, 20, 200), 10);
+			assert_eq!(
+				A::migrate_keys(
+					(2, 20, 200),
+					(
+						Box::new(|key| Blake2_256::hash(key).to_vec()),
+						Box::new(|key| Blake2_256::hash(key).to_vec()),
+						Box::new(|key| Twox128::hash(key).to_vec()),
+					),
+				),
+				Some(10)
+			);
+			assert_eq!(A::contains_key((2, 20, 200)), true);
+			assert_eq!(A::get((2, 20, 200)), Some(10));
+
+			A::insert((3, 30, 300), 10);
+			A::insert((4, 40, 400), 10);
+			A::remove_all();
+			assert_eq!(A::contains_key((3, 30, 300)), false);
+			assert_eq!(A::contains_key((4, 40, 400)), false);
+
+			A::insert((3, 30, 300), 10);
+			A::insert((4, 40, 400), 10);
+			assert_eq!(A::iter_values().collect::<Vec<_>>(), vec![10, 10]);
+
+			C::insert((3, 30, 300), 10);
+			C::insert((4, 40, 400), 10);
+			A::translate_values::<u8, _>(|v| Some((v * 2).into()));
+			assert_eq!(
+				A::iter().collect::<Vec<_>>(),
+				vec![((4, 40, 400), 20), ((3, 30, 300), 20)]
+			);
+
+			A::insert((3, 30, 300), 10);
+			A::insert((4, 40, 400), 10);
+			assert_eq!(
+				A::iter().collect::<Vec<_>>(),
+				vec![((4, 40, 400), 10), ((3, 30, 300), 10)]
+			);
+			assert_eq!(
+				A::drain().collect::<Vec<_>>(),
+				vec![((4, 40, 400), 10), ((3, 30, 300), 10)]
+			);
+			assert_eq!(A::iter().collect::<Vec<_>>(), vec![]);
+
+			C::insert((3, 30, 300), 10);
+			C::insert((4, 40, 400), 10);
+			A::translate::<u8, _>(|(k1, k2, k3), v| {
+				Some((k1 * k2 as u16 * v as u16 / k3 as u16).into())
+			});
+			assert_eq!(
+				A::iter().collect::<Vec<_>>(),
+				vec![((4, 40, 400), 4), ((3, 30, 300), 3)]
+			);
+
+			assert_eq!(A::MODIFIER, StorageEntryModifier::Optional);
+			assert_eq!(
+				AValueQueryWithAnOnEmpty::MODIFIER,
+				StorageEntryModifier::Default
+			);
+			assert_eq!(A::NAME, "foo");
+			assert_eq!(
+				AValueQueryWithAnOnEmpty::DEFAULT.0.default_byte(),
+				98u32.encode()
+			);
+			assert_eq!(A::DEFAULT.0.default_byte(), Option::<u32>::None.encode());
+
+			WithLen::remove_all();
+			assert_eq!(WithLen::decode_len((3, 30, 300)), None);
+			WithLen::append((0, 100, 1000), 10);
+			assert_eq!(WithLen::decode_len((0, 100, 1000)), Some(1));
+
+			A::insert((3, 30, 300), 11);
+			A::insert((3, 30, 301), 12);
+			A::insert((4, 40, 400), 13);
+			A::insert((4, 40, 401), 14);
+			assert_eq!(
+				A::iter_prefix_values((3,)).collect::<Vec<_>>(),
+				vec![11, 12]
+			);
+			assert_eq!(
+				A::iter_prefix_values((4,)).collect::<Vec<_>>(),
+				vec![14, 13]
+			);
+			assert_eq!(
+				A::iter_prefix_values((3, 30)).collect::<Vec<_>>(),
+				vec![11, 12]
+			);
+			assert_eq!(
+				A::iter_prefix_values((4, 40)).collect::<Vec<_>>(),
+				vec![14, 13]
+			);
+		});
+	}
+}