gistfile1.rs

// creating a state dump file (one time job), later on it's uploaded to S3 or something.
// We could build an API service with an archive node that can compute state dumps at any block you request and then cache them.
remote_externalities::Builder::new()
		.module("PhragmenElection")
		.uri(URI.to_owned())
		.at(now)
    .dump_to_file(format!("PhragmenElection-at-{}-polkadot.statedump", now, URI))
    .await;
    
/// actual test (can be re-run as many times you want, without the need to access an actual node)
remote_externalities::Builder::new()
     // this reads a local cached file (or requests from a S3 bucket if not found)
    .from_state_dump("./PhragmenElection-at-<hash>-polkadot.statedump")
		.build_async()
		.await
		.execute_with(|| {
			let random_voter = <StorageKeyIterator<AccountId, (Balance, Vec<AccountId>), frame_support::Twox64Concat>>::new(
				b"PhragmenElection",
				b"Voting",
			).skip(5)
			.take(1)
			.map(|(voter, (stake, votes))| (voter, stake, votes))
			.collect::<Vec<_>>();

			let (random_voter, stake, votes) = random_voter.first().unwrap();


			pallet_elections_phragmen::migrations::migrate_to_recorded_deposit::<Runtime>(1000_000);

			let voting = <Voting<Runtime>>::get(random_voter);
			assert_eq!(&voting.votes, votes);
			assert_eq!(voting.stake, *stake);
			assert_eq!(voting.deposit, 1000_000);
		})