How to use the .multi() method? (substrate-front-end-template)

Question

I finished the substrate kitty tutorial (backend and frontend). Now I would like to build a simple frontend for the RMRK pallet. Now I stuck on the .multi(...) method. Where can I find a documentation about this .multi-method?

      unsub = await api.query.rmrkCore.collections.multi(collectionIds, collections => {
          const collectionsMap = collections.map(collection => parseCollection(collection.unwrap()));
          setCollections(collectionsMap);
      });

The collection.unwrap() returns an error because it is [null,null]

--> However, with the following code I get a nice map<id,collections>

const entries = await api.query.rmrkCore.collections.entries();
const ids = entries.map(entry => entry[0]);
setCollectionIds(ids);

My goal is to parse the collections and store them (setCollections(collectionsMap))

I am grateful about any help :)

Answer 1

I found an example in the polkadot-js-docs. In short: it is a multi-query to retrieve several objects. I guess in the kitty-tutorial first the ids of the single objects are retrieved and second for each id the body is queried in a multi-query.

here the example from the docs:

// Import the API
const { ApiPromise } = require('@polkadot/api');

// Our address for Alice on the dev chain
const ALICE = '5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY';
const BOB = '5FHneW46xGXgs5mUiveU4sbTyGBzmstUspZC92UhjJM694ty';

async function main () {
  // Create our API with a default connection to the local node
  const api = await ApiPromise.create();

  // Retrieve the last block header, extracting the hash and parentHash
  const { hash, parentHash } = await api.rpc.chain.getHeader();

  console.log(`last header hash ${hash.toHex()}`);

  // Retrieve the balance at the preceding block for Alice. For at queries
  // the format is always `.at(<blockhash>, ...params)`
  const balance = await api.query.system.account.at(parentHash, ALICE);

  console.log(`Alice's balance at ${parentHash.toHex()} was ${balance.data.free}`);

  // Now perform a multi query, returning multiple balances at once
  const balances = await api.query.system.account.multi([ALICE, BOB]);

  console.log(`Current balances for Alice and Bob are ${balances[0].data.free} and ${balances[1].data.free}`);
}