Unable to return string using Oraclize

Question

I'm trying to do a simple coin flip using WolframAlpha and Oraclize, this is my smart contract

pragma solidity ^0.4.18;
import './usingOraclize.sol';

contract CoinFlipOracle is usingOraclize {

  string public result;
  bytes32 public oraclizeID;

  event Log(string text);

  function CoinFlipOracle() {
    Log("Contract created");
  }

  function flipCoin() payable {
    Log("Oraclize query was sent, waiting for response");
    oraclizeID = oraclize_query("WolframAlpha", "flip a coin", 5000000);
  }

  function __callback(bytes32 _oraclizeID, string _result) {
    require(msg.sender == oraclize_cbAddress());
    Log(_result);
    result = _result;
  }

  function getResult() view returns(string) {
    return result;
  }
}

On my frontend, I deploy contract, then I call flipCoin(), and finally I call getResult()

async flipCoin() {
  let contractInstance = await CoinFlipContract.at(this.state.contractAddress);
  await contractInstance.flipCoin();
  let result = await contractInstance.getResult();
  console.log(result);
}

I'm trying to return either "heads" or "tails", but all it returns is the txhash plus some other info. Am I using Oraclize correctly?

Answer 1

Riccardo from Oraclize here. Hope it is not too late.

I just tested your contract and the method flipCoin() works.

pragma solidity ^0.4.18;

import "./oraclize/usingOraclize.sol";


contract CoinFlipOracle is usingOraclize {

    string public result;
    bytes32 public oraclizeID;

    event Log(string text);

    function CoinFlipOracle() {
        Log("Contract created");
    }

    function flipCoin() payable {
        Log("Oraclize query was sent, waiting for response");
        oraclizeID = oraclize_query("WolframAlpha", "flip a coin", 5000000);
    }

    function __callback(bytes32 _oraclizeID, string _result) {
        require(msg.sender == oraclize_cbAddress());
        Log(_result);
        result = _result;
    }

    function getResult() view returns(string) {
        return result;
    }
}

I tested it in .js, and the test is returning me "heads" as it should. You have to be completely sure that your callback returns.

contract('CoinFlipOracle.flipCoin()', accounts => {
    const owner = accounts[0]
    const issuer1 = accounts[1]

    it("Should check if flipCoin() works", async () => {
        console.log("owner: ", owner);
        console.log("issuer1: ", issuer1);
        const coinFlipOracle = await CoinFlipOracle.deployed()
        let result = await coinFlipOracle.result.call({ from: issuer1 })
        console.log("result: ", result);
        await coinFlipOracle.flipCoin({ from: issuer1, value: web3.toWei(2, 'ether') })
        wait(20000); // **** WAIT FOR THE CALLBACK TO RETURN ***
        result = await coinFlipOracle.result.call({ from: issuer1 })
        console.log("result: ", result);
    })

})