How to do "erasStakers" in Rust?

Question

Very sorry for such a newbie question; just don't know where else to ask 🙏

Essentially, I want to programmatically dump from the network who won the most recent election. To do so, I'm issuing an erasStakers(...)[https://polkadot.js.org/docs/substrate/storage/#erasstakersu32-accountid32-palletstakingexposure] RPC call. So far as I understand it, there is no direct RPC API call for erasStakers(...), and one needs to interact with the node through getStorage(...) to learn the election result.

Yet the Polkadot Explorer somehow shows the human-readable output:

However, when I do in my Nodejs code:

await api.query.staking.erasStakers(635, null)
// => {"total":0,"own":0,"others":[]}

It seems the Polkadot Node just returns an empty object.

I also know about Parity's substrate-debug-kit[https://github.com/paritytech/substrate-debug-kit/blob/master/offline-election/src/subcommands/staking.rs] repo, which queries the Polkadot's node. I believe I could leverage sub_storage::read from that repo, but I can't guess the correct "Prefix" parameter and the return type to get erasStakers; it just says "get_pairs failed", whatever I try.

Is there a way to get either in my NodeJS or in my Rust code something akin to what I see in the picture — a human readable structure representing an erasStakers(...) call?

I'm new to Polkadot, so apologies if I mixed up terms, or misunderstood something trivial — any help would be much appreciated.

Answer 1

As per a comment above, to iterate over a map, you need to use the entries methods, for double maps there is also a cookbook example.

In the case of api.query.staking.erasStakers(...) it returns a specific entry, and only the value. In the case of api.query.staking.erasStakers.entries(...) it returns the full map, will all keys & values for each entry.

(This is obviously a much heavier operation, apart from the fact that the return type is different, since it needs to chunk the values and iterate on the Node RPC side)