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I want to query a smart contract which I deployed on the Aleph Zero testnet. The Polkadot.js documentation is pretty clear and easy to follow regarding connecting to the chain and querying general chain information. But I get a bit confused when it comes to querying information regarding deployed smart contracts.

Here is what I have so far:

// Required imports
import { ApiPromise, WsProvider } from '@polkadot/api';
import { ContractPromise } from '@polkadot/api-contract';

// Initialise the provider to connect to the local node
const provider = new WsProvider('wss://ws.test.azero.dev');

// Create the API and wait until ready
const api = await ApiPromise.create({ provider });
    
const contract = new ContractPromise(api, metadata, wasm);

I am a bit confused about what the metadata and wasm are in this context. The documentation just sends me in a circle of links here.

Also, when I have established the connection to the right deployed smart contract, how can I query or execute methods to get information about it?

1 Answer 1

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Following the Polkadot JS docs, I don't see where wasm is used tbh

import { ContractPromise } from '@polkadot/api-contract';

// The address is the actual on-chain address as ss58 or AccountId object.
const contract = new ContractPromise(api, metadata, address);

But anyways, the alephZero docs also mention the following aspects in this section:

The resulting files will be placed in mytoken/target/ink/ folder. If the compilation is successful you will find there the following 3 files:

  • mytoken.wasm is a binary WASM file with the compiled contract
  • metadata.json containing our contracts ABI (Application Binary Interface)
  • mytoken.contract which bundles the above two for more convenient interaction with the chain explorer

So, once you compile your smart contract, and because you mentioned that you've deployed it, I assume you have a metadata.json file, which is the one that you have to add here new ContractPromise(api, metadata, address);

Instead of a wasm file, the third argument address is the actual address of your smart contract deployed on the testnet, its identifier.

So let's assume you have a directory src where you store the metadata.json file and the address of your smart contract is 5DewKiJMRAaeRuYH48wmyXiTwzxedkq6zziRNr2EmirZATK3. You need to create the ContractPromise like:

... //snip
// Create the API and wait until ready
const api = await ApiPromise.create({ provider });
const scAddress = '5DewKiJMRAaeRuYH48wmyXiTwzxedkq6zziRNr2EmirZATK3';

const contract = new ContractPromise(api, 'src/metadata.json', scAddress);

Finally, in order to interact with your smart contract, you can follow this guide: https://polkadot.js.org/docs/api-contract/start/contract.read#reading-contract-values

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  • I successfully loaded the contract. Thank you for your answer. However, when trying to interact with the smart contract I always encounter this error. "Uncaught TypeError: contract.query.balanceOf is not a function". Do you have an idea what could be the reason for this?
    – rajohs
    Aug 8, 2022 at 16:58
  • If your smart contract does not have that function implemented, then it will throw that error. You can execute queries over your items defined under the macro#[ink(storage)], but I'm not sure at all.
    – andresvsm
    Aug 9, 2022 at 7:58
  • 1
    That was partially correct. I am implementing a PSP34 contract and it looks like you cannot query the methods implemented in the original PSP34 contract, but only the ones you defined yourself using #[ink(message)]
    – rajohs
    Aug 9, 2022 at 12:14

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